by Isaac Asimov
The lepton field is only about a hundred-billionth as strong as the electromagnetic field. The hadron field is therefore usually spoken of as the strong interaction, and the lepton field as the weak interaction. (Remember that the weak interaction, although weak in comparison with the strong and the electromagnetic interaction, is still about 10,000 trillion trillion times as strong as the gravitational interaction.)
These four interactions, as far as we now know, account for all particle behavior and, by way of it, for all measurable behavior of any kind. There is no indication as yet that any fifth interaction exists or can exist. (Of course, to say that these interactions account for all measurable behavior does not mean, by a long, long shot, that we can as yet understand all measurable behavior. The fact that you may know that a complex mathematical equation has a solution does not mean that you yourself can necessarily find the solution.)
The weak interaction was first dealt with mathematically in 1934 by Fermi; but for decades afterward, it remained the least known of the four interactions. For instance, all four interactions ought to have exchange particles through which the interactions are mediated. There is the photon for the electromagnetic interaction, the graviton for the gravitational interaction, the pion for the strong interaction at the proton-neutron level, and the gluon for the strong interaction at the quark level. Some such particle, called the W-particle (W for “weak,” of course), ought to exist for the weak interaction; but, for over half a century, that W-particle remained elusive.
THE CONSERVATION LAWS
Then, too, there is the question of the conservation laws that set up the rules by which one can judge which particle interactions are possible and which are not; and, therefore, more generally, what can happen in the universe and what cannot. Without the conservation laws, events in the universe would be anarchic and totally incomprehensible.
Nuclear physicists deal with about a dozen conservation laws. Some are the familiar conservation laws of nineteenth-century physics: the conservation of energy, the conservation of momentum, the conservation of angular momentum, and the conservation of electric charge. Then there are conservation laws that are less familiar: the conservation of strangeness, the conservation of baryon number, the conservation of isotopic spin, and so on.
The strong interactions seem to obey all these conservation laws; and in the early 1950s, physicists took it for granted that the laws were universal and irrevocable. But they were not. In the case of weak interactions, some of the conservation laws are not obeyed.
The particular conservation law that was first shattered was the conservation of parity. Parity is a strictly mathematical property that cannot be described in concrete terms; suffice it to say that the property refers to a mathematical function that has to do with the wave characteristics of a particle and its position in space. Parity has two possible values—odd and even. The key point is that parity has been considered a basic property that, like energy or momentum, is subject to the law of conservation: in any reaction or change, parity must be conserved. That is to say, when particles interact to form new particles, the parity on both sides of the equation (so it was thought) must balance, just as mass numbers must, or atomic numbers, or angular momentum.
Let me illustrate. If an odd-parity particle and an even-parity particle interact to form two other articles, one of the new particles must be odd parity and the other even parity. If two odd-parity particles form two new particles, both of the new ones must be odd or both even. Conversely, if an even-parity particle breaks down to form two particles, both must be even parity or both must be odd parity. If it forms three particles, either all three have even parity or one has even parity and the other two have odd parity. (You may be able to see this more clearly if you consider the odd and even numbers, which follow similar rules. For instance, an even number can only be the sum of two even numbers or of two odd numbers, but never the sum of an even number and an odd one.)
The beginning of the trouble came when it was found that K-mesons sometimes broke down to two pi mesons (which, since the pi meson has odd parity, added up to even parity) and sometimes gave rise to three pi mesons (adding up to odd parity). Physicists concluded that there were two types of K-meson, one of even parity and one of odd parity; they named the two theta meson and tau meson, respectively.
Now in every respect except the parity result, the two mesons were identical: the same mass, the same charge, the same stability, the same everything. It was hard to believe that there could be two particles with exactly the same properties. Was it possible that the two were actually the same and that there was something wrong with the idea of the conservation of parity? In 1956, two young Chinese physicists working in the United States, Tsung Dao Lee and Chen Ning Yang, made precisely that suggestion. They proposed that, although the conservation of parity held in strong interactions, it might break down in weak interactions, such as are involved in the decay of K-mesons.
As they worked out this possibility mathematically, it seemed to them that if the conservation of parity broke down, the particles involved in weak interractions should show handedness, something first pointed out in 1927 by the Hungarian physicist Eugene Wigner. Let me explain.
Your right hand and left hand are opposites. One can be considered the mirror image of the other: in a mirror the right hand looks like a left hand. If all hands were symmetrical in every respect, the mirror image would be no different from the direct image, and there would be no such distinction as “right” and “left” hand in principle (figure 7.9). Very well, then, let us apply this principle to a group of particles emitting electrons. If electrons come out in equal numbers in all directions, the particle in question has no handedness. But if most of them tend to go in a preferred direction—say up rather than down—then the particle is not symmetrical. It shows a handedness: if we look at the emissions in a mirror, the preferred direction will be reversed.
Figure 7.9. Mirror-image asymmetry and symmetry illustrated by hands.
The thing to do, therefore, was to observe a collection of particles that emit electrons in a weak interaction (say, some particle that decays by beta emission) and see if the electrons came out in a preferred direction. Lee and Yang asked an experimental physicist at Columbia University, Chien-Shiung Wu, to perform the experiment.
She set up the necessary conditions. All the electron-emitting atoms had to be lined up in the same direction .if a uniform direction of emission was to be detected; this was done by means of a magnetic field, and the material was kept at a temperature near absolute zero.
Within forty-eight hours, the experiment yielded the answer. The electrons were indeed emitted asymmetrically. The conservation of parity did break down in weak interactions. The theta meson and the tau meson were one and the same particle, breaking down with odd parity in some cases, with even parity in others. Other experimenters soon confirmed the overthrow of parity; and for their bold conjecture, the theoretical physicists Lee and Yang received the Nobel Prize in physics in 1957.
If symmetry breaks down with respect to weak interactions, perhaps it will break down elsewhere. The universe as a whole may be left-handed (or right-handed) after all. Alternatively, there may be two universes, one left-handed, the other right-handed: one composed of matter; the other, of antimatter.
Physicists are now viewing the conservation laws in general with a new cynicism. Anyone of them might, like conservation of parity, apply under some conditions and not under others.
Parity, after its fall, was combined with charge conjugation; another mathematical property assigned to subatomic particles, which governed its status as a particle or antiparticle; and the two together were spoken of as CP conservation, a deeper and more general conservation law than either the conservation of parity (P) or the conservation of charge conjugation (C) alone. (This sort of thing is not unprecedented. As we shall see in the next chapter, the law of conservation of mass gave way to the deeper and more general conservation of mass-ene
rgy.)
However, CP conservation proved inadequate, too. In 1964, two American physicists, Val Logsden Fitch and James Watson Cronin, showed that CP conservation was, on rare occasions, also violated in weak interactions. The question of the direction of time (T) was therefore added, and people now speak of CPT symmetry. For their work, Fitch and Cronin shared the 1980 Nobel Prize in physics.
A UNIFIED FIELD THEORY
Why should there be four different fields, four different ways in which particles might interact? There might be any number, of course, but the urge for simplicity is deeply ingrained in the scientific view. If there must be four (or any number), ought it not to be that all should be different aspects of a single field, a single interaction? If so, the best way of demonstrating this would be to find some mathematical relationship that would express them all, and that would then illuminate some aspects of their properties that would otherwise remain dark. For instance, over a hundred years ago, Maxwell worked out a set of mathematical equations that fit the workings of both electricity and magnetism and showed they were both aspects of a single phenomenon, which we now called the electromagnetic field. Might we now not go further?
Einstein began working on a unified field theory at a time when only the electromagnetic and gravitational fields were known. He spent decades on the task and failed; and while he was working, the two short-range fields were discovered, and the task was made all the harder.
In the late 1960s, however, the American physicist Steven Weinberg and the Pakistani-British physicist Abdus Salam, working independently, devised a mathematical treatment that covered both the electromagnetic field and the weak field, the two together being called the electroweak field. This treatment was then elaborated by the American physicist Sheldon Lee Glashow, who had been a high-school classmate of Weinberg. The theory made it necessary that both electromagnetic interactions and weak interactions should display neutral currents, certain particle interactions in which electric charge is not exchanged. Certain of these, not known previously, were found to exist exactly as predicted when searched for—a powerful piece of evidence in favor of the new theory. Weinberg, Salam, and Glashow all shared the 1979 Nobel Prize in physics.
The electroweak theory gave details as to what the missing exchange particles of the weak interaction (particles that had been sought in vain for half a century) ought to be. There ought to be not just a W-particle but three particles—a W+, a W–, and something labeled a Z0, or in other words, a positive, a negative, and a neutral particle. What’s more, some of the properties could be specified, if the electroweak theory was correct. They should be about 80 times as massive as the proton, for instance—a property that accounted for their being so elusive. It took enormous energies to bring them into existence and make them detectable. These huge masses, moreover, made the weak interaction very short-range, which made it unlikely that two particles should approach each other closely enough for the interaction to take place, which accounted for the weak interaction being so much weaker than the strong one.
By 1983, however, physicists had, at their disposal, energies sufficiently high for the task, and all three particles were finally detected—and with the predicted mass, too. That nailed the electroweak theory into place.
Meanwhile, the same mathematical scheme that seemed to cover both the electromagnetic field and the weak field seemed, to many physicists, to suffice (with some added complications) for the strong field as well. Several ways of doing so have been advanced. If the electroweak theory is a unified theory, one that would include the strong field as well would be a grand unified theory, usually abbreviated GUTs (because there is more than one).
If the strong field is to be brought under the GUTs umbrella, it would seem that there must be ultra massive exchange particles required beyond the gluons, no less than twelve of them. Because they are more massive than the W’s and Z’s, they will be harder to detect, and there is no hope for them right now. They will also be far shorter in range than anything that has yet been considered. The range of action of these ultramassive exchange particles of the strong field is less than 1 quadrillionth the diameter of the atomic nucleus.
Now if these ultra massive exchange particles exist, it is possible that one might pass from one quark to another within a proton. Such a passage might destroy one of the quarks, converting it to a lepton. With one of the quarks gone, the proton would become a meson, which would eventually decay to a positron.
However, in order for the exchange to take place, the quarks (which are point particles) must pass close enough to each other to be within the range of action of these ultramassive exchange particles. So incredibly tiny is the range that, even within the close confines of the proton, so close an approach is not likely.
In fact, it has been calculated that the necessary approach would happen so rarely that a proton would be destroyed only after 1031 years of existence, on the average. That many years is 600 million trillion times the total existence of the universe up to this point.
Of course, this is an average life span. Some protons would live much longer than that; and some much shorter. Indeed, if enough protons could be placed under study, a number of such proton-decays would take place every second. For instance, there might be about 3 billion proton-decays in Earth’s oceans every second. (That sounds like a lot but it is a totally insignificant quantity, of course, compared with the total number of protons in the ocean.)
Physicists are anxious to detect such decays and differentiate them clearly from other similar events that might be taking place in far greater numbers. If the decay could be detected, it would be a powerful piece of evidence in favor of the GUTs; but, as in the case of gravitational waves, the detection required is at the very limit of the possible, and it may take considerable time to settle the matter either way.
The theories involved in these new unifications can be used to work out the details of the big bang with which the universe started. It would seem that at the very start, when the universe had existed for less than a millionth of a trillionth of a trillionth of a trillionth of a second and was far tinier than a proton and had a temperature in the trillions of trillions of trillions of degrees, there was only one field and only one kind of particle interaction. As the universe expanded, and the temperature dropped, the different fields “froze out.”
Thus we could imagine the earth, if extremely hot, to be nothing but a gaseous sphere in which all the different kinds of atoms would be evenly mixed so that every portion of the gas would have the same properties as every other. As the gas cooled, however, different substances would separate out first as liquids, then as solids; and eventually there would be a sphere of many different substances existing separately.
So far, though, the gravitational interaction proves intransigent. There seems no way of including it under the umbrella of the kind of mathematics worked out by Weinberg and the rest. The unification that defeated Einstein has so far defeated all his successors as well.
Even so, the GUTs has produced something extremely interesting, indeed. Physicists have wondered how the big bang could produce a universe so lumpy as to have galaxies and stars. Why did not everything simply spread out into a vast haze of gas and dust in all directions? Then, too, why is the universe of such a density that we cannot be quite certain whether it is open or closed? It might have been distinctly open (negatively curved) or closed (positively curved). Instead, it is nearly flat.
An American physicist, Alan Guth, in the 1970s, used GUTs to argue that, when the big bang took place, there was an initial period of exceedingly rapid expansion or inflation. In such an inflationary universe, the temperature dropped so rapidly that there was no time for the different fields to separate out or for different particles to form. It is only later in the game, when the universe had become quite large, that the differentiation took place. Hence the flatness of the universe, and so, too, its lumpiness. The fact that GUTs, a theory developed from particles alone, should happen to ex
plain two puzzles that involve the birth of the universe is strong evidence in favor of GUTs being correct.
To be sure, the inflationary universe does not remove all problems, and different physicists have attempted to patch it in different ways to make a better match between predictions and reality—but it is early days yet, and there is considerable hope that some version of GUTs and inflation will work. Perhaps it will, when someone finally works out a way of including the gravitational interaction into the theory, and unification is at last complete.
Chapter 8
* * *
The Waves
Light
Until now, I have been dealing with material objects almost entirely—from the very large, such as galaxies, to the very small, such as electrons. Yet there are important immaterial objects, and of these the longest known and the most richly appreciated is light. According to the Bible, the first words of God were, “Let there be light,” and the sun and the moon were created primarily to serve as sources of light: “And let them be for lights in the firmament of the heaven to give light upon the earth.”
The scholars of ancient and medieval times were completely in the dark about the nature of light. They speculated that it consisted of particles emitted by the glowing object or perhaps by the eye itself. The only facts about it that they were able to establish were that light travels in a straight path, that it is reflected from a mirror at an angle equal to that at which the beam strikes the mirror, and that a light beam is bent (refracted) when it passes from air into glass, water, or some other transparent substance.
