Untitled 1/21/18, 11:31 AM Physics: What We Do and Don t Know Steven Weinberg NOVEMBER 7, 2013 ISSUE Speculations of this sort ran into an obvious dif

Transcription

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19 Untitled 1/21/18, 11:31 AM Physics: What We Do and Don t Know Steven Weinberg NOVEMBER 7, 2013 ISSUE Speculations of this sort ran into an obvious difficulty: photons have no mass, while any new particles such as W+, W-, and Z0 would have to be very heavy, or they would have been discovered decades earlier the heavier the particle, the higher the energy needed to create it in a particle accelerator, and the more expensive the accelerator. There was also the stubborn problem of infinities. The solution lay in an idea known as broken symmetry, which had been developed and successfully applied in other areas of particle physics since The equations of a theory may possess certain simplicities, such as relations among the photon, W+, W-, and Z0, which are not present in the solutions of the equations that describe what we actually observe.3 In the electroweak theory there is an exact symmetry between weak and electromagnetic forces, which would make the W+, W-, and Z0 massless, if it were not that the symmetry is broken by four proposed scalar fields4 that permeate the universe, from which the W+, W-, and Z0as well as the electron get masses. A new particle discovered last year appears to be the predicted quantum of one of these scalar fields. Because the equations of the electroweak theory are similar to those of quantum electrodynamics, it seemed likely that all infinities in the theory would cancel. This was proved in Effects of the exchange of Z0 particles were detected in 1973, and turned out to agree with the predictions of the electroweak theory. The W+, W-, and Z0particles themselves were discovered a decade later, all with Page 1 of 6

20 Untitled 1/21/18, 11:31 AM the expected properties. It took a little longer to understand another force, the strong nuclear force that holds protons and neutrons together inside atomic nuclei. Fifty years ago we had mountains of data about this force, and we could imagine any number of quantum field theories that could potentially describe it, but we had no way to use the data to pick out the right theory. Because this force is strong, every possible sequence of intermediate steps makes a significant contribution to whatever we are calculating. It was hopeless to add up these contributions even approximately, the way we could do in the electroweak theory. Worse yet, as time passed more and more types of particles were discovered that are affected by the strong nuclear force. It seemed unlikely that all these hundreds of particle types could be the quanta of different fields, i.e., bundles of the fields energy, one for each particle type. Some sense could be made of all these particles by supposing that they were composites of a few kinds of truly elementary particles, called quarks. Three quarks were supposed to combine to make up each proton and neutron in an atomic nucleus. But if so, why had experimenters been unable to find these quarks? I remember a widespread despairing doubt about whether the strong forces could be described by any quantum field theory. Then in the early 1970s the right theory was discovered. Like the successful electroweak theory, it turned out to resemble quantum electrodynamics, only now with a quantity called color taking the place of electric charge. In this theory, known as quantum chromodynamics, the strong forces between quarks are produced by the exchange of eight kinds of photon-like particles known as gluons. Quantum chromodynamics explained an experimental Page 2 of 6

21 Untitled 1/21/18, 11:31 AM result: the strong interactions among the quarks seem to become weaker when the quarks are studied at fine scales of distance, as when they are hit with high-energy electrons. This weakening of the force made it possible to do various approximate calculations like those done in the electroweak theory, and the results agreed with experiment, validating the theory. Gluons have never been found in any experiment. At first it was supposed that this was because these particles masses are too large for them to have been produced in existing accelerators. Gluons could get large masses through a symmetry breakdown in the same way that the W+, W-, and Z0 get large masses in the electroweak theory. Even if so, there would still be a mystery about why quarks had never been found. It was hard to believe that quarks were very heavy; they could hardly be much heavier than the particles like protons and neutrons that contain them. Then a few theorists suggested that since the strong force in quantum chromodynamics becomes weak when studied at small distance scales, perhaps it becomes very strong at large distances, so strong that it is never possible to pull colored particles like quarks and gluons apart. No one has proved mathematically that this is true, but most physicists believe it is, and there seems no prospect that isolated quarks or gluons will ever be found. So now we have a standard model of elementary particles. Its ingredients are quantum fields, and the various elementary particles that are the quanta of those fields: the photon, W+, W-, and Z0 particles, eight gluons, six types of quarks, the electron and two types of similar particles, and three kinds of nearly massless particles called neutrinos. The equations of this theory are not arbitrary; they are tightly constrained by various symmetry Page 3 of 6

22 Untitled 1/21/18, 11:31 AM principles and by the condition of cancellation of infinities. Even so, the standard model is clearly not the final theory. Its equations involve a score of numbers, like the masses of quarks, that have to be taken from experiment without our understanding why they are what they are. If the standard model were the whole story, it would require neutrinos to have zero mass, while in fact their masses are merely very small, less than a millionth the mass of an electron. Further, the standard model does not include the longestknown and most familiar force, the force of gravitation. We commonly describe gravitation using a field theory, the general theory of relativity, but this is not a quantum field theory in which infinities cancel as they do in the standard model. Since the 1980s a tremendous amount of mathematically sophisticated work has been devoted to the development of a quantum theory whose fundamental ingredients are not particles or fields but tiny strings, whose various modes of vibration we observe as the various kinds of elementary particle. One of these modes corresponds to the graviton, the quantum of the gravitational field. String theory if true would not invalidate field theories like the standard model or general relativity; they would just be demoted to effective field theories, approximations valid at the scales of distance and energy that we have been able to explore. String theory is attractive because it incorporates gravitation, it contains no infinities, and its structure is tightly constrained by conditions of mathematical consistency, so apparently there is just one string theory. Unfortunately, although we do not yet know the exact underlying equations of string theory, there are reasons to believe that whatever these equations are, they have a vast number of solutions. I have been a fan of string theory, but it is Page 4 of 6

23 Untitled 1/21/18, 11:31 AM disappointing that no one so far has succeeded in finding a solution that corresponds to the world we observe. The problems of elementary particle physics and cosmology have increasingly merged. There is a classic problem of cosmology: Why is the universe so nearly uniform? In the 13.8 billion years since the universe became transparent there has not been time for any physical influence to have connected parts of the universe that we see in opposite directions, and to have brought them to the homogeneity of density and temperature that we observe them to have. In the early 1980s it was found that in various quantum field theories, before atomic nuclei formed, there would have been an earlier period of inflation, during which the universe expanded exponentially. Highly uniform regions that were tiny during inflation would expand to become larger than the size of the present observed universe, remaining approximately uniform. This is highly speculative, but it has had an impressive success: calculations show that quantum fluctuations during inflation would trigger just the sort of chaotic sound waves, a few hundred thousand years later, whose imprint we now see in the cosmic radiation background. Inflation is naturally chaotic. Bubbles form in the expanding universe, each developing into a big or small bang, perhaps each with different values for what we usually call the constants of nature. The inhabitants (if any) of one bubble cannot observe other bubbles, so to them their bubble appears as the whole universe. The whole assembly of all these universes has come to be called the multiverse. These bubbles may realize all the different solutions of the equations of string theory. If this is true, then the hope of finding a rational explanation for the precise values of quark masses and other Page 5 of 6

24 Untitled 1/21/18, 11:31 AM constants of the standard model that we observe in our big bang is doomed, for their values would be an accident of the particular part of the multiverse in which we live. We would have to content ourselves with a crude anthropic explanation for some aspects of the universe we see: any beings like ourselves that are capable of studying the universe must be in a part of the universe in which the constants of nature allow the evolution of life and intelligence. Man may indeed be the measure of all things, though not quite in the sense intended by Protagoras. So far, this anthropic speculation seems to provide the only explanation of the observed value of the dark energy. In the standard model and all other known quantum field theories, the dark energy is just a constant of nature. It could have any value. If we didn t know any better we might expect the density of dark energy to be similar to the energy densities typical of elementary particle physics, such as the energy density in an atomic nucleus. But then the universe would have expanded so rapidly that no galaxies or stars or planets could have formed. For life to evolve, the dark energy could not be much larger than the value we observe, and there is no reason for it to be any smaller. Such crude anthropic explanations are not what we have hoped for in physics, but they may have to content us. Physical science has historically progressed not only by finding precise explanations of natural phenomena, but also by discovering what sorts of things can be precisely explained. These may be fewer than we had thought. Page 6 of 6