H-Physics-MatterEnergy

Biblical Prophets Revealed God’s World According to Their Beliefs

Recent Prophets Unravel Gods Real World

Fantastic Beyond Human Imagination.

Left: A uranium atom, with 92 electrons and 143 protons can be readily split, releasing substantial energy.

Right: Matter into energy: A cloud mushrooms from 11 kiloton atomic bomb blast above the Nevada desert test site, on September 14, 1957

The search to unravel the mysteries of matter and energy has been going on for over a millennia. While we have uncovered a vast store of knowledge, that search is being pushed even further by scientists in our own day. One thing we have learned clearly is tht mathematics is crucial for understanding many aspects of the natural world. But even the simplest parts of mathematics – those poarts taken easily and for granted today – were developed gradually by our ancestors, in response to their own needs and desires. Our story must begin with numbers.

The written language and mathematics developed in the same period in human history is no coincidence. Both are ways to describe the physical world, and both require the ability to put information regarding the physical world into a set of symbols. But symbols are the final step in the process that took humans millions of years. When it began, humans, likd many animals, most likely could recognize fewer and greater more than from less. They could count intuitively – like a bird that senses the number of eggs in its nest of the wasp that senses how many caterpillars it feeds it’s young – and could recognize that a pile of four stones was different from a pile of ten stones or that a distance of four steps was different from a distance of ten. What is hard for us to comprehend, however, is that although there were stones and steps, there was no "four," no "ten."

If there were any numerical truths recognized at all, they might have come out of self‑recognition: an understanding of the number one, for instance, through recognition of one body, one head, one mouth; or the truth of two, since as bilaterally symmetrical beings, we have two of most appendages. One indicated singularity; two might have indicated comparison or opposition: male and female, day and night, sun and moon, Earth and sky, water and land, hot and cold. In ritual and religious practice even today, some numbers are believed to speak potent and primal truths.

TOOLS FOR COUNTING AND MEASURING

When did the human mind begin to enlarge its store of such truths? Experience and tradition told preliterate migratory herders or hunters how to shape and size their tools and weapons, how close they needed to be to their prey, or how long their next journey would take. Did our ancient ancestors more than a million years ago count the number of times they struck a stone to turn it into a blade? Before they went off to hunt, did they count the number of stones they carried with them? Did they tally up their prey? Did they mark how many strides they had taken in their day's journey? If they did not do so a million years ago, when human brains were 37 cubic inches smaller than they are today, then when did such acts of outright counting begin?

Did the complexities of life in changing environments 50,000 years ago create a need to make superior calculations? Did making such calculations require our larger brains? Does the understanding of numbers‑both those that denote quantity, called cardinal numbers, and those that denote order, ordinal numbers ‑ signify the start of human culture?

Little evidence exists of when people developed the first forms of counting and arithmetic, or when they began to measure the regular movements of the sun, moon, stars, and planets. If they didn't wander aimlessly in search of food and shelter, did they take bearings of some kind to mark their journeys? Surely prehistoric humans noticed that shadows shifted over the course of the day and that a circle could be drawn around a stick placed in the ground and marked off at intervals to measure the day's progress. Such devices, called gnomons (Greek for "the one that knows"), are still used by tribes such as Africa's Bushmen. And the fact that some of the earliest systems of measurement known were based on parts of the human body‑the span of a hand, the length of an arm, foot, or stride‑indicates that these methods have long been in use.

600 B.C. Thales of Miletus concludes that the fundamental element of life and Earth is water.

500 B.C. The Pythagoreans study mathematics and greatly advance the study o geometry.

420 B.C. Greek philosopher Democritus argues that all matter is made of tiny, indivisible constituents that he calls atoms.

320‑260 B.C. Euclid collects all geometric knowledge and extends it in his famous work, Elements.

300 B.C. Mathematicians in India make first use of zero as a numeric placeholder.

287‑212 B.C. Archimedes determines that the buoyant force on a floating object is equal to the weight of the fluid it displaces.

50 B.C. The base‑ten numeral system is developed in India.

Left: Elaborate or simple, the sundial signifies how the human sense of time connects intimately with the natural world.

Center: Pictographs pressed to clay tablets served as account books in ancient Sumeria. Out of these wedge‑shaped symbols called cuneiform came the first written language. The ancient East's most important and wide spread writing, was cuneiform said to be the equivalent of the Latin alphabet in he ancient Middle East. The earliest known people to use cuneiform were the Sumerians.

Right: Lost homework? These figures likely represent a mathematics problem diagrammed on a typical hand‑size Babylonian clay tablet. The solution is written out above the problem in cuneiform.

Counting is a different matter. It is a great leap to go from looking at two trees, for instance, to putting two sticks on the ground to represent those trees, and a greater leap still to add groups of sticks together. Anthropologists and archaeologists surmise that the advent of agriculture and domestication of animals in the Middle East some 10,000 years ago moved this process along. Counting on fingers or toes or counting a few notches in a piece of wood worked for limited numbers. But once hunters and nomads settled in fertile lands to raise grain or cattle, they had to manipulate larger quantities of goods. Even in early settlements they had to account for grain, which they stored in communal silos. Such new quantities required a more advanced and streamlined counting methodology.

By 7500 B.C. in the land that was called Surneria, now southern Iraq, small clay tokens in various shapes were used to keep track of farmers' stores. Imagine the kind of tokens used in children's board games. A small clay marble would stand for a bushel of grain, a cylinder might represent an animal, and a little egg‑shaped token was a jar of oil. Such counters have been found in the shape of spheres, disks, and little pyramids.

These artifacts come from before the Bronze Age, a time when clay was fired at temperatures high enough to give it some lasting .hardness, a time when artful potery was being produced. Each token represented a single unit, and they could be counted up to assess one's holdings.

As more goods were manufactured‑cloth, perfumes, tools counting tokens became more elaborate and, as archaeologist Denise Schmandt‑Besserat theorized, could be used for a purpose new to civilization: tax collection. The tokens themselves were stored in clay globes, marked on the outside to indicate the shape of tokens being kept inside. Soon the marks that illustrated the tokens began to replace the tokens themselves, and the clay globes were flattened into more readily inscribed tablets. Instead of 50 egg‑shaped tokens, 50 egg‑shaped marks would be carved with a stylus on a clay tablet.

According to Schmandt‑Besserat, a great advance came around 3100 B.C., Instead of recording 50 bushels of grain with 50 images of grain tokens, a system was devised by which the symbol for a bushel of grain would be preceded by a special sign or combination of signs that represented the number 50. The number was still not completely abstract, in that it referred to some specific item. Two, for instance, still meant nothing on its own, only when used to denote two of something. But the fact that the same symbols could be used for quantities of different things was a giant step beyond simple counting. And although only two number symbols came into use to begin with, they still served to create a simple base‑ten system‑intuitive, perhaps, because of the number of fingers on the human hand. A wedge came to stand for one; a circle came to stand for ten.

By 3000 B.C. clay tokens were obsolete, replaced by durable clay accounting tablets and a new system of pictographic data storage that would be used and refined in the Middle East for the next 3,000 years. This period of history also saw the beginning of written language, using a new collection of inscribed symbols that became known as cuneiform, meaning "wedge‑shaped." This seemingly simple development happened over more than 4,000 years, yet it led to extraordinary advances in knowledge about the world, as human beings came to use numbers to measure and describe their new observations and ideas.

Earth, Air, Fire, Water

What is the world made of? The human senses appear to have provided diverse W ancient cultures with the same answer to this basic question, first, in China, around 2,000 B.C., then in ancient India, and finally in Greece, in the fifth century B.C.: All concluded that all matter was composed of a few basic elements. For the Chinese, the elements were water, metal, wood, fire, and earth; for the Greeks and Indians, they were earth, air, fire, and water. For the ancients, physics and metaphysics were rarely far apart, and these elements were imbued with conceptual as well as actual attributes.

In Greece, in the early fifth century B.C., Empedocles first articulated the idea of the four elements. Poet, philosopher, politician, and follower of Pythagoras, Empedocles concluded that earth, air, fire, and water form the rhizomata, or roots, of all life and matter. In opposition to atomists such as Democritus, who believed everything was composed of either atoms or void, and to Parmenicles, who believed that matter could be neither created nor destroyed, Empeclocies asserted that the elements were in constant flux and stress, acted upon by the two great forces of the cosmos, attraction and repulsion‑or, as he called, them, love and strife.

These forces created the world's recurrent cycle of creation and destruction.

Empedocles's idea that matter could be created then destroyed ran counter to Aristotle's view that all things were created with a purpose. Earth, air, water, and fire were combined within all matter, Aristotle agreed, but they combined in ways that advanced the perfection of all things along a chain of being from the low and inanimate to the human and divine. Aristotle worked out in intricate detail just how the four elements, paired in opposites‑earth to air, water to fire‑changed one into the other:

"Air, for example, will result from Fire if a single quality changes," he wrote, "for Fire, as we saw, is hot and dry, while Air is hot and moist, so that there will be Air if the dry be overcome by the moist.... The transformation of Fire into Water and of Air into Earth, and again of Water and Earth into Fire and Air, respectively, though possible, is more difficult, because it involves the change of more qualities."

The physician Hippocrates, in the fifth century BC, believed that health resulted from a balance of four bodily humors ‑ blood, phlegm, and two kinds of bile – which were analogous to four major organs, the four seasons, four ages of man, and the four elements. Throughout the Middle Ages, this system of symbolic qualities was as important as the elements themselves. The balance of wetness and dryness, heat and cold, signified spiritual and physical health.

Like so many of Aristotle's ideas, the theory of four elements and four corresponding bodily humors long went unchallenged. When alchemists in the Middle Ages began discovering the actual physical properties of materials, the theory of the Aristotelian elements came into question.

English chemist Robert Boyle disputed the nature of the four elements in 1661. True elements, he argued, can neither be decomposed into nor formed from other materials. By the time of the Renaissance, the interpretation of the elements was recognized as metaphorical, not scientific. In the late 18th century French chemist Antoine Lavoisier published a list of elements based on Boyle’s criteria, marking the starting point for today’s periodic table.

Left: Earth, Air, Fire, Water the four elements – link in this Italian woodcut dating from 1496

Center: Pythagoras is now chiefly known to the general public as the Man who explained the mathematical relationship among the sides of a right triangle. Whether he was the first to do so is unclear, but the concept spread far and wide.

Right: This document is a 13^th century Arabic rendition of Euclid's proof of the Pythagorean theorem.

Right: The Pythagorean Theorem is used and honored the world around. A coin from Uganda shows a right triangle, the formula for the theorem, and an image of Pythagoras. Pythagoras and his followers believed that reality was mathematical in nature. With intense study, the hidden harmonies and mathematical order of the cosmos could be discerned.

The Babylonians, a part of the eater Mesopotamian cultures of e Fertile Crescent, began refining k‑re advances of the Sumerians. They developed more number signs and created systems within which one unit could be converted into another, in the same way we convert ounces into pounds and pounds into tons. For instance, ten cones equaled one small circle. Six small circles equaled one big cone. Ten big cones equaled a big cone with a circle inside, and six of these were represented by a large circle. The Babylonians worked on a base unit of 60, comparable to our units of angles and time. A large circle with a small circle inside equaled ten large circles: 10 x 6 x 10 x 6 x 10, or 36,000 base units.

Different counting systems were applied to different things‑the system used for counting up grain stores was not the same as the system used for counting cattle and yet the symbols in each system could be identical.

Over the next thousand years, the complexities of this transitional period were streamlined into a system that used only two symbols: A wedge shape standing on its point represented numbers less than ten; a wedge pointing left represented the number 10. Addition, subtraction, multiplication, and division were done the same way we do them now, except that numbers were carried at 60 rather than 10. (Think, for example, of 53 minutes plus 20 minutes. The addition of 7 minutes makes one hour, and the 13 minutes are carried over.) To make things easier, the Babylonians created extensive multiplication and division tables, so that large calculations could be broken down into smaller ones that could then be added together. With this system, the Babylonians could accomplish sophisticated calculations.

The Babylonian penchant for creating mathematical tables did not seem to carry over to creating general mathematical formulas. They were able to accomplish algebraic calculations ‑ such as finding the base and height of a rectangle when their product and sum were known ‑ but they did this by a series of steps and not by working with what we would recognize as an equation.

The Greeks claimed that their early knowledge came from the Egyptians. Although Egypt may have seemed the more accomplished civilization at the time, when it came to mathematics it never extended its base‑ten system much beyond practical arithmetic. Still, the Greeks had established trade with Egypt by the seventh century B.C., and by the third century B.C., Egypt was part of the Greek empire.

Wherever mathematics developed in the ancient world ‑ and it developed independently in China and India, as well as among the Inca in the Western Hemisphere – it gave cultures a new way to express their increased wealth, productivity, and knowledge. Written language that developed at the same time served much the same purpose. Just as writing isn't necessarily philosophy, so measurement does not become science until it is used to investigate natural phenomena. Parlaying the language of mathematics into scientific discoveries required yet one more step: the conviction that nature itself was subject to mathematical laws.

How did this understanding come about? The earliest evidence points to Greece in the seventh century B.C., then a confederation of freethinking, politically capricious, and independent states stretching from the Greek mainland across the Aegean to the coast of Turkey.

In the middle of a trade network that brought goods and ideas from Africa, the Middle East, India, and China, this Ionian civilization arose quickly, inspired from the beginning with intellectual curiosity. Schools of philosophy sprang up on small islands or in the midst of growing cities and made their founders household names. In the late seventh and early sixth centuries, natural philosophers from the Ionian city of Miletus on the Turkish coast ‑ including Thales, his student Anaximander, and his student Anaximenes ‑ developed intricate cosmogonies. Their studies led them to the conclusion that simple mathematical ratios could in fact describe the universe.

The Pythagoreans in Italy took this idea to its extreme. Their work had a great impact on the course of Greek‑and, later, all scientific thought. Nothing of the actual teachings of Pythagoras remains, but his influence on mathematics, science, and all Western thought is undeniable.

Born in 560 B.C. on the island of Samos, just off the Turkish coast, Pythagoras traveled as a young man to Egypt and Babylonia. When he returned to Samos, he established a cult like order of men and women devoted to asceticism, vegetarianism, temperance, Politics, philosophy, astronomy, reincarnation, music, and mathematics. Pythagoras and his followers discovered the numerical ratios that determine the intervals between notes in a musical scale. From music, they extended their search for mathematical relationships to geometric forms and number progressions, always in search of harmonies and symmetries.

For us, the importance of these developments ties in the recognition that something as natural as sound was liable to mathematical law. The Pythagoreans made it a theoretical discipline: mathematical statements required rigorous proofs. For the Pythagoreans, "all things are number." In the regular movements of the stars and planets the Pythagoreans sought the “music of the spheres." It was their contention that geometry might be the way for humans to comprehend the universe and bring life itself into a state of harmony.

The Pythagoreans were awed by a strange discovery: They recognized that there was a constant ratio between the lengths of the hypotenuse and the two sides of any right triangle, but they could not find a way to express it as a fraction made with the numbers they knew.

What kind of a number was this that could not be expressed exactly? They were on the verge of discovering what we today call irrational numbers. Their investigation eventually led to the formula that we now call the Pythagorean theorem.

The theorem is expressed mathematically as a² + b² = c², where a and b are the lengths of the sides and c is the length of the hypotenuse. If the triangle's sides are one unit long, then using the theorem shows that c, the length of the hypotenuse, would be the square root of two. The Pythagoreans were right: There is no way to represent this value as a ratio of two other numbers. In our modern decimal notation, the numbers coming after the decimal point (1.414213 .... ) neither repeat in a pattern nor end. The ratio of the diameter to the circumference of a circle, called pi, was similarly a mathematical entity that could not be expressed in known numbers.

After Pythagoras, Greek thinkers found a spiritual allure in geometric forms as well. Plato argued that all things we observe or encounter in daily life are flawed reflections of timeless, ideal forms that exist outside the realm of earthly human experience. The Platonists, following the lead of the Pythagoreans, asserted that nature's forms and phenomena possessed inherent mathematical symmetries‑new truths‑just awaiting discovery.

The idea that mathematics could describe nature represents a huge conceptual advance. For example, it is generally believed that Plato set mathematical astronomy into motion by urging his students to uncover the regular, harmonious, mathematically perfect movements of the planets hidden behind their seemingly erratic heavenly paths. The Platonic view is summarized in a motto, repeated for centuries thereafter: "God always geometrizes." Almost two millennia later, Galileo would say essentially the same thing, declaring that the universe is a "great book ... written in the language of mathematics; its characters are triangles, circles and other geometric figures."

The search for geometrical truths inspired Euclid, who lived around 300 B.C., to record all the geometric knowledge up to his day, from ancient Babylon up to the work of Eudoxus, around 400 B.C.

In the resulting book, Elements, Euclid states that all of geometry derives from four basic postulates: that a straight line can be drawn between any two points; that any straight line can be extended indefinitely in a straight line; that given any straight line segment, a circle can be drawn with that line as its radius and one endpoint at its center; and, finally, that all right angles are congruent. Euclid's careful step-by‑step mathematical reasoning in Elements became the standard for mathematical proofs.

Euclid's book has served as a primary geometry text for more than 2,000 years. "Almost from the time of its writing and lasting almost to the present, the Elements has exerted a continuous and major influence on human affairs," wrote Dutch mathematician B. L. van der Waerden. "It was the primary source of geometric reasoning, theorems, and methods at least until the advent of non‑Euclidean geometry in the 19th century."

THE GREEKS: MIND AND MATTER

Alongside Greek advancements in geometry, mathematics, and astronomy, the study of nature called physics likewise draws its origins from the Greeks. These early natural philosophers questioned the world of matter. How did it come into being? What was the primary substance of the cosmos? Was it air, water, fire, or some combination of them all? Could matter come from nothing? Was there a creator? Did life have a purpose, or did things come into being by chance? Was the universe of matter always the same, or was it always changing? For many, these were metaphysical questions, not liable to mathematical solutions.

Some early philosophers of the seventh and sixth centuries B.C. developed their concepts of the cosmos around a primary element or combination of elements. That the forms of matter were so diverse and various‑from nonliving to living things‑meant that these elements must be in constant flux, at some point finding a balance between coming into being and passing out of being, over and over. These concepts governed the main stream of debate until the fifth century B.C., when the conversation diverged into two paths.

Parmenides of Elea, born around 515 B.C., argued that matter coming and going would mean that at a certain point, nonbeing must exist, and that this was not possible since being is, and, if it is, it remains as it is. As Parmenides wrote in his poem "The Way of Truth , ' "Never shall this prevail, that what is not is: Keep your thoughts from following that path!”

Empedocles, a contemporary and perhaps a pupil of Parmenides, agreed that an immutable cosmos once established, remains unchanged, but argued that change in the world in which we live was possible through the intricate interactions of the four “roots” of all matter: earth, water, air, and fire. These roots were together by love and strife – the names he gave to the forces of attration and repulsion. Each toot had it’s own characteristics, and all matter was made up of different combinations of these roots, soon called elements. Democritius of Abdera took a very different stand in the debate. Born circa 460 B.C., he came to be known as the laughing philosopher, for the attitude of amusement he maintained toward the human condition. He is rumored to have lived to be a hundred.

Left: The Greek mathematician Euclid is widely regarded as the greatest teacher of geometry, past and present.

Middle: Euclid’s teachings in geometry prompted attempts to locate the Earth in relationship to the stars, sun, moon, and planets. Ptolemy placed the Earth at the center of the cosmos, a mistake that proved difficult to correct.

Right: Greek Philosopher Democritus belived both space andmater were made up of an infinite number of indivisible and vanishing small units called atoms. Atoms remained in constant motion he believed and could combine into seemingly solid form.

Democritus saw no reason why being and nonbeing could not coexist. "No less aught is there than naught," he chided Parmenides's adherents. He envisioned the world as a great void in which there is a constant rain of imperceptibly tiny and impenetrable atoms (a word from the Greek, atomos, meaning unable to be cut or divided) of all different shapes and sizes. Random collisions of atoms form objects; those objects decompose when the atoms come apart.

While Plato directed his students to seek the eternal realities lurking behind the imperfect appearances of the world, his most famous student, Aristotle, greatly valued the direct observation of nature. Aristotle studied not only the exalted celestial motions celebrated by the Platonists and the Pythagoreans, but also the mundane particulars of the natural world, from worms to the creatures of the sea.

On logical grounds, Aristotle rejected the atomic theory and instead formulated a very detailed theory of matter using Empedocles's four elements of fire, water, earth, and air. These, in Aristotle's view, formed living and nonliving matter. He differed from Empedocles, though, in his belief that the elements could change. They could give up or take on wetness or dryness, heat or cold, and they could transform one into another.

Aristotle's observations of the natural world‑how fish are designed to swim, how cows are designed to chew‑told him that everything in nature had a design or purpose. In fact, life was organized on an ascending order of purpose, he believed, starting with inanimate things and going up to plants, which found perfection them to animals, which found perfection in finding food; and finally to humans. who found perfection in thought and happiness.

ANCIENT MATHEMATICIANS Perfectors of abstract reasoning

1900‑1600 B.C. Knowledge of what would later be called the "Pythagorean theorem" is present in Babylon,

580 B.C. Pythagoras is born on Samos.

535 B.C. Pythagoras travels to Egypt and is eventually captured by Cambyses II of Persia during an invasion of Egypt. He is soon brought to Persia and may have traveled to India.

532 B.C. Pythagoras moves to southern Italy and eventually establishes a school at Croton.

500 B.C. Pythagoras dies in Metapontum, in southern Italy.

400 B.C. Eudoxus is born in Cniclus, Asia Minor.

372 B.C. Eudoxus attends lectures in Athens for two months before leaving for Egypt, where he studies with priests.

350 B.C. After spending his life as a teacher and legislator, Eudoxus dies in Cniclus.

330 B.C. Euclid is born, most likely in Alexandria, Egypt.

300 B.C. Euclid completes the Elements, his seminal geometry text..

287 B.C. Archimedes is born in Syracuse, Sicily.

212 B.C. Archimedes is killed during the Roman sacl(ing of Syracuse.

260 B.C. Euclid dies.

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1600 English physician William Gilbert publishes his De magnete (On the Magnet), greatly advancing the understanding of magnetism.

1604,1609 Italian astronomer Galileo Gahlei performs experiments relating to gravity, acceleration, and velocity.

1621 Dutch astronomer Willebrord Snell discovers the law that determines the path of a refracted ray, eventually called Snell's law.

1654 French mathematician and philosopher Blaise Pascal states that force is transmitted through a fluid equally in all directions.

1662 English philosopher Robert Boyle determines that a gas held at constant temperature has a volume inversely proportional to its pressure, eventually called Boyle's law.

1668 English physicist Isaac Newton determines that the linear momentum of an object is equal to its mass multiplied by its velocity.

1687 Newton publishes Philosophiae Naturafis Principia Mathematica (or simply Principia), presenting his laws of motion and the law of universal gravitation.

1704 Newton publishes Opficks, his work on light and the spectrum.

Left: Archimedes's screw, above, used simple mechanics to lift water. In 1653, Rembrandt portrayed the ancient philosopher Aristotle, opposite, gazing on a bust of the even more ancient poet Homer.

Right: Legend has it that Archimedes's eureka moment came when he noticed how the displacement of water could help him determine whether the king's new crown was gold or alloy.

Aristotle's world is dynamic. Change and motion are its chief characteristics. He identified four causes, as he called them, that lie behind every object; once these are known, the object itself is understood. The first cause is the matter out of which the object is made. The second is the form taken on by this matter. The third, called the efficient cause, is the thing that makes the object. The fourth and last cause is the most important in Aristotle's worldview. Called the final cause, it expresses what a thing is for, what its purpose is, why it exists.

All natural things‑unlike the artificial works of human hands have within themselves a driving principle that pushes them on toward their goal or final cause,

An acorn, for example, needs no external impetus to drive it onward toward fulfilling its purpose, namely sprouting and growing into a mature oak tree. Thus, for Aristotle, material substances were always in flux, moving from one state to another, driven either by their internal principle of change toward their proper ends or diverted to other ends by the external hand of human artificers.

More than a century after Democritus, Epicurus returned to his atomic theory and contemplated a materialistic world made of atoms moving within a void without benefit of soul, gods, or creator. The atoms of Epicuru unlike those of Democritus, had weight; they might suddenly be diverted and collide; and, when endowed with certain shapes, they could evoke sensations, such as smell or taste. For Epicurus, natural events had no purpose; all was due to the chance actions of atoms. Even the mind was not any different from the body in being a structure of atoms. When death came, the atoms of both mind and body dispersed into the air.

The Greeks left behind basic physical concepts: Elements, atomic or otherwise, are the basic building blocks of all matter; matter is something that undergoes creation and growth, decay and destruction; since matter undergoes transformation, its forms are often impermanent. These are all ideas‑and in some cases, questions‑that still excite scientists and philosophers some 2,500 years later.

"EUREKA!"

Thanks to astonishing conquests by young Alexander the Great ‑ tutored by Aristotle himself ‑ in Africa and Asia, Greek knowledge and culture spread far and wide through the ancient world starting in the fourth century B.C. After the premature death of Alexander in 323, his empire was divided among his generals, but Hellenistic culture remained a unifying force.

Probably the most Hellenized, and certainly the most productive, area in terms of science and mathematics, was Egypt. There the city of Alexandria (one of over a dozen cities founded and named after the Macedonian general, and the only one to retain the name) became the ancient world's most cosmopolitan city. Merchants and scholars from various nations flocked to this teeming center.

The geometer Euclid worked out of Alexandria, and so, too, most likely, did the ancient world's prolific mathematician Archimedes. Born in Syracuse, Sicily, around 287 B.C., he is known by legend more than by fact. Most famously, he Jumped out of his bath and ran naked through the streets crying "Eureka!‑I have found it!" when he discovered that a submerged body displaces an amount of water equal to its volume. He was a thinker who could put his science to work alongside his practical genius, a thinker who wanted to know how processes occurred and to figure out which mathematical principles lay behind them.

His eureka moment was exemplary. The king of Syracuse had asked Archimedes to determine whether a certain crown was made of pure gold or of an alloy. The story that when he lowered himself into his ath, Archimedes noticed that the amount of water that overflowed the tub must be equal his own volume. Therefore, he had found way to measure the exact volume of the gold crown. Since gold is the densest metal, Archimedes had to do was to make a brick of pure gold that displaced the same amount of water as did the crown, then compare the weights of the crown and the brick. If the crown weighed less than the brick, then it must be made of a metal alloyed with a lighter metal. As it turned out, the weights were unequal, proving that the crown was not pure gold.

In his studies in geometry Archimedes advanced the work of Eudoxus and, by painstaking labor, calculated with great accuracy the ratio of the circumference of a circle to its diameter, pi. He also discovered formulas for the surface area and volume of spheres. (He took such pride in the latter that he asked for it to be inscribed on his gravestone.) He also developed a formula to determine an object's center of mass.

Archimedes's earliest mechanical invention appears to have been a device to move water uphill. Known as the Archimedean screw, it was a spiral‑shaped pipe that, when rotated, raised water from a stream up onto land. He detailed the principle of the lever, namely that things balance at distances from the fulcrum in inverse ratio to their weights, as well as those of the pulley, the wedge, and the windlass.

LORD KELVIN Discoverer of absolute zero

1824 Born as‑William Thomson on June,26, in Belfast, Ireland.

1834 Enters University of Glasgow.

1841 Enters Peterhouse College, University of Cambridge.

1845 Graduates with honors in mathematics, elected a fellow of Peterhouse College, Cambridge.

1846 Becomes professor of natural philosophy at University of Glasgow; a member of the Royal Philosophical Society.

1848 Publishes work On an Absolute Thermometric Scale; the proposed scale is later named the Kelvin scale.

1851 Elected as fellow of the Royal Society.

1852 In collaboration with James Prescott Joule, discovers tha gas temperatures change as gas containment changes; th discovery is later named the Joule‑Thomson effect.

1854 Produces his first patent, with brothers James and William Rankine, for improvements in telegraphic communication equipment.

1857 Embarks on venture to lay Atlantic telegraph cable from board H.M.S. Agamemnon and U.S.S. Niagara.

1860 Publishes important papers on the thermoelectric, thermomagnetic, and pyroelectric properties of materials

1907 Dies on December 17 near Ayrshire, Scotland.

Armed with the knowledge of his law of levers, Archimedes reputedly boasted: "Give me a place to stand on, and I will move the Earth."

A barometer's measures the level of atmospheric pressure. A barometer can also determine altitude, since air pressure corresponds to distance from sea level. When the barometer falls rapidly, a storm is usually expected; when it rises rapidly, fair weather is usually in the forecast. Archimedes and Pascal described fluids at rest, but mathematician Daniel Bernoulli found that fluids in motion had different physical properties. He explained his finding in Hydrodynamica, from the Greek for moving water.

The lever was not a new invention, but once Archimedes formulated the mathematics that made it work, he could calculate for any weight the length of lever needed to lift it. In a legendary demonstration, he set up a series of pulleys to show the king of Syracuse that he could move an enormous ship with no more than the strength of his hand.

Archimedes experimented with mirrors and reflected light, although the story that he used an array of burning mirrors to set fire to the Roman fleet is most likely apocryphal. In fact, the Romans sacked Syracuse, and Archimedes was killed in the melee. Even so, the Romans celebrated Archimedes for his brilliance and mechanical ingenuity, and his fame survived well beyond the age of antiquity. Archimedes's mathematical achievements were widely appreciated by Arab mathematicians, who rediscovered his work in the eighth century A.D. Archimedes enjoyed another surge of popularity in the Italian Renaissance and became the model for a new and enormously productive generation of Italian engineers in northern Italy, and from there across wider swaths of Europe. Archimedes's devices, as well as his theoretical investigations of such things as buoyancy, floating bodies, and hydraulics, continued to inspire analytical pursuits and inventiveness in the 16th and 17th centuries. French mathematical prodigy Blaise Pascal, for instance, drew upon Archimedes's work when he discovered in the mid 1600s that pressure applied to a confined liquid is transferred equally throughout the liquid. His experiments led to his invention of the syringe and the hydraulic press, still essential in hydraulic braking systems today.

Archimedes and Pascal studied hydrostatics, the science of fluids at rest. But fluids are often in motion, and in the 18th century the study of hydrodynamics emerged: the physics of flowing water, of water running through pipes, of sea currents and waves, and even of raindrops on windowpanes.

Daniel Bernoulli was to hydrodynamics what Archimedes was to hydrostatics. Bernoulli, born in Groningen, Netherlands in 1700, was the son of Swiss mathematician Johann Bernoulli, one of three brothers, all mathematicians. The father taught the son mathematics, but the son's work would soon exceed that of the father. The younger Bernoulli studied mathematics, medicine, and physics and in 1738 published his groundbreaking Hydrodynamica. In it, Daniel Bernoulli established the principle that bears his name, which says that pressure in a fluid decreases as its velocity increases and that became the basis of the science of fluid dynamics. It explains the movement of water through pipes as well as water moving through rivers. it begins to explain how bird and airplane wings work. Air is a fluid, too, and wings are shaped to reduce the air pressure above and create lift from below.

In 1734 Daniel Bernoulli and his father were declared joint winners of a grand prize in astronomy given the Paris Academy of Sciences. The elder Bernoulli was furious at having to share the prize with his son, and he threw Daniel out of the house. Johann Bernoulli even later tried to claim that his son's path breaking Hydrodynamica was copied from his own work, and went so far as to publish a similarly entitled book (based on Daniel's) with a false printing date to support his spurious claims.

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1738 Swiss mathematician Daniel Bernoulli determines that the faster a fluid moves, the lower its pressure, a principle that describes wings and makes human flight possible.

1785 French physicist Charles Augustin de Coulomb discovers the force between electrical charges, later named Coulomb's law.

1798 American‑born British physicist Benjamin Thompson studies heat and argues that it results from friction, not from any substance residing in material.

1800 Italian physicist Alessandro Volta invents the electric battery.

1803 English chemist John Dalton concludes that each element is made up of atoms and that these atoms combine to mal<e compounds.

1811 Italian physicist Amedeo Avogadro shows that equal volumes of gases at the same temperature contain an equal number of molecules.

1821‑1825 French physicist Andra Marie Ampere discovers that moving electrical charges create a magnetic field.

1827 German physicist Georg Simon Ohm publishes his law, which states that voltage is equal to the current multiplied by resistance.

THE ISLAMIC CONTRIBUTIONS

After the collapse of the Roman Empire and the disruption caused by barbarian invasions of Europe, the next great flourishing of scientific thought occurred in the newly ascendant Muslim world, particularly in the capital of Baghdad and Cordoba from about A.D. 800 to 1300. Sponsored in part by the caliphs, a great translation project turned hundreds of Greek texts into Arabic. The scholars of the Islamic world built extensively upon these Hellenistic foundations.

An exemplar of the golden age of Islamic science was Abu 'Ali al Hasan ibn al Haytham (Alhazen is the Latinized version), born in Basra (in today's Iraq) in about A.D. 965. As a young man, Albazen gave up a minor government post to study science. Especially interested in the work of Aristotle, he soon developed a reputation as a mathematician and natural philosopher. He eventually moved to Cairo at a time when Egypt was ruled by the Fatimids, an Islamic dynasty named for the prophet Muhammad’s daughter, Fatimah. The Fatimids ruled part of North Africa and Sicily, and they made Cairo the capital of their empire. The Fatimid caliph, al‑Hakim, had a great interest in science, and he had built a center of learning in Cairo that rivaled the House of Wisdom, Baghdad's library, archives, and record office.

After his arrival in Egypt, Alhazen considered the possibility of controlling the annual Nile floods. At some point he either sought al‑Hakim's patronage or was summoned by the caliph for the project. In either case, Alhazen found himself heading up the Nile with a team of engineers and a commission to construct controls to the great river's flow.

Once upstream, Alhazen realized that his plan wouldn't work. He returned to a displeased caliph, and although he was given a nonscience post in the government, Alhazen worried that the caliph was not done with him. Al‑Hakirn was a man used to having his way, and, as enlightened as he was, he could also be a dangerous eccentric. After the conquest of an enemy city, he had all the dogs killed because their barking annoyed him. Alhazen, a man of unquestionable scientific genius, decided to protect himself by pretending to go mad. He continued to do science but posed outwardly as a madman until al‑Hakim died in 1021.

Athazen, by then in his 50s, was able to accomplish major innovations in the science of optics, involving light, vision, color, and refraction. He rejected the Greek idea that vision emanated from the eye as a visual ray. He used mathematical models and experiments to argue that illuminated objects emit light in every direction. He posited that light travels in straight lines, which he called "rays”. The lens of the eye, he believed, received these rays, thus allowing us to see our surroundings. He even delved into anatomy, considering how the eye connected to the optic nerve.

Islamic scientists of the Middle Ages made original discoveries and crucial contributions in the fields of astronomy, medicine, early chemistry, and mechanics. one of the most wide‑ranging and important fields of Islamic scholarship was mathematics, both simple and complex. One key figure to whom we owe a great deal (even though the original Arabic versions of his most important works are now lost) is Muhammad ibn Musa al‑Khwarizmi.

Al‑Khwarizmi was born in Baghdad around 780. He became a scholar and translator of Greek manuscripts in the Baghdad House of Wisdom, built by caliph al Mamun after he came to power in Baghdad in the year 813. What al Khwarizmi eventually wrote, however, became at least as important as any of the texts he translated. His intention was to explicate "what is easiest and most useful in arithmetic, such as men constantly require in cases of inheritance, legacies, partition, lawsuits, and trade, and in all their dealings with one another, or where the measuring of lands, the digging of canals, geometrical computations, and other objects of various sorts and kinds are concerned." He would write about the practical applications of mathematics, in other words.

Left: Arabic Numerals The numerals 1, 2, 3, 4, 5, 6, 7, 8, 9, and 0 are commonly known as "Arabic" numerals, but their proper name is Hindu-Arabic." Indian Hindus developed most of this system in the third century B.C. It was not until the ninth century A.D. that the zero became known to Al Khwarizmi, an Arabic scholar. Although most of these numerals originated in India, the Arabs introduced them to Europe, and the Europeans misidentified them as "Arabic" numerals. Middle Eastern thinkers made substantial contributions to mathematics. An image from 1508 commemorates the triumph of arithmetical algorithms over the conventional abacus for calculation. The figure on the left is employing Arabic numerals, which Muslims transmitted to the West from India.

Right: Isaac Newton developed his reflecting telescope in 1668. Using mirrors rather than lenses to gather an object's light, he avoided the haze caused by the prism effects of light as it passes through glass.

To do this he offered the reader a way to state the terms of a problem so that all the functions are coordinated into statements that can be placed into an equation that can be solved. He called the first part of the process‑removing the negative terms from the equation ‑ al‑jabr, and the balancing of the equation al‑muqabala. From the former term we get our word algebra, and from a Latin corruption of al‑Khwarizmi's name we get the word algorithm.

Had this been al‑Khwarizmi's only accomplishment, it would have been sufficient, but his next book, on the Hindu Art of Reckoning, began the standardization of Arabic numerals and, just as important, introduced zero to mark a place in a written number. Although al-Khwarizmi's work contributed to advanced mathematics, it also, as he intended, brought mathematics into everyday use. For the next 700 years Arab mathematicians, astronomers, and makers of scientific instruments dominated the Western world, thanks to their mathematical and analytical proficiencies.

As often happens, science awaits a technological advance before it can move forward. Alhazen's work on optics had been precocious. Glass lenses were not yet well enough refined to demonstrate his theories. Galileo had worked to improve the telescope, trying to develop lenses that didn't present a view with color distortions at the edges of the image‑what's known as chromatic aberration. He finally gave up. Meanwhile, in the Middle Ages, Robert Grosseteste and Roger Bacon had both predicted that magnifying lenses would help researchers see things not only far away but also smaller than what could be seen through ordinary human vision. It took centuries to develop better grinding techniques and suitably transparent glass.

The earliest microscopes used only a single lens mounted between two pieces of metal, with a screw device that could be used to bring an object into focus. In the 1590s the Dutch lens maker Hans Jansen found that a compound microscope, with two lenses, would magnify things even more, but only once the lenses were painstakingly matched and properly aligned. In the late 17th century Antoni van Leeuwenhoek achieved magnifications of more than 250‑fold and astonished the scientific world by revealing the structure of many previously invisible objects.

In the 17th and 18th centuries advances in optics also inspired investigators to consider the very nature of light. They observed clearly that a ray of light bent as it passed through a lens or fluid, and in 1621 Willebrord Snel of Leiden developed a mathematical formula for the angle. Why light bent was another problem altogether. The search for a solution to that question would eventually come to alter the world's understanding of all forms of energy.

In the 17th century, though, light was considered to move at an infinite speed. In 1676 Ole Romer, a Danish astronomer, found that when Jupiter and the Earth were farthest apart, the regular eclipses of the Jovian moons occurred later than the calculations predicted, but when Jupiter and the Earth were close in space, this discrepancy did not occur. Romer proposed that the delav was due to the si)eed of light: The light took more time to cross over to us from the other side of the solar system. Romer calculated that light moved at a speed of 140,000 miles per second. He was wrong by about 25 percent but close enough to make the point.

At about the same time, Dutch mathematician Christiaan Huygens made another assertion about light. In 1659, he discovered the rings of Saturn, using a telescope with lenses that he and his brother had ground and polished. He believed that light traveled in waves that moved through the invisible but ubiquitous ether. The waves of light traveled outward like a circle of ripples from a rock dropped into a pond. When they met other objects, like ripples in a pond, they rebounded‑or, in the case of light, reflected‑and created what Huygens called "secondary wavelets.

Huygens's ideas did not go unquestioned. Light waves seemed to slow down when they entered a denser medium; how was it that they speeded up when they came out again? Isaac Newton, who was then a professor at Cambridge, England, asked why light waves did not simply bend around obstructions.

At about this time, Newton was beginning to lay out the broad outlines of his theory of gravity and his three laws of motion. He had become a professor at Cambridge at the age of 26, but outside of Cambridge, he was generally unknown‑until 1668, that is, when he designed a reflecting telescope that used a mirror as its main light‑gathering device.

Despite the acclaim he won for his telescope design, he remained determined to tackle the problem of chromatic aberrations in lenses, too. Where did the colors come from? At the time it was widely believed that pure light‑sunlight, for example‑contained no colors. Many people had seen that prisms, oil on water, and lenses could create a beam of light with a rainbow of hues, but the belief was that those colors came from the object that the light struck. Newton sought to prove that, far from being colorless, white light in fact contained a spectrum of many colors. He used a prism to break a ray of light into a spectrum, then used a second prism to combine the colors back into a single white beam. Going further, he passed a colored beam of light through the prism. The beam was not altered, proving that the individual colors of the spectrum were not further divisible. He also found that different colors are refracted their beams bent by passing through a prism‑to different angles.

Newton soon offered a view of light, different from that of Huygens and Robert Hooke, who also believed that light traveled in waves. Light, Newton proposed, was made up of particles. The different refractions of different colors, Newton said, would make it impossible to create lenses that had no chromatic aberrations. He published his Opticks in 1704. It was a remarkable work, and he was knighted by England's Queen Anne in 1705.

But on lenses, at least, Newton was soon proved wrong. An English lawyer, Chester Moor Hall, found that in a telescope made with two kinds of glass, one glass could cancel out the chromatic aberrations of the other. But the question still remained: what was light? Particles or waves?

Left: English physician William Gilbert investigated the properties of static electricity and contrasted them to magnetic effects. In this 19th‑century painting, Gilbert (standing, in black) demonstrates an electrical experiment before Queen Elizabeth I and various onlookers.

Right: In 1600 William Gilbert wrote the first book on magnetism, but the early Greeks knew the attractive force between lode stone and iron, and Lucretius, a Roman poet, hinted at magnetism in his work as well.

THE ATTRACTION OF MAGNETISM

The groundwork for exploring this question was set in place in 1600, when an English physician, William Gilbert‑Queen Elizabeths personal physician‑published De magnete, Magneticisque Corporibus et de Magno Magnete Tellure (On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth). Gilbert based his work solely on his own observations and measurements, studying a force that had been known since ancient times but that had never been so pains‑takingly investigated.

The ancient world knew the magnetic properties of naturally occurring lodestone, a form of iron oxide also known as magnetite, but to them, the forces involved seemed utterly inexplicable. The Greeks knew about lodestones, and in fact the very word "magnetism" comes from the name for lodestones found near Magnesia in Asia Minor.

According to science historian Colin Ronan, ancient Chinese fortune‑tellers used a divining board of two disks, a lower one that represented the Earth, and an upper, rotating one that represented the heavens. Both boards were marked with compass points. Symbolic objects would be tossed onto the board and the future divined by where they landed.

Sometime in the first century A.D. a rotating spoon, symbolic of the Big Dipper, replaced the up er board. Sometime later, diviners began making the board and spoon out of lodestone. When they did, they realized that the spoon's handle always pointed in the same direction. It seemed like magic. This "south‑pointing spoon" as it came to be called, according to Ronan, evolved into something more like a pointer. Eventually it was put to uses other than on the divining board.

Later, it was discovered that iron needles could be magnetized if stroked on a lodestone or if heated in a fire and allowed to cool while held in a south‑north direction. This important practical knowledge led to the use of compasses in determining the alignment of building sites and, by the tenth century, as a tool for navigation. Chinese scientists also discovered, 700 years before Western scientists, that magnetic north and south‑that is, the directions to which compasses point‑are not the same as geographic north and south.

By the 13th century, the Chinese magnetic compass had come west. In 1269 Pierre P6erin de Maricourt (latinized to Perigrinus), a French crusader, scholar, and military engineer, wrote Epistola de magnete (A Letter on Magnets), the first treatise of Western science on the properties of magnets. Peregrinus was the first to call the opposite ends of a magnet poles, the first to study how like poles repel each other, and the first to begin to consider the practical applications for magnets. He made a good start at the science of magnetism, especially considering that at the time he was a soldier serving in the army of Charles I of Anjou, besieging the Italian city of Lucera.

Left: A woodcut of a compass illustrated William Gilbert's De Magnete very well, since the magnetic forces of which he wrote were essential to the operation of this navigational tool.

Middle: Hans Christian Orsted observed that a magnetic needle, when brought near an electric current, turned at a right angle to the current. He noted the phenomenon but had no explanation for it.

Right: The eudiometer (from the Greek for measuring pure air) was an instrument invented by Italian physicist Allessandro Volta to measure gases after combustion. This model was developed by English chemist Henry Cavendish, who first identified hydrogen gas in 1766.

William Gilbert's De magnete, published in 1600, represented a new concept in science writing"Baconian science, based on experiment and observation rather than hearsay, being practised twenty years before the publication of Bacon's Novunt organum," as historians Stuart Malin and David Barraclough put it. Gilbert's text,, written in Latin, came out nine years before Kepler's Astronornia nova and ten years before Galileo set down his first astronomical observations in Sidereus nuncius.

For all the previous centuries of using and describing magnets, no one came as close as Gilbert to understanding how they worked. In the first of the six parts of De magnete, Gilbert concludes that the Earth itself is a great magnet. He then draws an important distinction (a controversial one, eventually proving to be an important similarity) between the attractive properties of magnets and what Gilbert called the "electric for e" produced when certain materials, such as amber, rub lightly against cloth or fur and then attract light objects. Gilbert used a spherical lodestone that he called a terrella, a miniature Earth, to study the effects of geomagnetism.

Two hundred years later, Hans Christian Orsted, a professor of science at the University of Copenhagen, demonstrated that when a wire carrying an electric current was held over a compass needle, the needle turned to stand at right angles to the wire. Why? Orsted published his results with no conclusion offered.

THE NEW ELEMENTS

Henry Cavendish was rich. With both the Duke of Devonshire and the Duke of Kent as grandfathers, he had an impeccable aristocratic pedigree. When he came into all his inheritances at age 52, in 1783, he held the largest individual account at the Bank of England. Although he was the man who discovered hydrogen and was first to analyze the composition of the atmosphere, he was ill‑dressed in a faded violet suit with frilled sleeves and a tricornered hat that had long gone out of fashion. He was so painfully shy that he had a special staircase built in his house so that he would never have to encounter the servants in person; he communicated with them by writing notes. He spent nothing on himself, and when asked to give to charity, found out who had given the most and matched that amount t o the penny. He never sat for a portrait and had to be persuaded to report his important discoveries in chemistry and physics.

Yet for all his peculiarities, Cavendish was a tireless experimenter. In the front of his house, he built a scaffold to allow him to climb into the trees to make astronomical observations. On his roof he installed a huge thermometer. He was eager to learn of the nature and composition of gases, a subject that drew in some of the best scientific minds of the time, from Scottish chemist Joseph Black, who first isolated carbon dioxide, to Joseph Priestley and Antoine Lavoisier, an equally wealthy Frenchman whose life was cut short, despite his scientific achievements, at the guillotine.

Cavendish created meticulous experiments to analyze the gases produced by reactions of solids and liquids. These "factitious" gases, as he called them‑isolable in the laboratory, but not found in nature could be isolated, contained, and

weighed. He discovered and finally reported to the Royal Society in 1766 one particular factitious gas that had no name. When it burned in a flask, water remained on the glass. Cavendish's first explanation was that all gases contained water.

But when news of the discovery reached Lavoisier, he ran experiments that produced not only water but also "inflammable air" when acids acted on metals. He finally was able to prove that there were two components of water: a gas he called oxygen and the gas identified by Cavendish. Lavoisier called the latter hydrogen‑water maker in Greek. After 3,000 years of

considering it one of the four basic elements, water became known as a compound of two gases.

Cavendish went on to conduct experiments in which he passed an electric spark through air, forcing it to combine to form nitrogen oxides. He then dissolved the oxides in water to produce nitrous acid. Working like a medieval alchemist, holed up in the laboratory of his great London villa, the ascetic Cavendish found that air was no element either, but a mixture of nitrogen and oxygen in a ratio he figured to be four to one, surprisingly close to the actual five‑to‑one ratio. He even found that, no matter how hard he tried to get all the nitrogen and oxygen in the air to combine, a tiny amount of something was left over that resisted any chemical reaction. This inert substance, as he called it, was in fact, argon‑an element whose existence would not be confirmed for another century.

By the end of the 18th century, chemistry was science's cutting edge. Lavoisier's and Cavendish's work led to a reorganization of chemical nomenclature based on a substance's composition. In the first years of the 19th century French chemist Joseph Louis Gay‑Lussac found that equal volumes of all gases expand equally with the same increase in temperature. GayLussac was a daredevil chemist who, in 1804, made hydrogen balloon ascents to an altitude of 22,000 feet, to measure the atmosphere. His more down‑to‑earth finding about expanding gases came to be called Charles's law, in honor of Jacques Charles, who had arrived at nearly the same conclusion 15 years earlier but had not published it. In 1808, Gay‑Lussac determined that different gases always seemed to combine in certain specific, whole‑number ratios by volume: two‑to‑one or five‑to‑three, for example. Why? And why, as Gay‑Lussac also found, when gases combined, did they seem to occupy less space?

The first answer would come from a Quaker teacher from Manchester, England, John Dalton, a man who had taught himself science and who, up to the age of 30, kept a daily record of the weather. He also performed a systematic study of color blindness, from which he suffered.

Dalton's considerations of the atmosphere led him to think further about Gay‑Lussac's problem in light of the compound nature of air, composed of two gases of different weight, as Cavendish had discovered. Why, for instance, didn't the heavier gas separate from the lighter one? Why did different gases dissolve in different amounts? The gases must not be combining chemically at all, he realized, but rather must be a mix of gas particles held together by heat. In deference to the ancient Greek Democritus, Dalton called these gas particles atoms. Democritus's atoms were all the same, though, adding up to a simple unified view of the natural elements. Dalton's atoms were all different.

The existence of atoms of different types, Dalton reasoned, would explain why compounds always combined in the same proportions by weight. Each gas, each element, had its own distinct atoms and distinct prop erties‑heavy gases had heavy atoms, for example. Atoms, he argued, could combine in different proportions by weight to produce different compounds. This law of multiple proportions, as he called it, meant that depending upon the proportions, a mixture f carbon and oxygen, for example, could create either carbon monoxide or carbon dioxide. Moreover, chemical reactions either combined or separated elementary particles; no new atoms were created, and none were destroyed.

The Periodic Table

It is the ubiquitous chart on the wall of the science lab, a colorful chart of squares, each bearing letters and numbers. For students, the periodic table of the elements is an imposing challenge. For scientists, it is an expression of the structure and behavior of all matter,

Dmitry Mendeleyev, a chemist. first charted the elements in 1869. He organized them in order of atomic weights, As he plotted out the 50 elements that had by then been identified and analyzed, he recognized that each element resembled the eighth element that followed it on his chart. For example, lithium resembled sodium, and both resembled potassium.

Left: In 1871 Dmitry Mendeleyev faced skepticism over his periodic table Right: John Dalton, a science teacher in Manchester, England, formulated the modern atomic theory and published this table of elements with their atomic weights in 1808.

As an explanation for this Mendeleyev; proposed what he called the periodic law. “The elements arranged according to the magnitude of atomic weights show a periodic change of properties."

Once he had distributed all the known elements according to their characteristics, Mendeleyev noticed that there were still empty in his table. So confident was he in his theory and the orderliness of nature, he predicted them were yet undiscovered elements and when they were found and analyzed, their properties would qualify them to fit into those spaces.

Remarkably, Mencleleyev proved to be right As elements were discovered, studied, and added they fit the table and matched the pattern conceptualized.

Over the next 50 years, the table was refined. When it was realized in 1911 that an elements atomic number ‑ the number of positive charges, or protons, in its nucleus ‑ distinguished its properties, that number replaced atomic weight in the chart. The periodic table demonstrates that atoms are constructed in an orderly fashion‑the identity of the elements, changes as successive protons are added to each nucleus.

The modern periodic table arranges elements by increasing order of atomic number from left to right The seven horizontal rows are called periods and the eight vertical rows am called groups The elements in each period begin with metals and progress to nonmetals with noble gases at the farthest night in each row. The periodic table now contains 92 natural elements and 20 man‑made elements (those that occur in nuclear reactions).

Mendeleyev's law still provides insight into the relationships of the elements to one another. It provides a physical representation of groupings of atoms with similar hardnesses, melting points, and densities, and helps express how, and how readily, elements bond with one another. With a thorough understanding of the periodic table one can tell at a glance how stable an element's atomic structure is and how well a given element will conduct electricity and heat. Mendeleyev’s insight gave scientists a new way to view and construct information regarding natural phenomena.

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Dalton delivered his first paper in 1803 to the Manchester Literary and Philosophical Society, but its repercussions reached very far. Assuming that a given volume of a given substance always contained the same number of atoms, weighing the substances in a chemical reaction would provide the relative weight of the atoms involved. Dalton created a table of relative atomic weights, giving hydrogen a weight of one unit and using it as the standard. We now use a similar system based on the atomic weight of carbon, 12.

As to why combined gases took up less space than the volumes of the gases separately, the answer came just a few years later, when Amedeo Avogadro of Turin hypothesized that when gases combined, they created groups of atoms that Avogadro called molecules, from the Latin, molecula, meaning small pile or heap.

Dalton was elected a fellow of the Royal Societies of both London and Edinburgh, elected to the French Academy of Sciences, and awarded an honorary degree from Oxford (which would never have admitted him as a student because he was not an Anglican). Eventually, he was summoned to an audience with the king. None of this seemed to change Dalton's habits. He continued to record the daily weather of his native Lake District, as he had since 1787. His last weather record was made on July 27, 1844, the day he died.

In 1800, the list of known elements numbered 30. By Dalton's death in 1844, it had nearly doubled. Soon chemists began asking whether the list was anything more than a checklist. In 1864 British chemist John Newlands wondered aloud why some elements showed similar chemical properties. His question implied some sort o c assi cation among the elements. The idea might have gone no further were it not for the recreational habits of an eccentric Siberian professor named Dmitry Mendeleyev, thrown out of St. Petersburg University for unconventional behavior, including supporting student protests.

At work on a new chemistry text, Mendeleyev made a set of about 60 cards on which he wrote the names and properties of the known elements. An avid solitaire player, he began to place them into patterns by order of weight and chemical properties. He found that one arrangement placed all the elements that behaved similarly in the same vertical rows. It was a visual display of the concept at which Newlands had hinted: that chemical properties recurred periodically. In 1869 Mendeleyev published his socalled periodic table, noting that the scheme seemed to suggest that there were still several elements yet unknown. He left blank spaces in the table but predicted the properties those elements would have based on their position.

In 1860 two scientists in Heidelberg, Germany, Gustav Kirchhoff and Robert Bunsen, had taken to improving on an idea first tried in 1814 by German optician Joseph von Fraunhofer. in the course of his experiments, Fraunhofer had found that the sun's spectrum was not a continuous array of colors but was shot through with hundreds of black lines of various widths. By the early 1820s he had determined that these lines were present, though in slightly different patterns, in light from bright stars, and that they also showed up when beams were split by a grating rather than a lens. Although he didn't know what they meant, he used the letters A through K to name the divisions of lines.

Kirchhoff and Bunsen, working with an instrument with better resolution than Fraunhofer's, repeated the work and found what by then were known as Fraunhofer's lines. Even more fascinating, they discovered that each element absorbed and emitted its own unique combination of wavelengths, a line pattern that served as a kind of spectral fingerprint.

Spectral analysis replaced long and tedious chemical analysis as a method for determining the components of any material. Once the spectra of the known elements were described, scientists began searching for new line patterns among substances in the laboratory, in the field, or even in light from space. They soon began to find new elements. And once they had analyzed the properties of those new elements, they found that they filled the very spaces that Dmitry Mendeleyev had reserved for them. His solitaire‑like table of elements, expanded to include more than 100 elements, hangs today in science classrooms around the world.

ELECTRICITY EVERYWHERE

William Gilbert's introduction of the word "electric" in association with the magnetic effects of a rubbed piece of amber would prove prophetic. One of the first to take a keen interest in furthering Gilbert's work was a German, Otto von Guericke, an ingenious scientific dabbler, born in Magdeburg in 1602, two years after Gilbert published his De magnete.

Left: To test his own color blindness‑and understand the phenomenon scientifically as wellEnglish atomic theorist John Dalton created this booklet of colored threads.

Right: Italian physicist Alessandro Volta's batteries found a host of uses, some of them less than entirely serious. This one, called a "Volta Pistol," consists of a cylindrical chamber into which explosive gases such as hydrogen or oxygen were placed. The barrel was plugged with a cork. When an electrical charge was applied, a spark ignited the mixture and the cork popped off.

What is air? Astute 18^th century scientist realized that although invisible , air had distinct gaseous components. We now know that it is approximately 78 percent nitrogen, 21 percent oxygen and 1 percent argon and carbon dioxide with tiny amounts of many other elements mixed in.

Von Guericke had little formal education, but he was popular enough in his hometown that at the age of 24 he was elected alderman, a post that he would hold for the next 50 years.

Von Guericke was deeply interested in the nature of space. He questioned whether a vacuum, a space containing no matter, could actually exist. Aristotle and Descartes had both denied it.

Two related questions were how the planets move in their orbits and how they interact with one another. Kepler and Gilbert had proposed a magnetic cause, and von Guericke set out to investigate the possibility. He devised a means of creating a partial vacuum, and by 1650 he had invented an efficient pump by which he could evacuate large volumes of air from vessels. He thereby showed the elasticity of air and the possibility of producing a vacuum. He explored the properties of vacuum as well. Combustion could not take place in a vacuum, he determined, but magnets in a vacuum still attracted metal.

In a famous experiment conducted at Madgeburg in 1657, von Guericke made a sphere of two copper hemispheres and demonstrated that by pumping the air out of the sphere, the two halves were sealed shut by the force of the surrounding air pressure. To prove the strength of the air pressure, two teams of eight horses each tried unsuccessfully to pull the spheres apart. Von Guericke took his dramatic experiment to the courts in Vienna and Berlin.

Having shown that magnetism traveled in a vacuum, von Guericke set out to see whether the same force might affect celestial bodies. Working from Gilbert's experiments with a lodestone terrella, von Guericke created a larger sphere composed of earthen materials including sulfur.

Gilbert found that by rotating the sphere and rubbing it with his hand, he could make it exhibit the effects that Gilbert had deemed electric. The sphere acquired attractive properties and gave off sparks, and the effects lasted even after the ball was no longer being rotated. Intrigued, von Guericke created a machine by which he could turn the ball with a crank; then he designed a belt‑driven machine that turned the sphere even faster. He was able to cause the sulfur sphere to glow. He had conducted the first experiment to demonstrate electroluminescence. Replicas of von Guericke's machine proved as popular for entertainment as for serious scientific study. During the first half of the 18th century, static electricity machines became ubiquitous, with variations made of glass spheres and disks and even of beer bottles.

In England, Stephen Gray discovered two things about static electricity: first, that the effluvium, or outpouring, of static electricity could be transmitted along a silk thread; and second, that objects brought near to the electrified source were themselves electrified.

Electromagnetism

Electromagnetism is a branch of physics that considers the relationship between electricity and magnetism, long sought but not demonstrated until 1819, when it was shown that an electric current or a changing electric field produces a magnetic field and, conversely, a changing magnetic field produces an electric field.

One of the first useful devices to come out of this discovery was the electromagnet A coil of wire usually wound around an iron core, produces magnetism in the iron core as long as an electric current flows through the wire. The basic device still works in doorbells, circuit breakers, telephone receivers and other devices today.

The force of magnetism can also produce an electric current through the process called electromagnetic induction. A changing magnetic field sets up an electric field within a conductor. In early experiments, a bar magnet moved through a coil of wire, changing the magnetic field. which in turn caused an electric current to flow along the wire. Today a coil of wire rotates between the poles of a strong magnet. The coil Is wired as part of a closed circuit, and as the wire coil rotates, electricity can be drawn off from it.

Electromagnetic induction is the basic process behind the electric generator, one of the most important inventions in history. Without generators all our lights would go out, all our electrical and electronic equipment would shut down, and all our industries would stop. In the case of the electric motor, another device of major importance in our technological lives, the procedure is reversed. With the same basic equipment as described for the generator, a current is sent through the conductor, which lies in a magnetic field; this makes the wire coil move ‑ and turns electrical energy into mechanical energy.

Starting around 1864, the Scottish mathematician and physicist James Clerk Maxwell considered much of the existing research on the relationship between electricity and magnetism. Maxwell proposed that not only are electricity and magnetism interrelated, but they act together to produce electromagnetic waves that propagate outward as radiant energy. He argued that visible light makes up only a small portion of a spectrum of electromagnetic wavelengths. Later experimental findings showed him to be quite correct.

1827 Scottish botanist Robert Brown discovers motion within small particles suspended in water, later named Brownian motion.

1835 French physicist Gaspard de Coriolis shows that in a rotating frame of reference, objects appear to move in a curved path due to the rotation; it is named the Coriolis effect.

1842 Austrian physicist Christian Johann Doppler predicts the shift in frequency of a wave dependent on the velocity of the source, later called the Doppler effect.

1848 Scottish physicist William Thomson (Lord Xelvin) discovers the ,absolute zero point f temperature.

1850’s First and second laws of thermodynamics developed by physicists William Rankine, Rudolf Clausius, and William Thomson (Lord Kelvin).

1859 Physicists Robert Bunsen and Gustav Kirchhoff discover that elements emit unique wavelengths of light with certain absorption lines missing from their spectra.

1864 Scottish physicist James Clerk Maxwell describes his four equations associated with electricity and magnetism.

Left: Scottish physicist James Maxwell concluded that light is a form of electromagnetic radiation

Center: Volta demonstrates his battery ‑ a "pile" of alternating layers of silver and zinc ‑ to Napoleon Bonaparte (seated) and other scientists in 1800. Napoleon was so impressed that he awarded Volta the medal of the Legion of Honor and made him a count.

Right: Benjamin Franklin's famous kite‑flying experiment not only demonstrated the electrical properties of lightning but also became an iconic image of the eccentric American printer, writer, statesman, and inventor.

In France, Charles Fran~ois de Cisternay Du Fay found that electrified bodies could attract or repel one another, which made him think that there were two kinds of electrical effluvia, which he called vitreous and resinous.

As these machines became more efficient, they produced greater amounts of static electricity. The only problem in continuing to increase the electrification was that it could not be stored. This problem was solved when two men‑a German inventor, Ewald G. von Kleist, in 1745, and a Dutch scientist, Pieter van Musschenbroek of the University of Leiden, in 1746 – independently developed devices to store electricity and invented the first capacitors.

A jar half filled with water was closed with a cork stopper. A wire was passed through the cork into the jar, so that it reached the water. The wire was then charged by bringing it near a static electricity generator. When the jar was taken away from the generator, it held the charge‑as anyone who touched the wire would find out. A letter published in the Philosophical Transactions of the Royal Society dated February 4, 1745, described someone who had: "He lost theuse of his breath for a few moments; and then he felt so intense a pain all along his right arm that he at first apprehended ill consequences,

Von Kleist improved the system by coating the glass with metal, so that its charge would pass through the glass into the water. In what was an early battle of technological oneupmanship, Musschenbroek coated both the inside and outside of the glass with metal, so that the metal from outside would charge the metal on the inside. In doing so he discovered that the thinner the glass between the layers of metal, the greater the spark emitted from the jar. What this seemed to indicate was that the electricity was a single flow, not two: a hypothesis that would be proved by American inventor Benjamin Franklin. Musschenbroek's device was named the Leyden jar, and some versions of it are still in use today.

By this time, the mid‑ 1700s, electricity was becoming the fashionable science. People invented an array of devices with armatures that rotated toward the charge and, once charged, were repelled. "Far from being an arcane preoccupation reserved for privileged intellectuals, electricity rapidly became a topic of conversation throughout society," writes science historian Patricia Fara. "Many wealthy families bought their own apparatuses, and aristocratic women produced miniature lightning flashes from their fingers and their whale‑bone petticoats, or titillated their admirers with a sensational‑if rather painful‑electric kiss.' Benjamin Franklin invented a bell that rang when exposed to static electricity. Charlatans promoted electrostatic charges as cures for everything from headaches to disease.

BENJAMIN FRANKLIN Father of American, science

1706 Born, on January 17 in Boston, Massachusetts.

1718‑1723 Apprenticed as a printer to his brother James.

1729 Begins publishing the Pennsylvania Gazette.

1730 Elected the official printer for Pennsylvania.

1732 'Publishes the first edition of his Poor Richard's Almanac.

1737 Elected postmaster of Philadelphia.

1744 Creates, the Franklin stove, a modified fireplace that more efficiently warms the home.

1746 Begins investigations into electrical phenomena.

1751 Publish6 Experiments and Observations on Elecricity

1753 Awarded the Copley Medal by the Royal Society of London Becomes deputy postmaster general for the northern colonies

1756 Elected A fellow‑of the Royal Society.

1770 Begins inquiries into meteorology the Gulf Stream.

1776 Signs the Declaration of Independence

1790, Dies on April 17 in Philadelphia, Pennsylvania, a hero to both the Americans and, the French.

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The dangerous possibilities of electricity became apparent as greater charges were built up in Leyden jars and when arrays of Leyden jars were connected to store large amounts of electricity for study. In 1750 Franklin was able to show that he could charge a Leyden jar by flying a kite outfitted with a metal tip and a silk string in a thunderstorm. Lightning, Franklin proved, was static electricity. Another person who tried charging his battery with lightning was killed, leaving him, as the coroner's report put it, with "a small hole in his forehead, a burnt left shoe and a blue spot at his foot": a harsh demonstration that electricity on Earth and from the skies was clearly one effluvium.

Joseph Priestley met Benjamin Franklin in 1765, and along with their discussions on politics‑both were liberal men of the Enlightenment‑they compared notes on their studies of electricity. Franklin encouraged Priestley to publish his work, and in 1767 Priestley published The History and Present State ofElectricity, with Original Experiments. He suggested, among other things, that the force of attraction or repulsion between electric charges varied according to the inverse square of the distance between them. This was exactly Newton's finding for gravitational attraction as well.

In 1785, French physicist Charles de Coulomb invented a sensitive mechanical apparatus that proved Priestley's hypothesis. Coulomb's law, as it later beca e known, states that the force between two electrical charges is proportional to the product of the charges and inversely proportional to the square of the distance between them. Coulomb also found that his law applied to the force of magnetic attraction.

But what was electricity? Each Leyden jar was capable of only one discharge, making any investigations difficult. These conditions changed, however, at the beginning of the 19th century, though, with the work of Italian physicist Alessandro Volta.

Thermodynamic Laws

Energy does not come and go at random. It follows the laws of thermodynamics, first set out by scientists in the middle of the 19th century.

The first law of thermodynamics ‑ also called the law of conservation of energy – states that the energy created equal to the energy used. The amount of heat you get from two sticks will be equal to the energy stored in the wood and the energy applied in rubbing them together. The amount of energy from a steam engine can't be greater than that of the coal burned to make it.

Thermodynamic law also observes the tendency toward equilibrium in a system. If you put hot coffee in a cold cup, the heat energy will flow from the coffee to the cup, and soon they'll both be the same temperature. But why doesn't the heat go from the to the cup and back again? Now the second law of thermodynamics comes into play: Energy can flow in only one direction. The calories your body burns to run a mile will not return back to your body when you stop running, any more than the energy you've dissipated in running will return. An engine needs more fuel; you need more food.

There is a second part of the second law, which states that in this irreversible process, some of the energy is not available for work. In the process of running, some of the energy from the calories you're burning will not be available for use. The calories represent potential energy, sitting in ordered equilibrium. When you transform that energy into kinetic energy when you're running, it's being moved about, and in the disorder, some of it will dissipate. This dissipation of energy is called entropy, from the Greek word entrope, meaning change.

Heat is caused by the motion of molecules, so if that motion is stopped, no heat can be transferred, and there will be no entropy. The third law of thermodynamics states that at absolute zero, entropy is zero. If there's no energy, there can be no loss of energy, in other words. But beyond this frozen state, in the world ‑ and the universe ‑ irreversible processes abound. If the laws of thermodynamics are correct, the longer these processes go on, the more entropy is introduced into every system, Eventually, collapse may be unavoidable.

Left: This schematic drawing shows the interior construction of James Watt's steam engine of 1788.

LCenter: English physicist Michael Faraday, seen here in his laboratory in 1831, found that he could induce an electric current by moving magnets along a coil of wire. With that realization, he created the first electromagnetic dynamo.

RCenter: In a rendering by John Eyre of 1886, Michael Faraday i's depicted here in one of a series of tiles celebrating famous scientists from the Cafe Royal in Edinburgh, Scotland..

Right: Faraday suspected that, just as an electric current could produce magnetism, a magnetic field must be able to produce a current. He proved that effect in 1831 with this apparatus, the first transformer.

Volta had been skeptical of the mysterious force, dubbed animal electricity, whose discovery had been announced by his friend and countryman Luigi Galvani. Galvani had probed frogs' legs with metal implements and found that the leg muscles twitched when touched. Galvani hypothesized that the metal liberated some sort of electrical

flow. Duplicating Galvani's experiments, Volta became convinced that the electricity was being generated not by the frogs' muscle tissue but by moist conditions and by the use of dissimilar metals in the probes.

To find out, Volta undertook a fairly direct experiment: He placed various combinations of metals, such as silver and tin, brass and iron, on his tongue. They produced bitter sensations, which

Volta speculated might be caused by a current flowing from metal to metal by way of the tongue's saliva. Different combinations of metals produced various intensities of bitterness, which Volta carefu y charted. He then created an artificial version of his experiment, stacking up silver and zinc disks, separated from one another with paper soaked in saltwater. The result was a continuous flow of electricity. He had invented the device that would come to be called the voltaic pile‑the first battery. It produced a current large enough to study, and it suddenly added chemistry as an essential factor in the investigation of the still imponderable force of electricity.

One of the first to exploit Volta's work was the British chemist Humphrey Davy. Known for his inventive experimentation with gases, Davy became intrigued by the implications of the voltaic pile. If chemical reactions produced electricity, he wondered if electricity could react with substances to separate them into their constituent elements.

Davy built an enormous voltaic pile and applied electrical current to compounds such as potash, the

material that results when ashes are soaked in a pot of water. He found that from a lump of potash, to which he had attached a wire from a battery, shiny metallic droplets began to form and explode into the air. He had discovered a new

element: potassium. Davy also isolated other elements: sodium, calcium, strontium, barium, mag

nesium, boron, and silicon. He was now certain that, as he put it in his 18th‑century language, "chemical

and electrical attractions are produced by the same cause, "

Among Humphrey Davy's many legacies were his belief that atoms were bound together in compounds by some arrangement of electrical forces and his championing of a young man named Faraday, whom he hired as an assistant after one of his experiments literally blew up in his face and temporarily blinded him.

Born in 1791, Michael Faraday was the son of a blacksmith. He was frequently ill and left school at 13 to become an apprentice to a bookbinder. He read many of the science books that he helped to produce, nurturing a hidden talent, and attended a series of lectures on chemistry, delivered by Humphrey Davy. Faraday sat rapt through the talks and made extensive notes.

When Davy suffered his accident and needed an assistant, Faraday was recommended. Based on the notes he had taken, which revealed his astuteness, Davy hired him for the job. Davy‑by then an internationally celebrated lecturer‑was just leaving for an 18‑month tour of the Continent, and he brought the 22‑year‑old Faraday along. Davy's wife treated the young man as a sort of valet. Faraday was happy just to attend Davy's lectures, observe his experiments, and meet many of the greatest scientific minds of Europe.

Faraday began conducting his own research, especially on the possible relationship between electricity and magnetism. In 1820, Hans Christian Orsted had just published his paper on how an electrical current near a magnet sets the magnet at right angles to the flow of the current.

French physicist Andre Marie Amppe had followed up Orsted's work, and in studies conducted between 1821 and 1825 found a basic relationship between electricity and magnetism. He found that two wires with electricity running through them in the same direction were magnetically attracted, while wires with the electricity running in opposite directions repelled one another. By making coils of wires and running an electric current through them, Amp&e found he could create an electromagnet whose strength increased with each added coil. Wrapping the coils around a piece of iron made the magnet all the stronger. Amp~re suggested that the magnetic force came from the electricity lining up all the atoms in the wire and iron.

With Orsted's and Ampere's work in mind (and having already created a little electric motor, based on Orsted's spinning compass), Faraday posed this question: If electricity could generate a magnetic effect, could magnetism generate electricity? Taking an iron rod wrapped with a coil of wireAmp~re's electric circuit without the electricity‑he moved a pair of strong magnets along it. A galvanometer, a device used to detect electricity, showed that a current had been induced in the wire coil. Then Faraday improved the model by keeping the magnets stationary and placing a copper disk between them. Rotating the disk between the magnets created electricity that was conducted to a wire, one end set close to the rim of the spinning disk, the other end connected to the spindle on which the disk rotated.

By doing this, Faraday had created the first electromagnetic dynamo. Over the course of the 19th century, this principle of electromagnetic induction would create a new world of engines and machines, altering the fields of transportation and communication in revolutionary ways.

How did the dynamo work? Faraday worked on this question for many years, but he had no knowledge of electrons, the particles whose movement constitutes electrical current. That was still more than a generation away. Nonetheless, Faraday hypothesized that when an electric current passed through a substance, it loaded up these atomic force fields with tension; and that the tension was relieved when the atoms passed it on to the next cluster. Electricity ran along lines of tension through a conducting substance just as a wave maintains its peak as it moves through the water: It's not the water moving toward the shore, but the energy. This, Faraday suggested, might also be the way lightning occurs, the way static electricity is created, the way current runs through a voltaic pile. He still had no clear sense of what electricity was‑but the answer seemed very dose at hand.

ATOMIC FORCES

As physicist Ernest Rutherford discovered, a single atom‑the basic unit of all matter‑is made up of mostly empty space. A positively charged nucleus, composed of protons and neutrons, occupies about one billionth part of that space. Surrounding the nucleus are negatively charged electrons, the lightest charged particles in nature. They are held in place by electrical force, but they are readily attracted to passing positive charges as well. A unique arrangement of electrons around the nucleus gives each element its distinctive chemical and physical properties. The atomic configuration determines how well it conducts heat or electricity, how quickly it melts, and how readily it will form compounds with other elements.

Rutherford modeled the atom as a central nucleus with electrons spinning around it, like central sun around which planets orbit. Although that is still the most popular depiction of an atom, in fact it is nearly a hundred years out of date. When scientists now describe the movements of electrons around a nucleus, they picture a kind of stationary wave‑pattern cloud, within which the actual location of any single electron is only one probability among every other possible location. Protons and neutrons within the nucleus also create wave patterns.

With the discovery of radioactive materials, it appeared as if there must be more to an atom than its protons, neutrons, and electrons. In 1932 the positron was found: a particle with the mass of an electron but with an opposite charge. Beginning in the 1960s, new particles were discovered within the nucleus itself and given the name quarks.

The basic force of electromagnetism helped scientists understand how the an atom was held together because it shed light on the relationships between charged particles. But it did not fully explain the forces at work within the fantastic subatomic world, because in that invisible yet discernible realm, the terms “mass" and "particle" have little substance or meaning.

Then investigators discovered two new basic force. One, called the strong force holds together the protons and neutrons in the nucleus. The other, called the weak force, alters the composite of the nucleus ‑ in radioactive decay, for instance ‑ and influences the comings and goings and interactions of subatomic entities. Of these four forces. the strongest is the strong force, which holds together the nuclei of all matter. Its effects, however, can be sustained only over and extremely short distance. Gravity, on the other hand, can act over great distances, as shown by the gravitational effects of the moon on Earth’s tides. Electromagnetic force is, strong as although not as strong as the strong force, has a range of distances over which it can be effective. The weak force, as its name indicates, is very weak. Like the strong force, it has a very limited range. Scientists are confident that one day they will discover how these four fundamental forces are unified. They already have a name for the discovery: the Grand Unified Theory.

James Clerk Maxwell Father of modern physics

1831 Born on June 13 in Edinburgh, Scotland.

1847 Attends Edinburgh University and studies natural philosophy, moral philosophy, and mental philosophy.

1854 Graduates in mathematics from Trinity College, Cambridge.

1855‑72 Publishes a series of investigations, Perception of Color‑Blindness

1859 Wins the Adams prize at Cambridge for his essay, "On the Stability of Saturn's Rings."

1860 Becomes professor at king's College in London, awarded the Rumford Medal from the Royal Society for work on colors.

1861 Elected a fellow of the Royal Society of London,

1864 Presents equations expressing the relationship of electricity and magnetism ‑ now known as Maxwell's equations ‑ to the Royal Society.

1865 Resigns as chair of physics and astronomy at King’s College.

1866 Formulates, independently of Ludwig Boltzmann, the Maxwell‑Boltzmann kinetic distribution law.

1871 Becomes the first Cavendish Professor of Physics at Cambridge.

1879 Dies on November 5 in Cambridge, He is buried in a small church yard in Parton, Scotland.

A FULL SPECTRUM

The 19th century witnessed a spectacular revival of the long‑standing debate of the nature of light. Was it made up of particles‑corpuscles, as Newton had called them‑or waves, as continental theorists such as Huygens had urged? The battle was revived as early as 1800, when British physician and physicist Thomas Young, interested in the physiology of vision, began his experiments.

Young shined a light through a tiny hole in a barrier. Beyond that barrier was a second one with two pinholes in it, beyond that a screen. The light that reached the screen was arranged in alternating bands of dark and bright areas, indicating that at some points, light waves were canceling each other out, creating dark bands; at others, the waves were reinforcing each other, producing bright bands.

The image dearly suggested wavelike interference patterns, not particle behavior. Newtor~s partisans in England didn't buy Young's revelations, but they were accepted on the Continent, where French physicist Augustin‑jean Fresnel confirmed Young's work.

In the 1850s, Scottish physicist James Clerk Maxwell (who had formed a friendship with Faraday through correspondence) sought ways of explaining Faraday's electric and magnetic fields. He found that an electric charge that moved back and forth would generate a pattern of connected, oscillating electrical and magnetic waves.

Over six years of intensive mathematical labors, Maxwell worked to accurately describe Faraday's and his own findings and to formulate a theory that would unify the forces of electricity and magnetism. Finally, in 1864, he presented his astonishing results: The equations describing a magnetic field created by an electric current were nearly identical to the equations used to describe the propagation of light waves. The results also showed that magnetic waves moved at 186,000 miles per second‑exactly the speed of light. Maxwell concluded that electricity and magnetism were one and the same, and that light was a form, but by no means the only form, of electromagnetic radiation. His equations suggested an invisible world of forces much larger than anyone had thought, and also predicted that there would be wavelengths both longer and shorter than visible light.

Left: Visible light and the colors we see make up only a small part of the electromagnetic spectrum. Beyond the range of human vision are short wavelength gamma rays and long wavelength radio waves.

Right: French physicist Marie Curie experimented with uranium, radium, and polonium to understand more about radioactive emissions. Curie did not know how toxic exposure to radioactivity could be.

Maxwell's theory electrified the scientific community, especially a young German physicist, Heinrich Hertz, who set out to test Maxwell's prediction that there were many different kinds of electromagnetic

radiation. By 1888, Hertz had collected the equipment he needed: an electrical circuit with a small gap in it and a metal device (now known as an antenna) designed to respond to electromagnetic waves. Closing the circuit would cause a spark to jump across the gap. That spark, Hertz reasoned, should generate waves that, although they could not be seen, would be detected by his antenna yards away‑and it happened just as he had predicted. Hertz then determined that these electromagnetic waves had a length of about a foot and that, as Maxwell predicted, they could‑like light and heat waves‑be reflected off walls, refracted by various substances, and even polarized (that is, made to vibrate in one plane). In addition, these waves, then called Hertzian but now called radio waves, seemed to move at the speed of light.

Maxwell had died ten years before, but Hertz's work confirmed his field theory. It also sparked the inventive genius of an Italian physicist, Guglielmo Marconi, who improved Hertz's apparatus piece by piece, controlling the spark by means of telegraph keys, enlarging the antenna, and using a device called a coherer to detect the Hertzian waves. Very quickly, Marconi was able to send and receive waves over a distance of a mile and a half. By 1901, he was successfully transmitting radio waves across the Atlantic.

But a major mystery remained. Water waves moved through water, sound waves moved by fluctuations in air pressure. What was the medium that was carrying all these electromagnetic waves‑especially those that brought starlight from space?

Since the time of Aristotle it had been pretty well considered universal knowledge that surrounding all things was an invisible medium, the fathomless ether through which all things moved. In 1887 two American physicists, Albert Michelson and Edward Morley, set out to measure the effects of this presumed ether. Michelson and Morley used an ingenious L‑shaped invention called an interferometer. The two arms of the instrument were perpendicular to one another. At the end of each arm was a mirror. In the center, where the two arms joined, was a light source and a device that could split a light beam in two, sending half to each mirror, and then recombine the two halves when they bounced back from the mirrors. They placed the interferometer with one arm pointing in the direction that the Earth was moving through space.

In theory, as the Earth sailed through the motionless ether, the beam moving in the direction of Eartl~s orbit would move faster, receiving a slight boost from the Earth's motion. The beam moving at a right angle would travel only at the speed of light. Each beam, therefore, would arrive back to the center at a slightly different time, and the light waves, thus out of phase, would be seen to interfere with one another. Yet every time the physicists measured, they found no difference at all. Science was finally forced to face the possibility that there was no ether‑or at least none that could be detected. Light and electromagnetic waves, it seemed, didn't need a medium through which to move.

By the end of the 19th century, many of the questions about electromagnetic radiation seemed to be solved‑all except the mystery of what it was. Then came a series of sudden, serendipitous discoveries that deepened both the mystery of radiation and science's knowledge of the nature of atoms.

In 1895, German physicist Wilhelm R6ntgen, like many of his colleagues, was investigating the puzzling phenomenon of cathode rays. Scientists had been observing the properties of a cathode, or a negative electrode, the piece through which the current leaves a battery or other electricity storage device. A cathode placed in a vacuum tube would give off a strange sort ‑of emission, detectable only,when it struck certain chemicals.

R6ntgen set up his cathode ray device in a darkened room, covering its tube in black cardboard. Then, by chance, he noticed something very odd. An object several feet away began to glow. When he turned off the cathode tube, the glowing stopped. Some beam was radiating from the tube.

This made no sense, because Rbntgen knew that cathode rayswhatever they were‑couldn't travel more than a few inches in air. He realized he must have discovered something different. He soon found that the beams would travel through his hand, casting a shadow with the outline of his bones on a screen. After weeks of meticulous experiments, he announced the existence of electromagnetic waves so peculiar that he called them simply "X.)'

Almost immediately, these x‑rays were being used in medicine. Still, scientists did not really know what they were or why they did what they did. Cambridge University physics professor Joseph John Thomson found that when the rays were beamed through a gas, it could conduct electricity. French physicist Antoine Henri Becquerel thought that the Rontgen rays might be involved in fluorescence, or the glow of certain compounds after being exposed to sunlight.

Becquerel placed a photographic plate wrapped in heavy black paper beneath a compound and planned to expose it to sunlight, surmising that if the compound, in this case uranium, gave off x‑rays while fluorescing, their emission would fog the plate.

But the weather turned cloudy, and Becquerel stuck the uranium and photographic plate in a drawer to await the sun. When he took them back out, he developed the plate anyway and, to his amazement, found that it was highly exposed. The uranium alone was generating some sort of radiation. But what was that? Becquerel found that the uranium rays, like Rontgen's xrays, allowed a gas to conduct an electrical current.

Polish‑born French physicist Marie Curie soon discovered that the property that she called radioactivity was common to uranium and another element, thorium. Testing other substances, she discovered that pitchblende, a uranium ore, had a higher level of radioactivity than pure uranium.

To figure out why, she and tier husband, Pierre, who was also a physicist, labored in a dismal laboratory in Paris, working through tons of the ore until, in 1898, they made the announcement that they had discovered two new substances: polonium and radium. Radium was the most radioactive substance known, and its strong emissions gave the surrounding air an electric charge.

MARIE CURIE Pioneer of radioactivity

1867 Born Maria Skloclowska on November 7, in Warsaw, Poland.

1893 Receives a degree in physics from the University of Paris.

1895 Marries Pierre Curie. Becomes a research scientist, studying, magnetic properties of tempered steel.

1896 Discovers that radiation is not a property of chemical reactions but of the atom.

1898 Along with husband, Pierre, discovers polonium an radium.

1903 Wins the Nobel Prize in physics, sharing it with Pierre Curie and Henri Becquerel.

1904 Receives her doctorate.

1906 Takes over as professor of physics at the University of Paris.

1910 Publishes Treatise on Radioactivity.

1911 Awarded the Nobel Prize in chemistry, becoming the only per1q, son to win two scientific Nobel Prizes.

1914‑19 Directs the Red Cross Radiology Service, organizing mobile x‑ray units for the French Army during World War I.

1918 Becomes director of her own laboratory in the Radium Institute at the University of Paris.

1934 Dies on July 4 near Sallanches, France.

Left: From his studies of human physiology, German physician and physicist Herman von Helmholtz drew the law of conservation of energy: The energy that comes out of any system is equal to the energy put in.

LCenter: William Thomson's sensitive mirror galvanometer, which he patented in 1858, made the longdistance telegraph cable possible.

RCenter: One of the great figures in thermodynamics was Hermann von Helmholtz, pictured here in 1881. In addition to formulating the law of the conservation of energy, he made substantial contributions to medicine, including the invention of the opthalmascope for examining the inner eye.

Right To cool hydrogen sufficiently to turn it to a liquid, engineers John Wood and A . J Schwernin built a bubble chamber in 1954. Cooled under pressure to –423F (‑252C), hydrogen gas becomes liquid,

THE SECRET LIFE OF HEAT

While studies and discoveries in electromagnetic radiation were stealing the scientific headlines, studies of another kind of energy were moving along at a quieter pace. That energy was heat, and perhaps so familiar that its study wasn't deemed newsworthy. After all, everyone knew that fire was hot, that it took heat to boil water and cook food, and that heat was needed to create the steam that, by the middle of the 18th century, ran engines and soon trains, boats, and machines.

For scientists, though, heat was as much a mystery as electricity. Based on the fact that materials expand as they get warmer, Galileo had devised a thermometer. He trapped a volume of air in a sort of upside‑down flask partly filled with liquid. As the temperature changed, the air expanded or contracted, moving the level of the liquid down or up. Without a scale against which to measure the movement of the fluid, the device showed only relative temperature changes.

That next step‑creating a scale with fixed points‑finally took place in the beginning of the 18th century, when Danish astronomer Ole Romer, who in 1675 had been first to measure the speed of light, developed a thermometer using alcohol as the liquid. He arbitrarily set 0' as the point where water froze and 60' as the point where water boiled.

In 1708, Polish‑born Dutch instrument maker Daniel Gabriel Fahrenheit visited Romer. He returned to the Netherlands and produced his own version of an alcohol thermometer, setting 0' as the temperature at which beer freezes and 100' as the temperature of the human body. By this scale, water freezes at 32' and boils at 212', quite different from the numbers set for those temperature points by Romer. Swedish astronomer Anders Celsius created a thermometer with exactly 100 degrees between the freezing and the boiling points of water. While Celsius originally set 0' as the boiling point and 100' as the freezing point, after Celsius's death in 1744, the Swedish biologist Carolus Linnaeus turned the scale around, resulting in the Celsius scale of temperatures that we use today. But what exactly were these thermometers measuring? Some thought heat came from vibrations set off in a substance. Others thought it was a weightless fluid‑they called it caloric‑that was contained in matter and flowed from place to place. At the end of the 18th century, Benjamin Thompson, an American living in England, stepped into the debate.

Born near Boston, Thompson had escaped his native country in 1776. He sided with the British during the early days of the American Revolution and served as a British military commander and a spy. Accusations of adultery and sodomy were leveled against him as well. Leaving his wife (some 20 years his elder) and daughter behind, Thompson expatriated to England and took up a scientific career. By 1779 he had been named to the Royal Society.

Thompson received an invitation to Bavaria from Prince Maximilian while traveling in France. Made major‑general of cavalry and privy councilor of state, he worked on improving the lot of the army and constructing English gardens outside of Munich. Eventually he earned the title of count and chose the name Count Rumford, after the town in New Hampshire where he had left his wife and child.

While at the Munich munitions works, Thompson noticed that the metal of the cannons became quite hot when they were being drilled out. He deduced that the amount of heat being produced was greater than the heat within the metal itself. Other‑wise, the heat in the metal would have melted away on its own. This proved that the idea of heat as caloric, a fluid contained within the metal, was not possible. Friction, he realized, was causing the heat. Motion was the key. Thompson even made estimates of how much heat a certain amount of motion would produce. He presented his Enquiry Concerning the Source of the Heat Which Is Excited by Friction to the Royal Society in 1798.

Thompson was a colorful character. When France and England were at war, each side considered him a spy‑and both may have been right. He chartered the Royal Institution in England and hired Humphry Davy as a lecturer. He pursued a career as an inventorredesigning fireplaces and stoves to conserve heat; developing central heating, a smokeless chimney, and an oven roaster; and experimenting with silk and the manufacture of thermal underwear. He made and lost fortunes and finally married the wealthy widow of the great French chemist Antoine Lavoisier. In a final act of irony, he established the Rumford Prize of the American Academy of Arts and Sciences and a Rumford Professorship at Harvard University, a center of pro‑British sentiment during the Revolutionary War.

Thompson's work was eventually taken up by James Prescott Joule, the son of a Manchester brewer. Of a conservative theological temperament, Joule believed that all forms of energy were one and the same and could be converted one into the other. It was a difficult proposition for a brewmaster to prove, even one who had studied under John Dalton, but Joule persisted.

He began with electricity. In 1840, Joule discovered that the rate of generation of heat by an electric circuit was proportional to the square of the current multiplied by the resistance. He then sought to determine whether both electrical current and mechanical motion could produce heat in predictable quantities, which Thompson had conceived but had only been able to estimate. For Joule, as for Thompson, the conversion of energy from one form into another could be explained without referring to hypothetical caloric fluids.

With so little mathematical training, Joule had trouble getting his ideas recognized. But other researchers were able to duplicate his careful experiments, notably his 1847 discovery of exactly how much mechanical force on a set of paddle blades it took to raise the temperature of water by one degree, using the Fahrenheit scale. Joule established that the amount of work done by a heat engine was proportional to the amount of heat lost in converting energy to work. To this day, a standard unit of work is called a joule.

Joule worked with William Thomson (later Lord Kelvin), who was also convinced that the study of heat and electromagnetism was leading toward a unified energy theory. He and joule shared research findings, and eventually Thomson reconsidered his belief in the caloric heat theory. A generous scientific collaborator, Thomson produced a great body of work on the mathematics of heat, electricity, and magnetism.

While joule was conducting his experiments, a German physiologist and later physicist, Hermann von Helmholtz, was formulating one of the most profound and useful ideas in physics, the law of the conservation of energy. It states that nature contains a fixed amount of energy that can be neither increased nor diminished. (The word energy comes from the Greek energia, meaning "in work.') This law applies even as energy is converted from one kind to another‑from heat energy to mechanical energy, chemical to electrical, kinetic to potential. The rule applies to the energy produced by a windmill or by flowing water as well as by burning fuel. It applies to the energy produced by the body, and would eventually be found true for gravitational, radiant, and nuclear energies as well. Measure the total energy at any point in a process, and it will always be the same.

Farenheit and Celsius each invented a scale for measuring temperature. A steady ration exists between the scales. The forumula to convert Celsius to Fahreheit is 9/5 x deg Celsius plus 32. To convert Farenheit to Celsius it is 5/9 x degrees Farenheit minus 32.

Transisiton points A melting point is the temperature at which a solid becomes liquid. A freezing point is the temperature at which a liquid becomes a solid. In theory the two are equal. A boiling point is the temperature at which a liquid becomes a gas.

Light Fantastic

Einstein may not have invented the laser, but he certainly helped. As early as 1916, he predicted that there were two ways for atoms to emit protons, or individual units of light.

One was spontaneous emission, which produces the familiar glow of a light bulb. When atoms get excited‑for example by electric current running through a tungsten filament in the bulb – their electrons jump to higher energy orbits. But nature always favors the lowest energy configuration (which is why water runs down hill), so the electrons almost immediately fall back to their original positions, shedding a photon to get rid if the extra energy . Those shed photons appear as light.

But if an atom was already excited, Einstein said , whacking it with a photon of precisely the right energy would cause the electron to emit two photons ‑ the original one that hit it plus a second one identical to the original. In theory, that could produce a very powerful beam of light, with all its photons of the same wave length and direction, precisely in phase, so that none cancel the others out.

In 1954 American physicist Charles Townes actually built a device in an effort to produce this effect. He called it a maser. Townes excited a cloud of ammonia atoms and then bombarded it with microwave radiation. The cloud emitted more microwaves than had gone into it, thus demonstrating Einstein's second concept of atomic emissions.

Six years later another American, Theodore Maiman, constructed a device that could do the same thing for visible light. He took a cylinder of ruby and excited it with a xenon flash tube, causing light to emit and thus producing the first laser. (The name for the device is actually an acronym for "light amplification by stimulated emission of radiation.") In 1960 it was a good idea with no apparent good use.

It did not take physicists long to figure out reasons for lasers. They made them in various strengths and wave lengths, finding uses in surgery, surveying, cutting materials, and printing, to name just a few. Laser probes have measured the distance to the moon; they sit at the center of every bar code reader in the grocery store.

The invention of fiber‑optic strands presented a new set of uses for the laser. Photons will move along a fiber‑optic cable considerably faster than a wave of electrons can shuffle down a wire. Now flashes of laser light carry signals essential to the operations of the telephone, television, computer modem, and many other information and communication devices, with greater speed and capacity than electric signals in copper wires ever could.

Joule and German physician and physicist Julius Mayer had already expressed related notions, but the law found its most valuable expression in Helmholtz's 1847 book,,,On the Conservation of Force. His concept of the conservation of energy proved key to the emerging science of thermodynamics and served as its first law. With the realization that energy could be transformed from one sort to another, many unexplained observations began to make sense.

French physicist Nicolas Sadi Carnot had found that the efficiency of a steam engine was related to the difference between the highest and lowest temperatures in the machine. In other words, the amount of work that you can get out of a heat engine depends on the temperature difference between the heat source, such as the steam from the boiler, and the temperature of the heat sink, the region of the engine into which heat was finally transferred. The relationship was known and accepted, but no one actually knew why. Carnot, using the caloric theory, had assumed that all the heat passed through the engine unchanged. if that were so argued German physicist Rudolf Clausius, the heat could be recycled and run the engine forever.

What Clausius concluded was that in nature, heat always flows spontaneously only in one direction – from hot to cold. The path is not reversible. If it were, a cup of coffee would stay hot all day, pulling heat out of the surrounding air. Moreover, Clausius observed, as time passes some fraction of energy in a always dissipates and is therefore unavailable for work. This disorder in the system accumulates; it came to be called entropy. Together these axioms constitute the second law of thermodynamics and were developed independently in England by William Thomson, Lord Kelvin,

(In a great twist, Lord Kelvin turned the second law inside out, that if a hot gas could cause mechanical force and dissipation of heat, then the reverse would also work. Using mechanical force to compress a gas could transfer heat from low- to high‑temperature areas. The proof of the idea spurred the growth of the early refrigeration industry in the 19th century.)

Following up on this second law, Viennese physicist Ludwig Edward Boltzmann posited that if energy is based on the motions of atoms thermodynamics could be analyzed mathematically. Boltzmann d equations for both the distribution of energy among molecules and for the effects of entropy.

The disorder in a system, he said can be measured, if not exactly then at least on probabilities. By establishing statistical relationships between the atomic structure of substances and all forms of energy that might affect them, Boltzmann became, with Maxwell, the key figure in articulating the relationships of matter and energy and synthesizing their studies in late 19th century. Those times also saw a reaction against atomic theory of matter, and Boltzmann, the theory's most energetic proponent, found himself entangled in academic battles. Exhausted and depressed, in 1906 he hanged himself.

THE STRANGE WORLD OF ABSOLUTE ZERO

With the nature of heat explained and brought into the fold with electrical and chemical energy, questions arose regarding the state of matter when it was cold. Lord Kelvin had established absolute zero ‑ some -470'F or -273'C ‑ as the point at which nothing could become any colder, and he predicted that as they neared such low temperatures, substances would increase in electrical resistance, becoming oblivious of energy.

The emerging studies in heat and thermodynamics brought Kelvin's prediction into question. Motion seemed not only to produce heat but also to affect liquids and gases similarly. Dutch physicist Johannes Diderik van der Waals established that the molecular state of liquids and gases depends not only on temperature but also on pressure and volume. As temperatures drop, the random motion of molecules that produces heat slows.

In 1877 physicists managed to cool oxygen to 90K (kelvins, a unit defined as equal to the number of degrees, Celsius scale, above absolute zero). At that point, the gas liquefied. just before the turn of the century, hydrogen was also liquefied, at around 20K; in 1908 Dutch physicist Heike Kamerlingh Onnes liquefied helium at 4.2K. Kamerlingh Onnes also found that, contrary to Kelvin's prediction, substances at such temperatures lost all resistance, becoming what are today called superconductors. Others lost all viscosity, becoming what we call superfluids. At 2.19K, For instance helium liquid will flow up the side a glass, over the top, and through the tiniest cracks.

What was this state called superconductivity? In the late 1950s, American physicists‑John Bardeen, John Schrieffer, and Leon Cooper ‑argued that in the supercold state, when atoms are arranged in distinct geometrical arrays, electron (essential components of atoms) form into pairs that emit and absorb energy equally, so there’s nothing to impair their movements. The atoms, for instance, in the 2.19K state all have the same momentum. Like runners tied together, if one moves, they all move. Heat is conducted so fast that it forms a wave through the material. If a magnetic field approaches a superconductor, it causes swirling electric currents in the outermost layers of the material that shove the field away. Superconducting materials will actually levitate above a magnetic field, a property now used to support trains above a track allow and them to move without the friction of wheels on rails. Super conductivity has inspired another technological race, for materials that will become superconducting at temperatures high enough that they can be used in everyday applications and machines.

INSIDE THE ATOM

British physicist Joseph John Thomson, in studying cathode rays, found that there seemed to be negatively charged particles that could be deflected by electric and magnetic fields. He believed they were about one thousandth the mass of the hydrogen atom. He called them corpuscles, but the name they came to be known by was coined by Irish physicist George Stoney, who called them electrons.

Scientists knew that atoms are electrically neutral. If they contain a negatively charged particle, they must also contain a positively charged particle. The leading theory was that electrons were embedded in surrounding, positively charged atomic matter, like raisins in a pudding. That notion was swiftly shattered by the New Zealand‑born physicist named Ernest Rutherford, who, along with others, had determined by 1900 that there were three kinds of radioactive emissions, which he called alpha, beta, and gamma, and that in the course of giving off such emanations, some elements transmuted into others.

The heavy alpha particles, decided Rutherford, were likely helium atoms stripped of their electrons. He designed an experiment to see what happened when he aimed alpha particles at metallic foils and measured how they scattered. Most of them bent only slightly. But one day in 1911, Rutherford's assistants reported amazing results: One out of about 8,000 alpha particles hit the foil and bounced back in nearly the same direction from which it had come. It was, recalled Rutherford, “quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you had fired a 15‑inch shell at a piece of tissue paper and it came back and hit you."

The reflected particles, Rutherford concluded, had struck the tiny nucleus of the atom. He believed that the nucleus contained nearly all of an atom's mass but took up a minuscule fraction of its volume. There was no escaping the logic: Atoms were almost entirely empty space. Moreover, he and others determined in 1919 that bombarding atoms with the right kind of radiation could knock key particles out of the nucleus, transmuting the target substance from one element to another. The ancient goal of the alchemists, to transmute matter, had been achieved. We now call those key particles protons.

Cracking the atom and examining its constituent parts posed a formidable challenge. Scientists knew that it was possible to accelerate particles such as protons and electrons by exposing them to electrical charges. But getting particles to move fast enough to smash a nucleus or do other interesting tricks required voltages that seemed unattainably high. Nature provided a few sources of high‑energy particles in the form of so‑called cosmic rays, which are not really rays at all but charged particles that come streaming through space from various sources and strike Earth. Because they strike with enough energy to tear the electrons off the atoms they hit, their effects can be observed by filling a jar with a gas (the cloud chamber or liquid (the bubble chamber), in which the incoming particles leave distinctive tracks.

But controlled study of particle collisions required artificial means of acceleration. In 1932 in England, physicists John Cockcroft Ernest Walton created an accelerator so mighty that it was able to blast protons into lithium atoms with such force that they could split the lithium into two helium nuclei. American physicist Robert Van de Graaff devised a generator that was capable of even higher voltages. Its design is still used today in many state‑of‑the-art facilities today. Soon scientists needed even more power.

1868 English physicist Joseph Norman Lockyer discovers heliurn on the sun by analyzing spectral absorption lines in sunlight.

1869 Russian chemist Dmitry Ivanovich Mendeleyev presents the organization of the elements in his periodic table.

1877 Austrian physicist Ludwig Boltzmann finds that the temperature and energy of atoms can be correlated.

1886 Radio waves are discovered by German physicist Heinrich Hertz.

1897 English physicist Joseph John Thomson discovers the component of the atom that is eventually named the electron.

1900 German physicist Max Planck determines that black bodies emit radiation at all wavelengths.

1905 German physicist Albert Einstein publishes his special theory of relativity.

1905 Third law of thermodynamics is developed.

Left When manufacturing mirrors and lenses that must be perfect, laser beams are used to detect defects.

Middle: Protected hands display a button of Uranium 235. The highly radioactive form of the element, which is used as a fuel inside nuclear reactors and as an explosive in nuclear weapons.

Right: Three electrons and three positrons from a cosmic ray leave tracks on the wall of a cloud chamber,

A particle accelerator generates and shoots a beam of very fast charged particles, either atomic or subatomic. Today this complex machine is used for radio carbon dating, cancer treatments, radioisotope production, and research into the nature of atomic nuclei

American physicist Ernest Lawrence saw a way to get it instead of accelerating particles in one quick blast, he reasoncd, it should be possible to keep them moving in a circle, by exploiting a convenient physical law: Charged particles in a magnetic field tend to move sideways. Lawrence figured that if he could cause a particle to circle a magnet, he could give it an extra kick of electric charge once or twice on every lap, eventually boosting it to tremendous levels of energy.

Lawrence's first cyclotron, as he called the machine he invented, was only five inches across, but it still did the job. Soon larger models were raising each particle to millions of volts.

In 1930, electrons and protons were the only two known atomic components. Scientists knew how many electrons surrounded the atoms of each element and knew that there must be an exactly corresponding number of protons. But if that were true, then atoms would be far too light.

Carbon, for example, has six electrons. So it must also have six protons. But it has a well established atomic weight equivalent to 12 protons. Where were the rest? Either there were six more proton‑electron pairs lurking in the nucleus, or atoms contained a nuclear particle with the same mass as the proton but with no electric charge. In 1932 British physicist James Chadwick determined that a mysterious form of nuclear emission that had been observed for decades but that fit none of the three categories of radiation was actually the long sought neutron.

A similar puzzle surrounded the neutrino, a nearly massless product of nuclear reactions. Its existence was suggested when the energy products of radioactivity didn't add up. In Italy, physicist Enrico Fermi named and described the particle in 1934, but it would take more than 20 years to detect one. Similarly, in 1930 British physicist Paul Dirac had predicted the existence of antimatter, a particle with the mass of an electron but with the opposite charge. Two years later such a thing was found, and it was named the positron.

By the late 1930s, the basic constituents of the atomic nucleus had been identified, and researchers were busily bombarding nuclei with protons to see what would happen. Fermi became interested in using neutrons instead. Protons were electrically repulsed by the other protons in the nucleus; but neutrons could slip in and produce radioactivity. Most likely, he thought, they would create new isotopes, variant atoms with a different number of neutrons. Before fleeing fascist Italy for the United States in 1938, he had discovered that he could slow down neutrons enough so that they were readily captured by some nuclei.

ENRIC0 FERMI Atomic physicist

1901 Born on September 29 in Rome, Italy‑

1922 Awarded a fellowship, receives doctorate degree in physics from the University of Pisa.

1924‑26 Lectures in mathematical physics and mechanics at the University of Florence.

1926 Discovers the statistical laws governing subatomic particles, now known as Fermi statistics.

1927 Elected professor of theoretical physics at the University of Rome.

1929 Elected to the Royal Academy of Italy.

1935‑36 Discovers slow neutrons, which leads to the discovery of nuclear fission.

1938 Awarded the Nobel Prize for his work in nuclear physics; leaves Italy and settles in the United States.

1939 Becomes a professor of physics at Columbia University.

1942 In charge of the Manhattan Project at the University of, Chicago, works on developing nuclear energy and the atomic bomb.

1945 Present in New Mexico at the first testing of the newly developed atomic bomb.

1954 Dies on November 28 in Chicago.

Quantum Mechanics

Named for packets of energy, called quanta, that move in an electromagnetic wave, quantum mechanics is the study of matter energy on the atomic and subatomic level. It is a study made difficult by the seemingly weird ways that energy and matter interact at this scale, and made even more difficult by the fact that within the quantum world exact measurements are impossible since the very act of measurement affects that which is being meas ured. Observation requireslight, light is energy, and light quanta will affect what you are trying to observe.

The amount of matter inside the atom is tiny in comparison with the amount of space. Shine a light on a solid brick of mater, and at the subatomic scale there's every probability that some of the light energy will get through. In quantum mechanics this process is called tunneling. Matter at this level can be imagined as an extra‑fine mesh, and quanta can be imagined as tiny blind fleas jumping toward it. Some fleas won't have the energy to reach the mesh. Most of the fleas will get caught up in it. A few fleas will pass through the mesh altogether. The configuration o f fleas left stuck to the mesh is comparable to the light's wave pattern.

Most people know rnatter and energy only on a macro scale, and therefore they will consider many of these ideas in quantum mechanics arcane if not downright bizarre. But just as studies of energy showed that electricity and magnetism were related so quantum mechanics has made it clear that at the subatomic level, the distinctions between matter and energy vanish. Both it appears, have the properties of particles – quanta ‑ and waves, and both bear the same burden of indeterminacy.

1913 Danish physicist Niels Bohr presents his quantum model of the atom.

1916 Albert Einstin publishes his general theory of relativity.

1924 Austrian born American physicist Wolfgang Pauli states that no two electrons can occupy the same state at the same time, now called the Pauli exclusion principle.

1925 Atomic theory of dispersion articulated stating that light travel at different speeds through different materials depending on the materials properties.

1932 English physicist James Chadwick discovers the neutron.

1932 American physicist Carl Anderson uses a cloud chamber to detect the existence of antiparticles.

1968 Gabriele Veneziano publishes his dual resonance model of the strong interactions, inspiring modern string theory.

1970 Stephen Hawking shows that black holes can emit radiation, later named Hawking radiation.

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That same year, German researchers Otto Hahn and Fritz Strassmann were using slow neutrons to blast uranium, the heaviest naturally occurring element. The results surprised them: The uranium seemed to have been changed into elements of lower atomic weight. That seemed impossible. Uranium was such a huge atom that it was simply assumed that nothing could split it.

Austrian‑born physicists Otto Frisch and Lise Meitner (who had worked with Hahn but as a Jew had fled Germany) heard of these results. They concluded that the uranium nucleus had fissioned, or split, into smaller nuclei, throwing off two neutrons in the process. The total fission products were slightly less massive than the original uranium atom. The missing mass, Meitner calculated, had converted to energy‑a lot of energy.

With war imminent, the fact that Germany had achieved atomic fission was worrisome, and the United States embarked on a high‑intensity, supersecret research program called the Manhattan Project to exploit and control nuclear fission. Fermi built the first nuclear reactor under the stadium walls of the University of Chicago. He wanted to confirm that neutrons emitted by fission would go on to cause other nuclei to split in a chain reaction. His reactor was packed with neutron‑absorbing carbon and cadmium, so the chain reaction took place in slow motion. Even then, it got intensely hot. If uncontrolled, a fission chain reaction would explode.

At Los Alamos, New Mexico, the secret team created the first atomic bomb, setting it off in the summer of 1945. A few weeks later, atomic bombs were moved to the Pacific and dropped on Japan. The war was over in days.

The bombs that obliterated Hiroshima and Nagasaki left the world in no doubt that the release of energy by nuclear fission was a stupendous source of power. By 1957, fission energy was being used to generate electricity in commercial power plants; it is still used for that purpose around the world.

STRING THEORY

In explaining the way the world works, physicists seem always to be driven by a sense that all of the matter, energy, and forces in the universe ought to be related at all scales. Electromagnetism provided the means to unify the theories of electricity, magnetism, and light. Quantum theory unified the electro‑magnetic world with the subatomic world of matter. Einstein's theory related light and gravity, time and energy. Then he spent the last 20 years of his life trying to connect his theory of general relativity, which elaborated the nature of gravity, to quantum theory, which described the atomic world of electromagnetic force. All his attempts at what was called a unified field theory failed, and physicists were left with different sets of formulas describing things on a very small scale and things on a large scale. Gravity, shown to be the result of curvatures of space surrounding mass, seemed to be on its own.

Is it now possible to unify all of these theories into one that describes all of matter and the forces of the subatomic and macrocosm? And if one created such a theory, how could it be proved? These questions face the proponents of a theory proposing that at the heart of all matter and energy are vibrating strings: infinitesimal filaments of energy. Each string has distinctive vibrations that relate it to a particular particle, just as the vibrations of a violin string relate to a single note.

These strings are small‑about 10³³ centimeters a point followed by 32 zeros and then 1 or a millionth of a billionth of a billionth of a billionth of a centimeter. As described by American physicist Brian Greene, one of the developers of string theory, "If an atom were magnified to the size of the solar system, a string would be the size of a tree."

String theory had its beginnings in the 1960s. Researchers were studying the strong force that holds protons and neutrons together in the atom's nucleus, and they found that the mathematical analysis resulting from their work seemed to describe energy as vibrating filaments.

According to string theory, the relationships between all the fundamental forces ‑ gravity, electromagnetic radiation the strong and weak forces ‑ take place by the resonance of these strings in many dimensions. They hypothesize as many as 11, all of them gut the three dimensions we experience so small that they can't be seen. The passage of vibrations through these dimensions, from one string to another, particle to particle, force to force, would unite them all. Gravity relates to these subatomic forces through the notion of gravitons, which as University of Maryland physicist Sylvester Gates puts it, are "waves of gravitational energy in space/time that are responsible for communicating the gravitational force."

Is this Einstein's unified theory? Many have their doubts, but string theorists continue to work on the idea, using complex mathematics. Empirical proof may not come until particle accelerators can find evidence of strings in action.

ALBERT EINSTEIN Father of the theory of relativity

1879 Born on March 14 in Ulm, Germany.

1896‑1900 Studies at the Zurich Polytechnic Institute.

190S Publishes paper on special relativity, Brownian motion, and the interactions of the quantum of light and the photoelectric effect.

1911 Becomes professor at Karl‑Ferclinand University, Prague, predicts the bending of light.

1914‑‑33 Becomes professor of physics and director of theoretical physics at Germany's Kaiser Wilhelm Physical Institutn

1915 Publishes worl< on the general theory of relativity.

192J Awarded Nobel Prize in physics for the photoelectric effect

1930 Produces model of the expanding universe.

1933 After Nazis gain power, leaves Germany and takes position Institute for Advanced Studies, Princeton, New Jersey.

1946 Serves as chairman of Emergency Committee of Atomic Scientists.

1952 Offered presidency of Israel; declines.

1953 Publishes The Meaning of Relativity .

1955 Dies on April 15 in Princeton.

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The atom had still more secrets to impart. One notion, propounded in the 1960s by American physicist Murray Gell‑Mann, was that protons and neutrons were made up of even smaller things, whimsically called quarks. Theory predicted six kinds, and the last of those six was finally detected in 1995.

Yet there maybe more. Today's accelerators have circular particle tracks that are miles long and accelerate their contents very close to the speed of light. With such devices, researchers have detected exotic particles that don't occur at the energy concentrations anywhere near those of our planet. The next generation of colliders may reveal particles that will help explain two of the deepest mysteries: why the big bang created a preponderance of matter over antimatter, and why nearly dimensionless, pointlike electrons and quarks have mass.

EINSTEIN'S ENERGY

Isaac Newton had proposed that light emanated in particles he called corpuscles. Thomas Young's experiments at the beginning of the 19th century seemed to clearly demonstrate that light must be waves. This remained the thinking all through the century, and it served well enough to solve many problems and help generate experimental and statistical evidence of the relationships between light, magnetism, and heat.

The first signs that there might be more to light than had met all these experimental eyes came after cathode and penetrating x‑rays provided evidence for the electron. Setting up another experiment to visualize the energizing of electrons, German physicist Philipp Lenard focused a beam of single frequency light onto a metal surface and found that the light ejected electrons from the metal plate. Another plate, connected to a sensitive current‑measuring device, collected the ejected electrons. An electrified grid was set up that could vary the voltage of the light beam as it passed through the grid to the first plate and affect the charge of the electrons that jumped toward the collecting plate. Lenard found that when he increased the voltage of the grid, the current measuring the electrons that hit the collecting plate‑the photoelectric effect‑decreased sharply. (The collecting plate now had a negative charge and repelled many of the negatively charged electrons.) At a certain point, the current disappeared altogether. When the light intensity was increased, to give the electrons more energy, however, it made no difference in the outcome of the experiment.

Left Physicists working to comprehend the forces of atomic particles use high‑level mathematics to express their findings

Middle British physicist Stephen Hawking shared the exhilaration of modern cosmology.

Right: Twice a Nobel winner, American chemist Linus Pauling displays the complexities of a protein molecule, using a wooden model.

In 1905, Austrian physicist Albert Einstein found an explanation. He knew that in 1900, German physicist Max Planck, working on the problem of why hot objects did not radiate with the predicted mix of infrared, visible, and ultraviolet light, had hit upon a mathematical formula based on Boltzmann's second law of thermodynamics that seemed to prove that energy was not released in a continuous stream of all magnitudes, but only in discrete bits, each of which was called a "quantum.' Einstein assumed that Lenard's light beam was made up of particles or photons, and that each of these transferred its energy to the electron on the first plate. As the electrons emitted from that plate worked their way toward the collecting plate through the charged grid, they used up energy. Some made it; some didn't: Most likely the ones closest to the surface, with less distance to travel, made it. Intensifying the light, then, wouldn't give the photons or the electrons any more energy; it would simply add more photons, allowing more electrons to escape. When the interfering voltage got high enough, no electrons would make it to the collecting plate.

So was light then made up of particles?

Based on the discovery of the electron and his conclusion that atoms were mostly empty space, in 1911 Rutherford conceived of a model atom that resembled a tiny solar system. A positively charged nucleus sat in the center‑occupying one billionth of the entire space but containing most of the mass of the atom. Negatively charged electrons orbited like planets around the nucleus.

In 1912, Danish physicist Niels Bohr suggested that if electrons worked this way, they would quickly dissipate their energy. With Planck and Einstein's work in mind, he proposed a way to revise the models' concept of electrons. The model needed to show that when electrons received heat or electromagnetic radiation, they responded in distinctive wavelengths, those spectral fingerprints that identify one substance from another. Electrons orbit the nucleus, but the orbits in which they can travel are fixed, different for each atom. When the atom receives light energy, it is distributed among the electrons, which then jump from one fixed orbit to another. When the electron jumps back to its original orbit, it releases light photons at the atom's distinctive wavelength.

LINUS PAULING Leader in biochemistry and physics

1901 Born February 28 in Portland, Oregon.

1925 Graduates from the California Institute of Technology with a Ph.D. in chemistry. Remains at Caltech for the next 38 years.

1933 Elected, youngest member ever, to the National Academy of Sciences.

1937 Appointed director of the Gates Laboratory and chairman of the Division of Chemistry and Chemical Engineering, Caltech.

1939 Publishes The Nature of the Chemical Bond, classic of chemistry and biochemistry.

1942 With Dan Campbell and David Pressman, announces successful formation of artificial antibodies.

1954 Receives Nobel Prize in chemistry for research on the chemical bond.

1955 With more than 50 other Nobel laureates, issues the Mainau Declaration, calling for an end to the use of nuclear weapons.

1958 Publishes No More War; leaves Caltech owing to antagonism from Caltech administrators.

1963 Receives Nobel Peace Prize.

1973 Founds the Institute of Orthomolecular Medicine, which later becomes The Linus Pauling Institute of Science & Medicine.

1994 Dies on August 19 at his California ranch.

Left: Particle Beam Fusion Accelerator II, operating in Albuquerque, New Mexico, was used in the 1990s to investigate what happens in the core of a nuclear explosion, on Earth or in space.

Center: At the Tokai Research and Development Center in Japan, neutrons generated by nuclear fission assist researchers exploring materials and life sciences.

Right: This x‑ray image of the starburst galaxy M82, taken by the Chandra X‑ray Observatory, is the first confirmed case of a black hole outside the nucleus of a galaxy‑possibly a new type of black hole.

These ideas suggested that light was both particle and wave. Soon a host of European theorists – including Werner Heisenberg from Germany, Erwin Schr6dinger from Austria, and Louis de Broglie from France ‑ took Bohr's quantum theory to its logical conclusion: Particles actually behaved like waves, which explained their quantum nature; but waves of light also had particle like properties, such as momentum. In fact, matter was so wavelike that it was, in principle, impossible to tell where an electron was at any given instant. It didn't actually exist in any particular place until it was measured, and there was no way to measure it. Instead, it was everywhere at once and had only a probability of having a certain position or speed. Even Einstein found it difficult to conceive of this idea. "God does not play dice" with the universe, he declared. As mind‑boggling as it was, though, quantum mechanics would soon prove an accurate physical theory.

EINSTEIN'S SPACE‑TIME

The September 1905 issue of the German physics journal Annalen der Physik proved to be, according to physicist Max Born, "one of the most remarkable volumes in the whole scientific literature." In it, 26 year old Albert Einstein, then an examiner at the Swiss Patent Office in Bern, published three papers that together pulled back the curtains on the long‑established Galilean/Newtonian world view, revealing a universe hidden until then, in which time, space, matter, energy, and gravity performed seemingly impossible feats.

How does one begin to appreciate Einstein's groundbreaking theories? First, it's best to recognize that the world of relativity is not the world with which we are familiar, in which time, speed, space, place, and matter are concrete realities. We mark time by the clock, travel our fastest in a car or plane, recognize our place by a set of basic coordinates, and know that matter is solid. As Einstein himself put it, in our daily lives, "All our thoughts and concepts are called up by sense‑experiences and have a meaning only in reference to sense ‑experiences. " Einstein's theories seem difficult because they take us outside our sense experiences. When he published them in 1905, his theories seemed outside the experience of most physicists as well.

Although by the turn of the 20th century much had been learned about the nature of light, physicists puzzled over why the speed of light remained the same even when beamed from a moving object. If you're swimming along at a mile an hour in a current that's moving two miles an hour, your total speed will be three miles an hour. If you swim across the current, your speed will be back to one mile an hour.

In the 19th century, scientists believed that, analogously, the Earth moved through invisible but fluid like ether. Einstein argued that there was no ether, and that the speed of light was constant whether or not the object from which it was beamed was at rest or in motion. For light, at least, this view argued against Newtonian physics, in which velocities were simply added together. Einstein imagined what would happen if one actually traveled at the speed of light.

RICHARD FEYNMAN Theoretical, physicist

1918 Born May 11 in Queens, New York.

1939 Graduates with bachelor of science degree from the Massachusetts Institute of Technology.

1941 Begins work on the atomic bomb project at Princeton University; continues later at Los Alamos, New Mexico.

1945 Observes detonation of atomic bomb in New Mexico; appointed professor of theoretical physics at Cornell University, where he studies the fundamentals of quantum electrodynamics.

1950 Accepts position as professor of theoretical physics at the California Institute of Technology.

1950s Provides quantum mechanics explanation for theory of superfluidity; develops theory to account for the weak force associated with radioactive decay.

1959 Appointed Richard Chace Tolman Professor of Theoretical Physics at Caltech.

1961 Publishes Quantum Electrodynamics and Theory of Fundamental Processes

1965 Awarded the Nobel Prize for fundamental work in quantum thermodynamics; elected fellow of the Royal Society.

1986 Serves on the commission investigating the Challenger space shuttle accident.

1988 Dies on February 15 in Los Angeles, California.

Our sense experience tells us that time everywhere moves at the same rate. If we're standing on the street and a plane flies overhead, we have no doubt that the passenger's watches are moving at the same speed as our own. Further, if we see lightning strike the ground in front of the plane and behind the plane at the same time, we assume that anyone in the plane would have seen the same thing, since time is the same for both of us. Einstein realized that all these things depend on where we are and how fast we, or another person seeing the same thing, are moving.

If I'm driving at ten miles per hour, the light from my headlights still moves at 186,000 miles per second. If someone else is driving at 20 miles per hour, the light from her headlights is still moving at 186,00 miles per second. If neither could move faster than the speed of light, and speed is distance divided by time (miles per hour), then the only way to account for the constant speed of light from cars moving at different speeds is that distance and time must change.

Einstein's theory says just this: The faster an object travels, the more slowly time passes for that object. Neil Tyson, director of the Hayden Planetarium at the American Museum of Natural History, created a chart showing that at 25 percent of the speed of light ‑ 47,000 miles per second‑a second will lengthen by 0.03 seconds. At 50 percent of the speed of light, a second will lengthen by 0. 15 second; and at 99 percent of the speed of light, one second will be 7.09 times as long as a second experienced on Earth. At 99.99999999 percent of the speed of light, a second will be 19.6 hours long relative to a second on Earth.

This is not just a concept ‑ evidence of Einstein's theory can be measured in a fast plane, and it becomes an important reality to factor into the consideration of objects, such as atomic particles, whose velocity does approach the speed of light. In an atomic accelerator, the mass of such particles appears to increase as well, to the point that at the speed of light, an object's mass would be infinite. Such technology wasn't available for Einstein, yet his thoughts did turn to mass and its relationship with energy.

A few months after he published his special theory of relativity ‑ in the very next volume of Annalen der Physik, in fact Einstein published the beginnings of his general theory of relativity. "Does the Inertia of a Body Depend on Its Energy Content?" was a very subtle title for his groundbreaking paper, which went up against the well established theory that neither matter nor energy could be created or destroyed. Magnetic energy might become electrical energy, liquid might become a gas, but the laws of the conservation of matter and energy seemed inviolable.

Einstein thought otherwise. Matter and energy, he stated, were two sides of the same coin. Energy could be created out of matter, and matter out of energy. He even gave an equation that described the transaction: E = mc², in which E is energy, m is mass, and c is the speed of light. Since the speed of light squared is such a huge number‑something close to 448,900,000,000,000,000 in miles per hour squared – a great amount of energy can be produced from a small bit of mass.

STEPHEN HAWKING Theorist of black holes

1942 Born January 8 in Oxford, England.

1962 Graduates with honors from Oxford University; enrolls at Cambridge University to pursue a Ph.D. in cosmology.

1963 Diagnosed with annyotrophic lateral sclerosis, also known as motor neuron or Lou Gehrig's disease,

1966 Completes doctorate; awarded fellowship at Gonville and Caius College, Cambridge.

1970 Shows that black holes can emit radiation, a phenomenon later named Hawking radiation.

1974 Elected one of youngest fellows ever of the Royal Society.

1979 Appointed Lucasian Professor of Mathematics at Cambridge, chair held by Isaac Newton in 1669.

1985 Contracts pneumonia, undergoes tracheotomy, and is left entirely without speech. Begins communicating by computer, which allows him to write and synthesize speech.

1988 Publishes A Brief History of Time.

1998 Publishes Stephen Hawking's Universe: The Cosmos Explained.

2004 Announces solution to black hole paradox; presents findings at international conference on general relativity and gravitation in Dublin, Ireland.

2005 Awarded the Smithsonian Bicentennial Mdal

ALBERT EINSTEIN AND THE ATOMIC BOMB Although Albert Einstein was not directly involved with the developrnent of the atomic bomb in the United State, his name will forever be associated with the atomic age. Einstein was a pacifist, but when he found that German scientists were in the midst of a research project capable of producing a devastating weapon, he informed President Franklin Roosevelt. This led to the Manhattan Project and the use of atomic bombs on Japan.

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Einstein realized that part of the radium that Marie Curie studied, was in fact being transformed to energy. What we now call radioactivity is mass being converted to energy, and Einstein's theory measures it exactly. Knowledge of this equation led to the development of nuclear fission, a process by which atoms are split in order to release energy from their mass. This technology, much to Einstein's regret, led to the atom bomb.

Even greater amounts of energy could be produced by nuclear fusion ‑ the combining of atoms. In the sun, intense heat tears apart hydrogen atoms, separating positively charged nuclei and negatively charged electrons. These particles collide and create helium atoms, four hydrogen atoms making one helium. The mass of the resulting helium, however, is less than the combined mass of the hydrogen. The differential changes into smaller nuclear particles ‑ and energy. Scientists began trying to replicate this process, which powers the sun and stars.

Einstein's special theory raised another question. if the sun~s gravity, for instance, were acting on the Earth, it would mean that gravity would have to be traveling at 93 million miles in an instant‑faster than the speed of light. Since Einstein had concluded that nothing could move faster than the speed of light, either something besides gravity was keeping Earth in orbit or gravity was not what Newton thought it was.

Einstein calculated that what causes the force called gravity is the distortion of space around a mass. Newton's law states that the force of gravity depends on the distance between two bodies; Einstein's general relativity theory states that such distances are along curves caused by the matter's distortion of space.

When large masses are involved, space bends so that gravity pulls light beams along its curve. The mass of the sun can bend light rays, but it is hardly noticeable compared with the bending of space when a star collapses and its mass is squeezed into a small volume. The warp in space becomes so great and the gravity intense that even light cannot escape. Such a whirlpool of acceleration known as a black hole.

Einstein's special and general theories of relativity altered the way physicists understand the universe, but it also changed the way all thinking humans imagine our place in it. Einstein was aware of the philosophical impact of his theories. He wrote: "The non mathematician is seized by a mysterious shuddering when he hears of four‑dimensional' things, by a feeling not unlike that awakened by thoughts of the occult. And yet there is no more commonplace statement than that the world in which we live is a four‑dimensional space‑time continuum.'

QUANTUM MECHANICS

Researchers began using the theories of quantum mechanics to analyze how atoms worked in the real, although unseen, world. Among the many question posed by quantum theory was how, within the new understanding of atomic structure, chemical bonds could form.

In the 1920s, American chemist Linus Pauling had found that, just as the atom has no fixed structure, combinations of atoms in chemical compounds exist in intermediate states between one structural form and another, a phenomenon called resonance. In 1929 Pauling was able to set down rules by which relationships among electrons in such bonds could be discovered. Through those, he was able to understand more of the properties of the compounds that they form.

Pauling's work was distinguished by his ability to combine theoretical and practical chemistry, as well as by his understanding that chemical bonds could be both stable and variable. This led him to investigate sickle‑cell anemia, and he discovered that the disease resulted from a variation in the structure of hemoglobin molecules. His paper, "Sickle Cell Haemoglobin: A Molecular Disease' " contributed significantly to interest in the genetic causes of disease, not only sickle‑cell anemia but many others as well.

RICHARD FEYNIVIAN Nobel Prize‑winning American physicist Richard Feynman stands in front of a blackboard strewn with his own notations in Los Angeles, California, in March 1983.

Pauling had also been attempting to model the structure of a DNA molecule. In 1953, with crystallographer Robert Corey as co‑author, he published images and a discussion of his proposed three‑dimensional version of DNA, with three twisted strands. In 1954, Pauling received a Nobel Prize for his work on chemical bonds. In 1963, he won a second Nobel, this time the Nobel Peace Prize for his work on behalf of nuclear disarmament. Had he envisioned one fewer strand in his DNA molecule, he might have won a third Nobel‑but in the end that went to Francis Crick and James Watson, who ultimately discovered the double, not triple, helix as the structural backbone of the DNA molecule.

Another researcher inspired by quantum physics was American physicist Richard Feynman. The indeterminism of quantum physics seemed to inspire his innate sense of independence. Working with the difficult mathematics of quantum mechanics, Feynman was able to picture the relationships of subatomic forces at work in electromagnetic radiation ‑ how, within the indefinite structure of the atom, photons interact with electrons and their positively charged opposite particles, positrons. Moreover, Feynman was able to illustrate the exchange of force and the collisions of particles, using drawings that came to be known as Feynman diagrams. Feynman was awarded a Nobel Prize for his groundbreaking work in quantum electrodynamics.

Working with physicist Murray Gell‑Mann, Richard Feynman also succeeded in describing the forces at work in the process of radioactive decay. Called the weak force, it provided a glimpse into the smallest particles now theorized to exist within the atom ‑ fermions, bosons, W particles, and Z particles. Because they are often slow to react, these particles can, under extreme heat and pressure, trigger larger reactions. These particles have been found to lie at the heart of nuclear fusion. Charismatic and genial, Feynman became a great popularizer of science. He made use of his prodigious talents as a storyteller, and he managed to fascinate scientists and nonscientists alike with his descriptions of the logic and the implications of advanced work in physics.

In the 21st century, there are students of energy and matter who return to the ancient study of cosmology. English physicist Stephen Hawking works to unify quantum physics with Einstein's general theory of relativity. Hawking combines these two concepts‑the first dealing with subatomic realms and the second dealing with large masses ‑ and uses the intellectual synthesis as a way to understand such unfathornables as the beginning of the universe and black holes, where gravity is so strong it keeps light from escaping. Hawking describes black holes as billions of tons of mass, compressed into a volume the size of a single proton. In such a state, the particle would act according to quantum theory: It would emit radiation and then gradually dissipate and disappear.

In a later formulation Hawking, along with Thomas Hertog of the European Organization for Nuclear Research, made a bold suggestion. We cannot know the exact momentum or location of any particle at any moment, but if the particle that was the early universe complied with quantum theory, the universe itself is a quantum event. If that is the case, then, as Hertog put it: "The universe doesn't have a single history, but every possible history, each with its own probability."

The ancients began by gazing up into the skies and wondering about matter and energy. Today, given the elegance of general relativity and the inexplicable truths of quantum mechanics, we still find ourselves in a similar state of wonder.