By the end of the nineteenth century after more than two thousand years of intellectual struggle that began with the Greek philosophers, physical scientists had reason to believe that they were beginning to understand the universe. Their theories of matter and energy, of electricity and magnetism, of heat and sound and light were confirmed in laboratories throughout the world with increasing precision. Experimentation was the method and mathematics the language of a powerful coherent body of knowledge called classical physics.
For a few years before and after the turn of the century, the world was taking a breather from war and rebellion. The monumental achievements of science, technology, and industry such as the installation of a transatlantic telegraph cable, inspired hopes for a peaceful and prosperous future. But beneath the calm surface, in politics as well as in science, the roots of future turmoil were quietly gathering strength. Even the sturdy foundations of classical physics were developing alarming cracks. Some discrepancies were found when experiments clashed with theory.
Perhaps the most unsettling of these was the failure to discover the ether. When their results plainly contradicted the ether hypothesis, physicists were dismayed. How could there be vibrations without something to do the vibrations? Other puzzles cropped up by accident. On November 8, 1885 the German physicist Wilhelm Conrad Rontgen stumbled upon a way to make strange rays with the power to penetrate black paper, and even living flesh. Since x is the unknown in algebra, Rontgen called them X rays.
By December, he had used them to take pictures of human bones, and within a year their practical value was well understood. The rapid spread of use of X rays throughout the world foreshadowed the way scientists, engineers, and inventors would turn fundamental discoveries into technological applications in the coming century but no one knew where X rays came from. The chance discovery of radioactivity finally signaled the beginning of a new era in physics. As the element polonium, identified by Polish-born Marie Curie in 1898, emits radiation it changes spontaneously into lead.
This discovery shattered the belief inherited from the Greeks that the elements are immutable and their atoms indestructible. What forces are at work inside them? Such questions were new to physics, and were to remain at its cutting edge throughout the twentieth century. The answers would affect our lives in ways no one could imagine in the year of 1900. The twentieth century began with a flurry of innovations such as the airplane, the mass-produced automobile, and transatlantic radio communication. They transformed the world, but the changes sweeping over physics at the same time were far more radical.
Those brought about not just different lifestyles, but new ways of thinking. Modern physics grew out of classical physics and rest of three pillars: the quantum theory, which describes atoms and their nuclei, Special Relativity, which deals with the relationship between space and time and General Relativity, which explains gravity. The latter two were the sole creations of Albert Einstein and even the former received a crucial early contribution from him. Einstein’s miracle year came in 1905 he was 26 years old and working as a patent examiner in Bern, Switzerland.
In March he submitted a paper in which he proposed that light, which was classical physics treats as a wave phenomenon, could also be thought of as consisting of discrete bits of energy he called quanta. The implied wave/particle duality of light became the cornerstone of the new quantum theory. In May, Einstein explained the erratic motion of pollen suspended in water as due to the jostling of innumerable invisible molecules. When this theory was verified in the laboratory, even the most skeptical of physicists were forced to accept atoms, which until then had been mere conjecture as real material objects.
For four years, from 1914 to 1918, World War I engulfed the world and overshadowed scientific pursuits. When it was over, and scientists could resume their research, they immediately began applying the conceptual tools perfected before the war to push the frontiers of physics both outward, and inward. In the cosmic realm, General Relativity provided the theoretical framework, and the giant telescopes built in the clear air of California the observational foundation, for the emergence of physical cosmology– the science of the structure and history of the universe. An essential preliminary step was a test of General Relativity.
In 1919, when the bending of starlight by the sun, as predicted by Einstein, was observed, he instantly became an international media star. The curvature of of space time, while not fully comprehensible to most people, nevertheless seemed to be such a profound insight into the structure of the universe that it caught the imagination of a wide public. Since then, General Relativity has been confirmed by numerous observations, and remains in place as the correct theory of gravity.
But the study of cosmology itself would turn up wonderful surprises in the coming decades. In the atomic realm. nfusion reigned. Despite heroic efforts by physicists throughout the world, Bohr’s theory had only very limited success in accounting for the properties of the light, and X rays emitted by atoms. Quantum theory seemed headed for failure, or a revolution. The vital clue was found more by guessing than by deduction. in 1923 the French physicist Louis de Brogile was writing his doctoral dissertation at the age of 31. Deeply impressed with Einstein’s interpretation of light as both wave-like and particle-like, he wondered whether this strange wave-particle duality could apply to particles of matter was well.
Specifically, he proposed that electrons too have wave-like characteristics, and even suggested a formula for their wavelengths. Most of his colleagues ignored these wild and unsupported claims, but Einstein, who had a reliable intuition about physics, wrote “I believe it is the first feeble ray of light on this worst of our physics enigmas. ” Einstein turned out to be right” de Brogile had discovered the secret of matter. But as de Brogile himself admitted in his thesis, unless experimental proof could be found, his theory would remain useless speculation.
The roaring twenties were a boisterous era of prosperity, fast cars, jazz, popular radio, and illegal drinking. Before they ended with the crash of the stock market in 1929, which triggered the Great Depression, the twenties produced such human and technological accomplishments as the invention of television and the jet engine, and the first transatlantic solo flight by Charles Lindbergh in 1927. Out of range of public clamor, this exhilaration atmosphere also produced what might be called the greatest achievement of quantum mechanics.
Frustrated by the inconsistencies of the patchwork quantum theory pioneered by Einstein and Bohr, the 23 year old German physicist Werner Heisenberg started from scratch. In the summer of 1925 he decided that atoms should be described without assuming anything about unmeasurable quantities such as the positions and speeds of electrons inside atoms. instead, he arranged the measurable quantities, such as the discrete frequencies of light emitted by the atom, in arrays of number not unlike spreadsheets.
By manipulating these spreadsheets, which mathematicians call matrices, Heisenberg was able to recover the successes of the older quantum theory, without encountering its contradictions. Heisenberg’s matrices give the right answers, but convey no visual image of the interior of the atom. in the winter of 1925-26 the Austrian physicist Erwin Schrodinger succeeded in finding a more intuitively appealing description. in this approach, de Broglie’s waves are solutions of an equation which came to be called the Schrodinger equation.
Discrete colors of light emitted by glowing matter reflect the fact that electron waves confined in an atom have specific frequencies, just as sound waves inside a flute can only have discrete frequencies. The birth of the new theory culminated in Schrondinger’s amazing proof, in March 1926, that his and Heisenberg’s formulations, which appeared so different, are actually mathematically equivalent.
Henceforth Quantum Mechanics in either guise, replaced Newtonian mechanics as the correct description of atomic particles. incorporates wave/particle duality and substitutes probability for certainly in dealing with the building blocks of matter. it broke with classical physics even more radically than Special and General Relativity — and for three quarters of a century it has passed all experimental tests. Throughout the nineteen thirties, while America struggled with the Great Depression and Adolf Hitler’s Nazis rose to absolute power in Germany, physicists quietly quietly continued to collaborate across national boundaries.
Quantum mechanics proved to be a reliable framework for the study of solid matter, of molecules, and of atoms. The discovery of the neutron and the invention of particle accelerators launched the new science of nuclear physics. Although the size of the nucleus is 100,000 times smaller than that of an atom, and its internal energy higher by the same factor, quantum mechanics continued to work perfectly. The future of physics looked promising, but in 1939 World Wary II broke out and swept the whole world, including the physics community, into its wake.
Physicists and engineers helped to win the air-borne Battle of Britain by developing Radar, and their German counterparts designed the V-2 rockets that terrorized London. Of greater historical significance, though, was the construction of the atomic bomb. As soon as nuclear fission was discovered in Europe, it became apparent that if a way could be found to release its energy in a bomb, the course of the war would be altered.
In America a number of physicists, many of European origin, worried that Hitler might acquire such a weapon and persuaded the normally pacifistic Albert Einstein to warn President Franklin D. Roosevelt. In an urgent letter dated August 2, 1939, he explained the danger by writing: “It is conceivable… that extremely powerful bombs of a new type may thus be constructed. ” Einstein’s letter did not have an immediate effect, but eventually helped to persuade the United States to begin the monumental task of building an atom bomb. The man chosen to direct the project was the theoretical physicist Robert Oppenheimer. Although he had no industrial or even experimental experience, he proved to be a remarkably effective leader.
His team on a remote mesa in New Mexico, and smaller groups in other secret laboratories, included most of the nation’s best physicists. By the force of his towering intellect Oppenheimer managed to unite this diverse group in a common effort to design and build a bomb, and to test it successfully in July 1945. By then, Germany had already surrendered, but its ally Japan was still at war. In August 1945, two atomic bombs dropped on the Japanese cities of Hiroshima and Nagasaki contributed to a quick end of World War II. Their chief legacy, however, was to be felt for a long time.
For almost half a century the Cold War’s nuclear stand-off between the United States and the Soviet Union held the world in its grip. In the united States the successful conclusion of World War II inspired a heady sense of optimism and self-confidence which was further bolstered by the end of the Great Depression. Renewed prosperity, in turn, allowed America to contribute generously to the reconstruction of the world’s shattered nations. neither the emerging Cold War, nor the sudden eruption of the Korean conflict in 1951, could dampen the good spirits.
Scientists returned to their universities and industrial labs, full of new ideas picked up in the course of their war work, and eager to get on with their careers. Far from closing down, weapons laboratories developed into permanent national research centers devoted to both military and civilian research. For the first time, the federal government undertook the systematic support of basic science.
One of the theoreticians who came down from oppenheimer’s mesa in New Mexico was Richard Phenomena, a native New Yorker just three years past his Ph. D. Brilliant, irreverent, and ambitious, he distrusted authority and insisted on figuring things out in his own way. His particular strength was his visual imagination. For example he developed an elegant code for representing complex equations by simple diagrams that allowed him to let his physical intuition guide his mathematical calculations toward quick, accurate solutions. Feynman brought this unorthodox technique to bear on what was at the time the principal problem of theoretical physics: the quantum mechanics of light.
Photons had been recognized for almost half a century, but a detailed description of how they are emitted and absorbed by electrons was lacking. Together with American colleagues and Japanese physicists who had worked along similar lines while they were out of touch with the West during the way, Feynman solved the problem by creating Quantum Electrodynamics(QED). QED proved to be of such unprecedented precision and scope that it set a standard of excellence against which all future fundamental theories of elementary particles would come to be measured.
In contrast to QED, which applies to the outer shell of the atom where the electrons reside, theoretical descriptions of the atomic nucleus remained rudimentary. Even as the list of so-called elementary particles produced by accelerators grew into the hundreds, theories proliferated, but none were mathematically satisfactory. Neither the aging giants, such as Werner heisenberg, nor the young geniuses, such as Feynman, knew which way to turn. The tantalizing success of QED only added to their frustration.
By mid-century the new physics was beginning to pay off in a wide range of applications. Its scope extended vertically, as it were, from the unimaginably small interior of the nucleus up to the incomprehensibly vast stretches of the universe. At the same time, physics also had a powerful horizontal impact on other branches of science, often by way of novel instrumentation, and on technology. For biology, physical methods brought about spectacular results. The discovery of the double helix of the DNA molecule, revealed by X-ray/images of crystallized DNA, triggered a revolution in genetics.
Henceforth the mechanisms of heredity could be understood in tangible, material terms, and eventually even manipulated. Medicine a acquired an important technique when Rosalyn Sussman Yalow, an American nuclear physicist, invented a way to use radioactivity for detecting minute amounts of a huge variety of materials, ranging from nicotine to viruses, in the human body. Though its name twists the tongue, her method, called radioimmunoassay, relied on simple principle.
If you count six red-eyed fruit flies in a jar and you know that the incidence of red eyes is one in a thousand, you conclude that there are 6000 flies in the jar — without the tedium of counting them. Radioimmunoassay counts molecules rather than flies, and measures radioactivity, rather than eye color. Chemistry gained a valuable diagnostic tool with nuclear magnetic resonance. Radar research had led to instruments that can identify nuclei by the way they absorb microwaves.
For chemists, whose usual province is the electron cloud of the atom, this descent to the nucleus opened new opportunities. clear magnetic resonance itself would later develop into magnetic resonance imaging, with a name chosen purposely to avoid the connotations of the word nuclear. Geology also adopted an instrument based on 20th century physics. The SQUID(Superconducting Quantum Interference Device), which relies on a peculiar quantum effect discovered in 1962, can detect otherwise imperceptible changes in magnetic fields induced by the presence of mineral deposits. In these countless other ways physics began to stimulate research in its sister sciences.
The most influential achievements of the 1950’s, however, were the invention of the laser and the development of the transistor. Both of these devices, which are direct applications of quantum mechanics, would transform science and spawn entire industries devoted to new technologies. Even as the Vietnam War was tearing at the fabric of America, the Beatles conquered the world, and the first astronauts landed on the Moon, two unrelated discoveries on opposite coasts of the country announced the opening of a new chapter in the history of physics.
At Bell Telephone Laboratories in New Jersey, Arno Penzias and Robert Wilson were annoyed by a persistent hissing noise in their sensitive microwave receiver. Unable to suppress it no matter what they tried, they tracked it back to its surprising source: the cosmic background of microwave radiation that has been gradually cooling off since the Big Bang to a temperature of about three degrees Celsius above absolute zero. This astonishing observation reinvigorated research on cosmology, with General Relativity as its firm theoretical backbone.
Four years later Jerome Friedman, Henry Kendall, and Richard Taylor, working at the Stanford Linear Accelerator Center in California, found the first experimental evidence for the existence of quarks, which had been proposed on theoretical grounds earlier in the decade. Protons and neutrons, it now appeared, are not elementary like photons and electrons, but composed of quarks. Here at last was reason for hope that a fundamental theory, as compelling as Quantum Electrodynamics, might one day be constructed for nuclear physics.
On the face of it, the discoveries of the cosmic background and of quarks seemed to take place at opposite ends of the scale of distances. Indeed, the size of the observable universe is about forty-five powers of ten larger than that of a quark. Nevertheless, the two realms turn out to be closely related. The cosmic background radiation provides evidence of the universe the way it was about 1010 years ago, before it had expanded to its present size. A fraction of a second after the Big Bang, its particles were packed tighter than they are in an atom, so it was ruled by quantum mechanics.
At one time, furthermore, the universe consisted not of atoms but of quarks. in this way cosmology brings the physics of the immensely large back to join the physics of the immeasurably small. The scope of modern physics may be symbolized by a circle that extends from quarks, past atoms, molecules, and boulders, past planets, stars, and galaxies, out to the universe and, via the Big Bang back to its elementary building blocks. Cosmology also relates how the universe has evolved in time.
When this story is combined with stellar evolution and the geological history of the planet, with biological and cultural evolution, and finally with recorded history, a coherent narrative of truly epic proportions emerges. To be sure, the story is riddled with gaps and puzzles — including the mystery of the origin of life — but its broad outlines are firmly in place. In the nineteen sixties, physicists first began to draft the opening chapters of this story. Future generations may well look back on this ambitious undertaking as the principal accomplishment of physics in the twentieth century.
On July 20, 1976, a couple of weeks after the two hundredth birthday of the United States, an automated spacecraft landed on Mars and beamed back images of its red soil. The world held its breath as a robot searched for extraterrestrial life (and found none). As significant as the experiment itself was the manner in which the news was reported to the public. As a result of the universal spread of color TV, visual images began to supplant the written and spoken word which had been the principal carrier of news since antiquity. Science itself has long recognized the value of human vision enhanced by technology.
To the telescopes, microscopes, and cameras of classical physics the twentieth century has added television, holography, and, most importantly, the computer. The field of computer graphics, which builds upon discoveries in modern physics, has in turn become an indispensable tool for basic research. In 1981 an ancient dream came true when the outlines of individual atoms were revealed to the human eye for the first time. The instrument that made this possible, called the Scanning Tunneling Microscope(STM), consists of a fine needle whose tip gently scans a surface the way a blind person’s fingertip might scan an unfamiliar face.
The digitized contours are fed into a computer which organizes them into a picture resembling the underside of an egg carton: each bump represents a single atom. Synthetic color coding adds to the contrast and helps to identify atoms of different species. The resulting map of the invisible atomic landscape we inhabit is imbued with a haunting beauty. In medicine the combination of computers with different probes has yielded equally dramatic results.
Views of the brain produced by pencil-thin beams of X rays — useless when considered individually — are assembled by Computerized Axial Tomography(CAT) scanners into three-dimensional color-coded images that have revolutionized neurosurgery. Ultrasound images of fetuses have benefited obstetrics. Other techniques for peering into the human body include Magnetic Resonance Imaging(MRI), which produced the picture at right, and Positron Emission Tomography(PET scanning), which records the radiation emitted when positrons from radioactive materials administered to the body combine with electrons in nearby cells.
Even pure mathematics, the queen of the sciences aloof from the material world, has embraced computer graphics. The Mandlebrot set, for example, a mathematical structure whose delicate beauty and complexity fascinates mathematicians, artists, and computer whizzes, owed its discovery in 1979 to the emerging image-making capability of the computer. The generations of physicists after 1975 will not look at the world through glass lenses, but at its image on a computer monitor. What will they see?
The news of the fall of the Berlin Wall (1989), the reunification of Germany (1990), and the disintegration of the Soviet Union (1991) rang out over an astonished world like so many peals of a great bell celebrating the end of the Cold War. As international tensions diminished and global trade flourished, the world seemed determined to make a fresh start. Physics, too, entered a new phase. The headlong rush into new discoveries that had started after World War II was slowing down, mainly because over the years science had grown cumbersome.
Theories were becoming so complex that even super computers couldn’t keep up with the calculational demands made upon them. Experiments in some branches of physics took years to plan and carry out, simply because they required enormous research teams, scientific instruments, and financial resources. Physicists took advantage of the new, more measured pace by going back to take a second, harder look at what had been discovered earlier in the century — sometimes with surprising results. Since 1925 quantum mechanics had been an infallible guide to the atomic world, universally accepted, but difficult to interpret.
Nobody understands quantum mechanics,” grumbled Richard Feynman. The advent of lasers, computers, and fast electronics led to the substitution of real experiments for mere thought experiments. Observations of the behavior of the individual photons and atoms brought about increasingly convincing proofs that nature really is as bizarre as quantum mechanics makes it appear. To describe nuclear and particle physics, a consistent theory based on quantum mechanics, relativity, and quarks had in two decades been refined to such a degree that it acquired the name Standard Model.
Although it left many questions unanswered, it successfully accounted for all known particles and forces except gravity. The Standard Model confidently predicted the existence of a sixth and last quark named “top. ” But when the top quark was finally found in 1995, its huge mass turned out to be so grotesquely out of proportion with the others that it became a new enigma itself. On the human scale, the phenomenon of superconductivity, which had been discovered in 1911 and explained in 1957, also produced a bombshell: the detection of superconductivity at much higher temperatures than had been thought possible.
What’s more, the old explanation did not fit the new observations, so theorists had to start all over again. On the cosmic scale, new instruments mapped out the microwave background radiation discovered in 1965 at an unprecedented level of detail. The new date encouraged cosmologists, including the British physicist Stephen Hawking, to tackle a theory of “quantum cosmology”, which deals with the wave function of the entire universe, and the beginning of time. If successful, it will be the ultimate union of atomic and cosmic physics.
A look back over a century of physics reveals an era of vigorous growth, not only in depth and scope, but also in sheer volume. The membership of the American Physical Society, for example, increased 400-fold from about a hundred in 1900 to over forty thousand in 1997. In part this growth reflects ballooning university enrollments, but it is also a symptom of the evolution of the scientific enterprise from a genteel academic pursuit into a robust component of the world’s economy. The story of the transistor illustrates the transformation. Life without computers is now as unthinkable as a computer without miniaturized transistors.
Those, in turn are products of a vast applied research effort at university and industrial laboratories that was rooted in pure, basic research. The lineage of today’s laptop leads straight back to Werner Heisenberg’s discovery of quantum mechanics in 1925. Turning to the coming century, and trying to anticipate the future directions of science, it helps to remember that the great discoveries are rarely the outcomes of deliberate searches for universal answers, but more often the unanticipated dividends of careful research focused on modest, specific questions.
Nearly four hundred years ago, for example the German astronomer Johannes Kepler struggled for four years to remove a tiny discrepancy in the calculated orbit of Mars — and discovered the laws that govern that motions of all the planets in the universe. In this century, Ernest Rutherford was investigating the details of the passage of charged particles through matter, when he hit upon the atomic nucleus. in the 21st century the passionate pursuit of particular problems will likewise yield wonderfully unexpected universal insights. And what are the profound insights physicists could hope for?
We may soon know what dark matter is, and whether the universe will continue to expand forever, but how did time begin? General Relativity teaches us what gravity is, but where does mass, which measures inertia even in the absence of gravity, come from? How should we describe turbulence, the chaotic swirl of liquids and gases that has defied mathematical physicists for a century? If we knew, would we be able to predict weather patters and heart attacks? Can consciousness be explained in terms of electrical currents in neural networks, and possibly quantum mechanics, or is there more to it?
For that matter, do we have to accept the strange laws of quantum mechanics without question, or will someone discover the clue that makes the quantum obvious, as Albert Einstein never stopped hoping? How did life begin? Are we alone in the universe? Until we can answer such questions with confidence, we cannot claim to have understood the world. Looking back we realize that we have learned much in this century, but of mysteries there is no end. The most impenetrable of them all is to predict what the next discovery will be.