The Scientist as Rebel (28 page)

Young people like me saw all these famous old men making fools of themselves, and so we became conservatives. The chief young players then were Julian Schwinger and Richard Feynman in America and Sin-Itiro Tomonaga in Japan. Anyone who knew Feynman might be surprised to hear him labeled a conservative, but the label is accurate. Feynman’s style was ebullient and wonderfully original, but the substance of his science was conservative. He and Schwinger and Tomonaga understood that the physics they had inherited from the quantum revolution was pretty good. The physical ideas were basically correct. They did not need to start another revolution. They only needed to take the existing physical theories and clean up the details. I helped them with the later stages of the cleanup. The result of our efforts was the modern theory of quantum electrodynamics, the theory that accurately describes the way atoms and radiation behave.

This theory was a triumph of conservatism. We took the theories that Dirac and Heisenberg had invented in the 1920s, and changed as little as possible to make the theories self-consistent and user-friendly. Nature smiled on our efforts. When new experiments were done to test the theory, the results agreed with the theory to eleven decimal places. But the old revolutionaries were still not convinced. After the results of the first experiments had been announced, I brashly accosted Dirac and asked him whether he was happy with the big success of the theory that he had created twenty-five years earlier.

Dirac, as usual, stayed silent for a while before replying. “I might have thought that the new ideas were correct,” he said, “if they had
not been so ugly.” That was the end of the conversation. Einstein too was unimpressed by our success. During the time that the young physicists at the Institute for Advanced Study in Princeton were deeply engaged in developing the new electrodynamics, Einstein was working in the same building and walking every day past our windows on his way to and from the institute. He never came to our seminars and never asked us about our work. To the end of his life, he remained faithful to his unified field theory.

Looking back on this history, I feel no shame in being a conservative today. I belong to a generation that saw conservatism triumph, and I remain faithful to our ideals just as Einstein remained faithful to his. But now my generation is passing from the scene, and I am wondering what the next cycle of history will bring. After the revolutionaries of string theory have grown old, what will the next generation think of them? Will there be another generation of young revolutionaries? Or shall we again have an inversion of the normal state of things, with a new generation of young conservatives in rebellion against the elderly pioneers of string theory? My generation will not be around to see these questions answered.

One of the main themes in Greene’s book is the disconnect between Einstein’s theory of general relativity and quantum mechanics, the two discoveries that revolutionized physics at the beginning of the twentieth century. Einstein’s theory is primarily a theory of gravity, describing the gravitational field as a curvature of space-time, and describing the fall of an apple as the response of the apple to the curvature of space-time induced by the mass of the earth. Einstein’s theory treats the apple and the earth as classical objects with precisely defined positions and velocities, paying no attention to the uncertainties introduced by quantum mechanics. The apple and the earth are large enough so that the quantum uncertainties are negligible.

On the other hand, quantum mechanics describes the behavior of
atoms and elementary particles, for which the quantum uncertainties have a dominating influence, and pays no attention to gravity. The atoms and particles are small enough so that any gravitational fields that they induce are negligible. The two theories divide the universe of physics between them without overlapping, general relativity taking care of large objects from apples to galaxies, and quantum mechanics taking care of small objects from molecules to light-quanta. General relativity is important for astronomy and cosmology, while quantum mechanics is important for atomic physics and chemistry. This division of the universe works well for all practical purposes. It works well because the gravitational effects of single atoms or particles are unobservably small.

Greene takes it for granted, and here the great majority of physicists agree with him, that the division of physics into separate theories for large and small objects is unacceptable. General relativity is based on the idea that space-time is a flexible structure pulled and pushed by material objects. Quantum mechanics is based on the idea that space-time is a rigid framework within which observations are made. The two theories are mathematically incompatible. Greene believes that there is an urgent need to find a theory of quantum gravity that applies to large and small objects alike. Quantum gravity means a unified theory that works like general relativity for large objects and like quantum mechanics for small objects. In spite of heroic efforts by many people, no consistent theory of quantum gravity was found until string theory came along. The first and greatest triumph of string theory was its success in unifying general relativity with quantum mechanics. That success gave its discoverers some justification for claiming that it could be a “theory of everything.” String theory is still incomplete and far from ready for practical application, but it does in principle provide us with a theory of quantum gravity.

As a conservative, I do not agree that a division of physics into separate theories for large and small is unacceptable. I am happy with the
situation in which we have lived for the last eighty years, with separate theories for the classical world of stars and planets and the quantum world of atoms and electrons. Instead of insisting dogmatically on unification, I prefer to ask the question whether a unified theory would have any real physical meaning. The essence of any theory of quantum gravity is that there exists a particle called the graviton which is a quantum of gravity, just like the photon which is a quantum of light. Such a particle is necessary in quantum gravity, because energy is carried in discrete little packets called quanta, and a quantum of gravitational energy would behave like a particle.

The question that I am asking is whether there is any conceivable way in which we could detect the existence of individual gravitons. It is easy to detect individual photons, as Einstein showed, by observing the behavior of electrons kicked out of metal surfaces by light incident on the metal. The difference between photons and gravitons is that gravitational interactions are enormously weaker than electromagnetic interactions. If you try to detect individual gravitons by observing electrons kicked out of a metal surface by incident gravitational waves, you find that you have to wait longer than the age of the universe before you are likely to see a graviton. I looked at various possible ways of detecting gravitons and did not find a single one that worked. Because of the extreme weakness of the gravitational interaction, any putative detector of gravitons has to be extravagantly massive. If the detector has normal density, most of it is too far from the source of gravitons to be effective, and if it is compressed to a high density around the source it collapses into a black hole. There seems to be a conspiracy of nature to prevent the detector from working.

I propose as a hypothesis to be tested that it is impossible in principle to observe the existence of individual gravitons. I do not claim that this hypothesis is true, only that I can find no evidence against it. If it is true, quantum gravity is physically meaningless. If individual gravitons cannot be observed in any conceivable experiment, then
they have no physical reality and we might as well consider them nonexistent. They are like the ether, the elastic solid medium which nineteenth-century physicists imagined filling space. Electric and magnetic fields were supposed to be tensions in the ether, and light was supposed to be a vibration of the ether. Einstein built his theory of relativity without the ether, and showed that the ether would be unobservable if it existed. He was happy to get rid of the ether, and I feel the same way about gravitons.

According to my hypothesis, the gravitational field described by Einstein’s theory of general relativity is a purely classical field without any quantum behavior. Gravitational waves exist and can be detected, but they are classical waves and not collections of gravitons. If this hypothesis is true, we have two separate worlds, the classical world of gravitation and the quantum world of atoms, described by separate theories. The two theories are mathematically different and cannot be applied simultaneously. But no inconsistency can arise from using both theories, because any differences between their predictions are physically undetectable.

Another major theme of Greene’s book is the interpretation of quantum mechanics and the weird phenomena of quantum entanglement. He devotes two long chapters, “Entangling Space” and “Time and the Quantum,” to this theme. He makes a valiant attempt to clarify a notoriously foggy subject. But he makes his task more difficult by insisting that quantum mechanics must include everything. He rejects without any serious discussion the dualistic interpretation of quantum mechanics, the idea that there are two separate worlds, the classical world and the quantum world, each following its own rules. The dualistic view, limiting the scope of quantum mechanics to well-defined experimental situations, makes the problems of interpretation much simpler.

The dualistic interpretation of quantum mechanics says that the classical world is a world of facts while the quantum world is a world
of probabilities. Quantum mechanics predicts what is likely to happen while classical mechanics records what did happen. This division of the world was invented by Niels Bohr, the great contemporary of Einstein who presided over the birth of quantum mechanics. Lawrence Bragg, another great contemporary, expressed Bohr’s idea more simply: “Everything in the future is a wave, everything in the past is a particle.” Since the greater part of our knowledge is knowledge of the past, Bohr’s division limits the scope of quantum mechanics to a small part of science. I like Bohr’s division, because it allows the possibility that gravitons may not exist. If the scope of quantum theory is limited, gravity may legitimately be excluded from it. But Greene will not accept any such limitation. After briefly describing Bohr’s point of view, he says:

I prefer the calmative effect of Bohr’s perspective on the mind, while Greene prefers the hidden reality. In his first chapter, Greene shows us what he means by hidden reality:

The next-to-last chapter, “Teleporters and Time Machines,” is a pleasant interlude, describing some possible engineering applications of quantum entanglement and general relativity. The teleporter is a device that can scan an object at one place and reproduce a precise copy of it at another place far away, using quantum entanglement to ensure that the reproduction is exact. The good news is that such a device is in principle possible. The bad news is that it inevitably destroys the object that it copies. The time machine is a tunnel through hyperspace connecting two portals that exist at different places and times in our universe. If you can find the portal that is later in time, you can walk through the tunnel to emerge in your own past. The good news is that such a tunnel is a possible solution of the equations of general relativity. The bad news is that a tunnel large enough to walk through would require more than the total energy output of the sun to hold it open. Neither the teleporter nor the time machine is likely to contribute much to the welfare of our descendants. Greene describes these fantasies with a proper mixture of scientific accuracy and irony.

In January 2001, I was invited to the World Economic Forum in Davos, Switzerland. Brian Greene was also invited, and we were asked to hold a public debate on the question “When will we know it all?” In other words, when will the last big problems of science be solved? The audience consisted mainly of industrial and political
tycoons. Our debate was intended to entertain the tycoons, not to give them a serious scientific education. To make it more amusing, Greene was asked to take an extreme position saying “Soon,” and I was asked to take an extreme position saying “Never.”

Here is my version of Greene’s opening statement, reconstructed from my unreliable memory after we came back from Switzerland. He said that this generation of scientists is amazingly lucky. Within a few years or decades, we will discover the fundamental laws of nature. The fundamental laws will be a finite set of equations, like Maxwell’s equations of electrodynamics or Einstein’s equations of gravitation. Everything else will then follow from these equations. Once we have the fundamental equations, we are done. If we are not smart enough to find the equations, then we will leave it to our grandchildren to finish the job. Either way, the end of fundamental science is near.