Fear of Physics (17 page)

Standing upright on Sunday night, with the sun low in the sky directly behind it, it would cast the image of a rectangle. On Monday, after being tossed on its side by the collectors, it would cast the image of a circle. A three-dimensional object may, when viewed from different angles, cast very different two-dimensional projections. With this leap of inspiration, not only was a puzzle solved but different phenomena could now be understood to represent merely different reflections of the same thing.

Because of such realignments, physics does not progress as wheels within wheels; greater complexity does not always follow from finer detail. More often, new discoveries reflect sudden shifts in perception such as in the cave example. Sometimes the hidden realities that are exposed connect formerly disparate ideas, creating
less
from more. Sometimes, instead, they connect formerly disparate physical quantities and thus open up new areas of inquiry and understanding.

I have already introduced the unification that heralded the era of modern physics: James Clerk Maxwell’s crowning intellectual achievement of the nineteenth century, the theory of electromagnetism and, with it, the “prediction” of the existence of light. It is appropriate that in the story of Genesis, light is created before anything else. It has also been the doorway to modern physics. The odd behavior of light caused Einstein to speculate about a new relationship between space and time. It also caused the founders of quantum mechanics to reinvent the rules of behavior at small scales, to accommodate the possibility that waves and particles might sometimes be the same thing. Finally, the quantum theory of light completed in the middle of this century formed the basis of our current understanding of all the known forces in nature, including the remarkable unification between
electromagnetism and the weak interaction in the last twenty-five years. The understanding of light itself began with the more basic realization that two very different forces, electricity and magnetism, were really one and the same thing.

Earlier I sketched the discoveries of Faraday and Henry that established the connections between electricity and magnetism, but I don’t think these give you as direct an idea of the depth or origin of such a connection as I would like. Instead, a thought experiment demonstrates directly how it is that electricity and magnetism are really different aspects of the same thing. As far as I know, this thought experiment was never actually performed prior to the experimental discoveries that led to the insights, but, with hindsight, it is very simple.

Thought experiments are an essential part of doing physics because they allow you to “witness” events from different perspectives at the same time. You may recall Akira Kurosawa’s classic film
Rashomon,
in which a single event is viewed several different times and interpreted separately by each of the people present. Each different perspective gives us a new clue to intuit a broader, perhaps more objective, relationship among the events. Because it is impossible for one observer to have two vantage points at once, physicists take advantage of thought experiments of the type I will describe, following a tradition established by Galileo and brought to perfection under Einstein.

To perform this thought experiment, there are two facts you need to know. The first is that the only force a charge particle at rest feels, other than gravity, is an electric force. You can put the strongest magnet in the world next to such a particle and it will just sit there, oblivious. On the other hand, if you move a charged particle in the presence of a magnet, the particle will experience a
force that changes its motion. This is called the Lorentz force, after the Dutch physicist Henrik Lorentz, who came close to formulating special relativity before Einstein. It has a most peculiar form. If a charged particle moves
horizontally
between the poles of a magnet, as shown below, the force on the particle will be upward, perpendicular to its original motion:

These two general features are sufficient to allow us to demonstrate that an electric force to one person is a magnetic force to another. Electricity and magnetism are thus as closely related as the circle and rectangle on the cave wall. To see this, consider the particle in the previous diagram. If we are observing it in the laboratory, watching it move and get deflected, we know the force acting on it is the magnetic Lorentz force. But imagine instead that you are in a laboratory traveling at a constant velocity
along with
the particle. In this case, the particle will not be moving relative to you, but the magnet will be. You will instead see:

Because a charged particle at rest can feel only an electric force, the force acting on the particle in this frame must be electric. Also, since Galileo we have known that the laws of physics must appear the same for any two observers moving at a constant relative velocity. Thus, there is no way to prove, absolutely, that it is the particle that is moving and the magnets that are standing still, or vice versa. Rather, we can merely conclude that the particle and the magnets are moving relative to each other. But we have just seen that in the frame in which the magnets are standing still and the particle is moving, the particle will be deflected upward due to the magnetic force. In the other frame, in which the particle is at rest, this upward motion must be attributed to an electric force. As promised, one person’s magnetic force is another’s electric force. Electricity and magnetism are the different “shadows” of a single force, electromagnetism, as viewed from different vantage points, which depend upon your relative state of motion!

Next I want to jump to the present, to view how a much more recent development in physics—one that took place within the
last twenty-five years and to which I alluded at the end of the last chapter—looks in this light. When I discussed the surprising relationship between superconductivity and the Supercollider, I described how one might understand the origin of mass itself as an “accident” of our restricted circumstances. Namely, we associate mass with only some particles because of the fact that we may be living amid a universal background “field” that preferentially “retards” their motion so as to make them appear heavy. I remind you that exactly the same thing happens for light in a superconductor. If we lived inside a superconductor, we would think that the carrier of light, the photon, was massive. Because we don’t, we understand that the only reason light appears massive inside a superconductor is because of its interactions with the particular state of matter therein.

This is the trick. Stuck in a metaphorical cave, like a superconductor, how can we make the leap of inspiration to realize what actually exists outside our limited sphere of experience in a way that might unify otherwise diverse and apparently unrelated phenomena? I don’t think there is any universal rule, but when we do make the leap, everything comes into focus so clearly that we know we have made the right one.

Such a leap began in the late 1950s, and ended in the early 1970s in particle physics. It slowly became clear that the theory that achieved completion after the discussions at Shelter Island, involving the quantum mechanics of electromagnetism, might also form the basis of the quantum theory of the other known forces in nature. As I earlier indicated, the mathematical frameworks behind both electromagnetism and the weak interactions responsible for most nuclear reactions are extremely similar. The only difference is that the particles that transmit the weak force are heavy, and the photon, the carrier of electromagnetism, is
massless. In fact, it was shown in 1961 by Sheldon Glashow that these two different forces could in fact be unified into a single theory in which the electromagnetic force and the weak force were different manifestations of the same thing, but for the problem of the vast mass difference between the particles that transmit these forces, the photon and the W and Z particles.

Once it was recognized, however, that space itself could act like a vast superconductor, in the sense that a background “field” could effectively make particles “act” massive, it was soon proposed, in 1967, by Steven Weinberg and, independently, Abdus Salam, that this is exactly what happens to the W and Z particles, as I described at the end of the last chapter.

What is of interest here is not that a mechanism to give the W and Z particles mass had been discovered, but that in the absence of such a mechanism, it was now understood that the weak and electromagnetic forces are merely different manifestations of the same underyling physical theory. Once again, the observed considerable difference between two forces in nature is an accident of our situation. If we did not live in a space filled by the appropriate coherent state of particles, electromagnetism and the weak interactions would appear the same. Somehow it was managed, from disparate reflections on the wall, to discover the underlying unity present beyond the direct evidence of our senses.

In 1971, the Dutch physicist Gerard ’t Hooft, then a graduate student, demonstrated that the mechanism proposed to give the W’s and Z’s mass was mathematically and physically consistent. In 1979, Glashow, Salam, and Weinberg were awarded the Nobel Prize for their theory, and in 1984, the particles that transmit the weak force, the W and Z particles, were discovered experimentally at the large accelerator at the Centre Européen pour Recherche Nucléaire (CERN) in Geneva, with their predicted
masses. And finally, in 1999 ’t Hooft himself, along with his thesis supervisor, Martinus Veltman, were awarded the Nobel Prize for their original work that showed the Glashow-Weinberg-Salam theory was in fact sensible after all.

This is not the only result of this new perspective. The success of viewing the weak and electromagnetic forces in a single framework that both mimics and extends the “simple” quantum theory of electromagnetism provided motivation to consider whether all the forces in nature could fall within this framework. The theory of the strong interactions, developed and confirmed after the theoretical discovery of asymptotic freedom I described in chapter 1, is of exactly the same general form, known as a “gauge” theory. Even this name has a history steeped in the notion of viewing different forces as different manifestations of the same underlying physics. Back in 1918, the physicist/mathematician Herman Weyl used one of the many similarities between gravity and electromagnetism to propose that they might be unified together into a single theory. He called the feature that related them a gauge symmetry—related to the fact that in general relativity, as we shall soon see, the
gauge,
or length scale, of local rulers used by different observers can be varied arbitrarily without affecting the underlying nature of the gravitational force. A mathematically similar change can be applied to the way different observers measure electric charge in the theory of electromagnetism. Weyl’s proposal, which related classical electromagnetism and gravity, did not succeed in its original form. However, his mathematical rule turned out to play a vital role in the quantum theory of electromagnetism, and it is this property that is shared in common in the theories of the weak and strong interactions. It also turns out to be closely related to much of the current effort to develop a quantum theory of gravity and to unify it with the other known forces.

The “electroweak” theory, as it is now known, along with the theory of the strong interaction based on asymptotic freedom, have together become known as the Standard Model in particle physics. All existing experiments that have been performed in the last twenty years have been in perfect agreement with the predictions of these theories. All that remains to complete the unification of the weak and electromagnetic interactions in particular is to discover the exact nature of the coherent background quantum state that surrounds us and that we believe gives masses to the W’s and Z’s. We also want to know whether this same phenomenon is responsible for giving masses to all other observed particles in nature. This is what we hope the LHC will do for us.

Being a theoretical physicist, I am easily excited by these striking, if esoteric, realities hidden in the world of particle physics. Yet I know from conversations with my wife that these may seem too removed from everyday life for most people to get excited about. Nevertheless, they actually are directly tied to our own existence. If the known particles did not get exactly the masses they do, with the neutron being only one part in a thousand heavier than the proton, life as we know it would not have been possible. The fact that the proton is lighter than the neutron means that the proton is stable, at least on a time scale of the present age of the universe. Thus, hydrogen, made of a single proton and an electron and the most abundant element in the universe as well as being the fuel of stars such as our sun and the basis of organic molecules, is stable. Moreover, if the mass difference between the neutron and proton were different, this would have changed the sensitive equilibrium in the early universe that produced all the light elements we see. This combination of light elements contributed to the evolution of the first stars that formed, which not only contributed to the formation of our own sun some 5 billion
to 10 billion years later but also produced all of the material inside our own bodies. It never ceases to amaze me that every atom in our own bodies originated in the fiery furnace of a distant exploding star! In this direct sense, we are all star children. In a related vein, in the core of our sun, it is the mass difference between elementary particles that determines the rate of the energy-producing reactions there that fuel our own existence. Finally, it is the masses of these elementary particles that combine together to produce the readings on our bathroom scales that many of us dread stepping on.