In the happy years of the 1920s, Bohr played the diffident, sometimes disapproving father, Heisenberg the rebellious and brilliant prodigy. In his three years at Copenhagen, from 1924 through 1927, Heisenberg proved his worth. His first major contribution was a formula that figured the energy states within an atom. Max Born and Pascual Jordan extended the formula into a true matrix mechanics with which all frequencies of the spectrum could be figured. Heisenberg's second contribution was less formalistic and much more incendiary. The uncertainty principle by its very nature contradicted classical physics and, in a way, challenged the very essence of modern science. Throughout the development of classical physics, it was assumed that perfectly accurate measurement was possible, in ideal conditions. Only the crudeness of our measuring apparatus stood between our results and the object's true dimensions. Heisenberg said no, it is impossible to “see” sufficiently into the atom to know for certain what processes—specifically, wave or light—one is measuring. Further, whatever means we use to measure will inevitably disturb the element under scrutiny. Thus, the “observer” will affect and distort the “observed.”
For Bohr, Heisenberg's uncertainty principle was too limited. It focused only on conditions that occurred during observation, and in effect ignored the “wave” state. Bohr insisted (vehemently) on much more. Little was gained, he argued, from ignoring empirical evidence.
The wave-particle duality that so vexed quantum physics had a very solid empirical history. It began with an experiment, now replicated in physics classrooms throughout the world, called the “double-slit experiment.”
In 1801, Thomas Young (a physicist, physician, and Egyptologist who in his spare time deciphered the Rosetta Stone) devised a mechanism to analyze light: “I made a small hole in a window shutter, and covered it with a thick piece of paper, which I perforated with a fine needle,” he told the Royal Society in 1803. Thus began what has been called the most beautiful experiment in physics.30 When Young “split” the sunlight by dividing it with a thin card, he observed, projected onto the wall, “fringes of colour on either side of the shadow”—clear evidence of interference or diffraction. He was astonished. Light, according to Newton, was made of particles. Yet as it traveled past Young's thin card, it diffracted, just like a wave breaking on a jetty. Startling as this was for Young, a 1927 variation on the double-slit experiment came up with an even more astounding result: Electron beams from a nickel crystal produced diffraction. Matter, like light, was shown to behave like waves.
Young's experiment was the basis for much resistance to Einstein's theory of photons. After all, Young had proved that light was wave, not particle. Einstein countered with proof of light's particle nature. In 1915, Robert Millikan, after ten years of experimentation, reluctantly concluded that Einstein's equations were correct. In 1923, Arthur Compton confirmed the particle nature of electromagnetic quanta by observing the scattering of electrons from X-rays.
Thus, the paradox: Light and, as de Broglie proved, matter are both wave and particle. If physics had challenged our intuitive sense of the world with relativity, it now seemed to have done away with intuition altogether. After all, these states—wave and particle—are quite different. A wave is continuous and nonlocal, spread out over a large area. Particles are discrete, indivisible, and local. Richard Feynman calls the uncertainty principle a “logical tightrope on which we must walk if we wish to describe nature successfully.”31 We must think anew, says Feynman, “in a certain special way” to avoid “inconsistencies.” The awkward phrasing is unusually revealing. Quantum physics challenges not only our intuitive perception, but the limits of language. When Bohr and Heisenberg argued over the uncertainty principle, more was at stake than the utility of the principle itself. In order for a theory to “work,” it must not only explain evidence; it must also gain acceptance among practitioners. Bohr was older than Heisenberg and certainly more empathetic to the general state of alarm over quantum mechanics. Perhaps, too, he was more understanding of our psychological need to visualize (that is, imagine, in its etymological sense) our world. Something in addition to de Broglie's elegant proofs was needed to bridge the gap between particle/discontinuity (the side favored by Heisenberg) on the one side and wave/continuity on the other.
The answer, Bohr came to believe, was “complementarity.” Rather than see particle and wave in opposition, as mutually exclusive, might we not accept both as true? Particle and wave are interdependent; so, too, are classical physics and quantum theory. Here, seemingly, was an attempt to forge connections. Yet beneath the gentle-sounding word, complementarity posed a radical idea. Not only was exact measurement impossible, but the ambiguity lay in the properties themselves. Even before measurement, the atomic system is uncertain. All we can hope to attain by way of information lies in the realm of probability.
We have seen that quantum physics developed not through the genius of a single thinker, but through a series of conversations among both believers and nonbelievers. The most intense of these took place between Einstein and Bohr. It began well before Bohr's landmark presentation of the Copenhagen interpretation in 1927. Indeed, Einstein had contributed to the theory himself, not only by “discovering” the quanta, but also in collaborating with S. N. Bose to develop Bose-Einstein statistics (a set of formulae defining the statistical distribution of particles called bosons).
Einstein, unwilling to set foot in Fascist Italy, did not attend Bohr's lecture in Como. Within a few weeks, however, Bohr was scheduled to speak at the 1927 Solvay Conference in Brussels. Einstein came to the conference full of misgivings. What he heard was Bohr's paper, “The Quantum Postulate and the Recent Development of Atomic Theory.” He was horrified. In Bohr's words, “Einstein… expressed a deep concern over the extent to which causal account in space and time was abandoned in quantum mechanics.”32 Thus began the famous dialogue between Bohr and Einstein, one that would help Bohr refine his theory of complementarity and spur Einstein ever forward in his search for a theory that would subsume quantum mechanics and encompass both atomic physics and astrophysics.
EINSTEIN AND UNIFIED THEORY: CHASING THE RAINBOW
By 1927, Einstein had begun his assault on quantum mechanics. He never let up. It led to the most famous dispute of twentieth-century physics. Two eminent physicists, Albert Einstein and Niels Bohr, remained locked in battle for years, raising questions that remain unsettled even today.
Einstein's stubborn criticism struck many, particularly of the younger generation of Pauli and Heisenberg, as folly, waste, even provocation. In their view, the greatest physicist and boldest pioneer of the age had become a reactionary. The young Robert Oppenheimer visited Princeton in 1935 and described Einstein as “completely cuckoo.”33
Yet the old Einstein was only acting like the young Einstein. In 1905, he had opposed the orthodoxy of those who thought that light flowed through the invisible material called “aether.” Now, he was a “heretic,” cast out by the new quantum orthodoxy. He became a prophet in the wilderness, preaching the need to refound all of physics on a revivified “classical” basis, closer to Newton than to Heisenberg. This time, though, he would not succeed.
He failed in part because subatomic physics was in its infancy, unable to yield sufficient data. Physics had to wait years before “powerful atom smashers would clarify the nature of subatomic matter,” notes Michio Kaku in his reverential Einstein's Cosmos.34 Had Einstein had our wealth of data, Kaku hazards, he might have succeeded. As it was, knowing nothing of the bosons, gluons, muon neutrinos, partons, leptons, and quarks—the whole atomic stew, as it were—he could not form a “picture” to guide his mathematics.35
Yet Einstein's grand project had an agenda, one that skewed his method. He not only disagreed with the new quantum theory; he detested it. Not only did quantum mechanics turn the universe into a game of dice by replacing causality with probability, but it also seemed to him inelegant and ungainly. He was especially repelled by Heisenberg's uncertainty principle and its
implied threshold beyond which we must remain ignorant. In his effort to unify electromagnetism and gravity, Einstein remained within the fold of classical physics. However revolutionary his own notion of the relativity of space and time had been, his unified field theory would succeed not by accepting the newest revolution, but by subsuming it.
With anyone less sane and generous than Einstein—and Bohr—the dispute over quantum mechanics could have turned acrimonious. The issues were not technical, but philosophic. Einstein on the one hand, and Bohr, Pauli, and Einstein's close friend Max Born on the other, were arguing from their deepest convictions about what physics should be. When Paul Ehrenfest had to choose Bohr over Einstein, he began to sob. Yet no real personal bitterness or resentment surfaced in this dispute, which, while intense, was respectful, though it lasted for over thirty years. Unable to change each other's mind, Einstein and Bohr finally talked past each other. By the late 1930s, they were exhausted. Afterwards, when they met, they exchanged pleasantries, but no more. Einstein and Born continued to argue for decades. In 1926, Born wrote about the new quantum mechanics to Einstein, and got this reply:
Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us closer to the secret of the “old one.”36
Einstein's confident appeal to an “inner voice” upset the rigorous Born, but their friendship never wavered. In 1944, Einstein wrote Born that they had become “antipodean” in their scientific beliefs: “You believe in a God who plays dice, and I in complete law and order in a world which objectively exists.”37 Born answered a month later, suggesting that Einstein was not really up to date with the arguments. “[G]ive Pauli a cue,” wrote Born, “and he will trot them out.” On one of Born's later manuscripts on the topic, Einstein wrote in the margins such remarks as “Blush, Born, blush!” and “Ugh.” To which the astonished Born replied: “Do you really believe that the whole of quantum mechanics is just a phantom?”38
In their letters, mutual affection mingles with frustration, especially from Born. In 1948, Einstein and Born exchanged what appear to be plaintive cries in the night:
(Einstein): I am sending you a short essay which, at Pauli's suggestion, I have sent to Switzerland to be printed. I beg you please to overcome your aversion long enough in this instance to read this brief piece as if you had not yet formed any opinion of your own, but had only just arrived as a visitor from Mars…. I am… inclined to believe that the description of quantum mechanics… has to be regarded as an incomplete and indirect description of reality, to be replaced at some later date by a more complete and direct one…. (Born):…. Your example is too abstract for me and insufficiently precise to be useful as a beginning…. I am, of course, of a completely different opinion from you. For progress in physics has always moved from the intuitive towards the abstract…. Quantum mechanics and the quantum field theory both fail in important respsects. But it seems to me that all the signs indicate that one has to be prepared for things which we older people will not like.39
They kept sparring, and in March 1954, Pauli finally intervened. He was visiting in Princeton again, and Einstein had given Pauli an article Born had written. Pauli, as usual, was blunt in his letter to Born:
[Einstein] is not at all annoyed with you, but only said you were a person who will not listen. This agrees with the impression I have formed insofar as I was unable to recognize Einstein whenever you talked about him in either your letter or your manuscript. It seemed to me as if you had erected some dummy Einstein for yourself, which you then knocked down with great pomp.40
Pauli's explanation of Einstein's “classical” view was “simple and striking,” Born remarked, but as usual, no mind was changed. As quantum theory developed from 1925 until his death in 1955, Einstein went his own way, ignoring the new findings and searching for a unified theory according to his original plan.
Only in Newton do we find a parallel for the first half of Einstein's life: his discovery at age twenty-six of relativity, the identity of mass and energy, and the quantum nature of light, and at age thirty-six of gravitational theory. But no one, not even Newton, rivals the later Einstein. Newton spent the last half of his life pondering theology and running the Royal Mint. Einstein was in his mid-forties when he began his search for the unified field. At that age, Bohr, Planck, and Rutherford, their great discoveries behind them, were slipping away from science into administration or teaching. To the day he died, Einstein toiled away at a problem more ambitious than any he had faced before, as if he still had his greatest discovery in front of him.
Today, Bohr's Copenhagen interpretation, as is the common fate of orthodoxies, has undergone dramatic revision. In a sense, quantum mechanics has been reworked into a near-unified theory called the “standard model,” a quantum field theory that is consistent for special relativity and quantum mechanics—that is, consistent for every point in space. Although early proponents of quantum mechanics tried their hand at field theories, nothing succeeded fully until Richard Feynman and others successfully manipulated the necessary mathematics. The result was quantum electrodynamics, or QED, which described the electromagnetic field and the interactions of particles precisely to ten decimal places.
When Einstein started his quest, he meant to unify gravitation and electromagnetism. Since then, the additional forces, strong and weak, have been unified with electromagnetism under QED. Gravity is the odd force out. It has yet to be folded into the other three forces. Any “grand unified theory” (dubbed GUT) or “theory of everything” must connect all four forces: gravitation, electromagnetism, and the two forces, strong and weak, that stabilize the nucleus. These forces have rightly been called primordial. They have existed from the beginning, or as near to it as can be imagined. When matter came to be, it found those primordial forces waiting to turn elemental matter into the universe we know. We do not know why gravity or electromagnetism or the strong or weak forces exist; like the universe, they simply do.
The holy grail of unified theory continues to attract believers. String theories, attempts to tweak the standard model into a form compatible with gravity, have long been contenders. Some hope that an ultimate “superstring” theory will be a “theory of everything.” Not yet. Among other problems, no variation of string theory can be verified experimentally. Michio Kaku dismisses the need for experiments:
The real problem is purely theoretical: if we are smart enough to completely solve the theory, we should find all of its solutions…. So far, no one on Earth is smart enough.41
In a way, modern physics has always been driven by the ambition to unify. In the view of one recent account, “modern physics is best summed up a series of unifications.”42 In the late Middle Ages, it was assumed that matter on the earth and moon differed from that of the celestial bodies, that each domain was governed by different laws, that each element of the “universe,” from the earth to the farthest quasar, was sui generis. Modern physics was born with Copernicus, Kepler, Galileo, and Newton. Each searched for a single system that would apply to all space, time, and matter.
For a time, the solution was at hand. Newton's mechanical system, complete and quite successful, prevailed for more than two centuries. Then, in 1819, Hans Christian Oersted, a Danish professor, noticed the deflection of a compass needle towards a source of electricity. Magnetism and electricity were, somehow, one and the same. Everything changed. Electromagnetism, theorized in its classical form by James Clerk Maxwell, involved phenomena quite different from Newton's solid bodies. Maxwell even demonstrated that light is an electromagnetic wave. Newton's mechanical model began to crumble around the edges. The struggle to reconcile electromagnetism with the forces of gravity, space, and time gave rise to relativity on the one hand and new atomic theories on the other.
Pass a magnet over a heap of iron filings, and the filings line up in a certain direction, as if an enveloping force has moved them all—which is exactly what has happe
ned. The field is the magnet's sphere of influence. Gravity is another such field; its power extends to all objects in its region, pressing us to the earth and preventing us (and everything in our world) from levitating. The clustering of the filings around the magnet's poles illustrates the anatomy of a field. The lines of force tell the filings where to move and how quickly. The field itself generates the power that moves the filings. In Newton, particular forces “told” each body how to move and at what speed and strength—like billiard balls struck by a cue. Maxwell's field, however, simplifies things greatly: It is both particle and force at once—the vehicle and the source of energy.
Einstein sided with Maxwell and against Newton. Indeed, relativity was born from Faraday and Maxwell's “new type of law.” In Newton, one body was assumed to affect another instantaneously, no matter how vast the distance. But how could a force be transmitted “instantly” when nothing can go faster than the speed of light? And besides, what “medium” carried forces from the sun to the earth or from the earth to the moon? Until Einstein, physicists postulated an ether through which electromagnetic waves and light traversed. From Maxwell, Einstein began to see that there were a myriad of “local” fields everywhere in space, transmitting energy—a crucial step in the journey toward general relativity.
Einstein's Genius Club Page 16