by Isaac Asimov
If protons were the only charged particles present, then the electromagnetic interaction should be represented by very strong repulsions between the protons that were pushed tightly together into the tiny nucleus. Any atomic nucleus should explode with shattering force the instant it was formed (if it ever could be formed in the first place).
Clearly, some other type of interaction must be involved, something much stronger than the electromagnetic interaction and capable of overpowering it. In 1930, the only other interaction known was the gravitational interaction, which is so much weaker than the electromagnetic interaction that it can actually be neglected in the consideration of subatomic events, and nobody misses it. No, there must be some nuclear interaction, one hitherto unknown but very strong.
The superior strength of the nuclear interaction can be demonstrated by the following consideration. The two electrons of a helium atom can be removed from the nucleus by the application of 54 electron volts of energy. That quantity of energy suffices to handle a strong manifestation of electromagnetic interaction.
On the other hand, the proton and neutron making up a deuteron, one of the most weakly bound of all nuclei, require Z million electron volts for disruption. Making allowance for the fact that particles within the nucleus are much closer to one another than atoms within a molecule, it is still fair to conclude that the nuclear interaction is about 130 times as strong as the electromagnetic interaction.
But what is the nature of this nuclear interaction? The first fruitful lead came in 1932 when Werner Heisenberg suggested that the protons were held together by exchange forces. He pictured the protons and neutrons in the nucleus as continually interchanging identity, so that any given particle is first a proton, then a neutron, then a proton, and so on. This process might keep the nucleus stable in the same way that one holds a hot potato by tossing it quickly from hand to hand. Before a proton could “realize” (so to speak) that it was a proton and try to flee its neighbor protons, it had become a neutron and could stay where it was. Naturally it could get away with this only if the changes took place exceedingly quickly, say within a trillionth of a trillionth of a second.
Another way of looking at this interaction is to imagine two particles, exchanging a third. Each time particle A emits the exchange particle, it moves backward to conserve momentum. Each time particle B accepts the exchange particle, it is pushed backward for the same reason. As the exchange particle bounces back and forth, particles A and B move farther and farther apart so that they seem to experience a repulsion. If, on the other hand, the exchange particle moves around boomerang-fashion, from the rear of particle A to the rear of particle B, then the two particles would be pushed closer together and seem to experience an attraction.
It would seem by Heisenberg’s theory that all forces of attraction and repulsion would be the result of exchange particles. In the case of electromagnetic attraction and repulsion, the exchange particle is the photon; and in the case of gravitational attraction (there is apparently no repulsion in the gravitational interaction), the exchange particle is the graviton.
Both the photon and the graviton are without mass, a-id it is apparently for this reason that electromagnetism and gravitation are forces that decrease only as the square of the distance and can therefore be felt across enormous gaps.
The gravitational interaction and the electromagnetic interaction are long-distance interactions and, as far as we know to this day, the only ones of this type that exist.
The nuclear interaction-assuming it existed-could not be one of this type. It had to be very strong within the nucleus if the nucleus were to remain in existence; but it was virtually indetectable outside the nucleus, or it would have been discovered long before. Therefore, the strength of the nuclear interaction dropped very rapidly with distance. With each doubling of distance, it might drop to less than 1/100 of what it was—rather than to merely ¼, as was the case with the electromagnetic and gravitational interactions. For that reason, no massless exchange particle would do.
THE MUON
In 1935, the Japanese physicist Hideki Yukawa mathematically analyzed the problem. An exchange particle possessing mass would produce a shortrange force-field. The mass would be in inverse ratio to the range: the greater the mass, the shorter the range. It turned out that the mass of the appropriate particle lay somewhere between that of the proton and the electron; Yukawa estimated it to be between 200 and 300 times the mass of an electron.
Barely a year later, this very kind of particle was discovered. At the California Institute of Technology, Carl Anderson (the discoverer of the positron), investigating the tracks left by secondary cosmic rays, came across a short track that was more curved than a proton’s and less curved than an electron’s. In other words, the particle had an intermediate mass. Soon more such tracks were detected, and the particles were named mesotrons, or mesons for short.
Eventually other particles in this intermediate mass range were discovered, and this first one was distinguished as the mu meson, or the muon. (“Mu” is one of the letters of the Greek alphabet; almost all of which have now been used in naming subatomic particles.) As in the case of the particles mentioned earlier, the muon comes in two varieties, negative and positive.
The negative muon, 206.77 times as massive as the electron (and therefore about one-ninth as massive as a proton) is the particle; the positive muon is the antiparticle. The negative muon and positive muon correspond to the electron and positron, respectively. Indeed, by 1960, it had become evident .that the negative muon was identical with the electron in almost every way except mass. It was a heavy electron. Similarly, the positive muon was a heavy positron.
Positive and negative muons will undergo mutual annihilation and may briefly circle about a mutual center of force before doing so—just as is true of positive and negative electrons. A variation of this situation was discovered in 1960 by the American physicist Vernon Willard Hughes. He detected a system in which the electron circled a positive muon, a system he called muonium. (A positron circling a negative muon would be antimuonium.)
The muonium atom (if it may be called that) is quite analogous to hydrogen 1, in which an electron circles a positive proton, and the two are similar in many of their properties. Although muons and electrons seem to be identical except for mass, that mass difference is enough to keep the electron and the positive muon from being true opposites, so that one will not annihilate the other. Muonium, therefore, does not have the kind of instability that positronium has. Muonium endures longer and would endure forever (if undisturbed from without) were it not for the fact that the muon itself does not endure since, as I shall shortly point out, it is very unstable.
Another similarity between muons and electrons is this: just as heavy particles may produce electrons plus antineutrinos (as when a neutron is converted to a proton) or positrons plus neutrinos (as when a proton is converted to a neutron), so heavy particles can interact to form negative muons plus antineutrinos or positive muons plus neutrinos. For years, physicists took it for granted that the neutrinos that accompany electrons and positrons and those that accompany negative and positive muons were identical. In 1962, however, it was found that the neutrinos do not cross over, so to speak; the electron’s neutrino is not involved in any interaction that would form a muon, and the muon’s neutrino is not involved in any interaction that would form an electron or positron.
In short, physicists found themselves with two pairs of chargeless, massless particles, the electron’s antineutrino and the positron’s neutrino plus the negative muon’s antineutrino and the positive muon’s neutrino. What the difference between the two neutrinos and between the two anti neutrinos might be is more than anyone can tell at the moment, but they are different.
The muons differ from the electron and positron in another respect, that of stability. The electron or positron, left to itself, will remain unchanged indefinitely. The muon is unstable, however, and breaks down after an average lifetime
of a couple of millionths of a second. The negative muon breaks down to an electron (plus an antineutrino of the electron variety and a neutrino of the muon variety); while the positive muon does the same in reverse, producing a positron, an electron-neutrino, and a muon-anti neutrino.
When a muon decays, then, it forms an electron (or positron) with less than 1/200 of its mass, and a couple of neutrinos with no mass at all. What happens to the remaining 99.5 percent of the mass? Clearly, it turns to energy which may be emitted as photons or expended in the formation of other particles.
In reverse, if enough energy is concentrated on a tiny volume of space, then instead of forming an electron-positron pair, a more bloated pair may form; a pair just like the electron-positron pair except for the energy-bloat which makes its appearance as mass. The adherence of the extra mass to the basic electron or positron is not very strong, so the muon is unstable and quickly sheds that mass and becomes an electron or positron.
THE TAUON
Naturally, if still more energy is concentrated on a tiny volume, a still more massive electron will form. In California, Martin L. Perl made use of an accelerator that smashed high-energy electrons into high-energy positrons head on; and, in 1974, evidence was detected of such a superheavy electron. This he called a tau electron (tau being another letter of the Greek alphabet), and it is frequently called a tauon for short.
As might be expected, the tauon is about 17 times as massive as a muon and, therefore, about 3,500 times as massive as an electron. In fact, the tauon is twice as massive as a proton or a neutron. Despite its mass, the tauon is a lepton for, except for its mass and instability, it has all the properties of an electron. With all its mass, it might be expected to be far more unstable than the muon, and it is. The tauon lasts for only about a trillionth of a second before breaking down to a muon (and then to an electron).
There is, of course, a negative tauon and a positive tauon, and physicists take it for granted that associated with these is a third kind of neutrino and anti neutrino, even though these have not yet actually been detected.
THE NEUTRINO’S MASS
There are now twelve leptons known, then: the negative and positive electron (the latter being the positron), the negative and positive muon, the negative and positive tauon, the electron neutrino and antineutrino, the muon neutrino and antineutrino, and the tauon neutrino and antineutrino. Clearly, these are divided into three levels (or, as physicists now say, flavors). There is the electron and associated neutrino and their antiparticles; the muon and associated neutrino and their antiparticles; and the tauon and associated neutrino and their antiparticles.
Having these three flavors, there is no reason why there should not be others. It may be that if the amount of energy that could be used could be increased indefinitely, more and more flavorsof leptons could be formed, each flavor more massive and more unstable than the one before. Although there may be no theoretical limit to the number of flavors, there would, of course, be a practical limit. Eventually, it might simply take all the energy in the universe to form a lepton of a particularly high level, and there would be no going beyond; and eventually, such a particle would be so unstable that its existence would be meaningless in any sense.
If we confine ourselves to the three flavors now known, the mystery of the neutrinos is compounded. How can there be three massless, chargeless fermion pairs, each distinctly different as far as particle interactions go and yet with no distinguishing property as far as we can tell?
Perhaps, there is a distinguishing property, but we have not looked for it properly. For instance, all three flavors of neutrino are supposed to have zero mass and therefore to be moving, always, at the speed of light. Suppose, though, that each flavor of neutrino has a very tiny mass, different from that of the other two. In that case, their properties would naturally be slightly different one from the other. For instance, they would each travel at very slightly less than the speed of light, and the amount by which that speed would fall short would be slightly different for each.
There are theoretical reasons for arguing, in this case, that any neutrino, as it travels, shifts its identity, being an electron-neutrino at one time, a muon-neutrino at another, and a tauon-neutrino at still another. These shifts represent neutrino oscillations—first suggested as a possibility in 1963 by a group of Japanese physicists.
In the late 1970s, Frederick Reines, one of the original detectors of the neutrino, along with Henry W. Sobel and Elaine Pasierb of the University of California, set out to test the matter. They used about 600 pounds of very pure heavy water and bombarded it with neutrinos arising from fissioning uranium. This process should produce only electron-neutrinos.
The neutrinos can bring about either of two events. A neutrino can strike the proton-neutron combination of the heavy hydrogen nucleus in the heavy water, splitting them apart and continuing to move on. This is a neutral-current reaction, and any of the neutrino flavors can do it. Second, the neutrino, on striking the proton-neutron combination, can induce a change of the proton into a neutron, producing an electron; in this case, the neutrino ceases to exist. This is a charged-current reaction, and only electron-neutrinos can bring it about.
One can calculate how many of each type of event should take place if the neutrinos did not oscillate and remained only electron-neutrinos, and how many if the neutrinos did oscillate and some had changed over. In 1980, Reines announced that his experiment seemed to demonstrate the existence of neutrino oscillation. (I say “seemed” because the experiment was very nearly at the limit of the detectable, and because other experimenters checking the matter have reported that they have not detected signs of such oscillation.)
The matter remains in doubt, but experiments by physicists in Moscow, involving a point that has nothing to do with oscillations, seem to show that the electron-neutrino may have a mass of possibly as much as 40 electron volts. This would give it a mass 1/13,000 that of an electron, so it is no wonder the particle has passed for massless.
If Reines is correct, then, and there is neutrino oscillation, it would explain the shortage of neutrinos from the sun, which I mentioned earlier in the chapter and which is so puzzling to scientists. The device used by Davis to detect solar neutrinos would detect electron-neutrinos only. If the neutrinos emitted from the sun oscillate so that they arrive at Earth in a mixture of the three flavors in perhaps equal quantities, it is no wonder we detect only one-third of the neutrinos we expect.
Then, too, if neutrinos have a small amount of mass, even only 1/13,000 of an electron, there are so many neutrinos in space that all together it is possible to calculate that they far outmatch all the protons and neutrons. More than 99 percent of the mass of the universe would be neutrinos, and they could easily represent the “missing mass” I spoke of in chapter 2. In fact, there would be enough neutrino mass in the universe to close it and to ensure that eventually the expansion would stop and the universe would begin to contract again.
That is, if Reines is correct. We do not know yet.
Hadrons and Quarks
Since the muon is a kind of heavy electron, it cannot very well be the nuclear cement Yukawa was looking for. Electrons are not found within the nucleus, and therefore neither should the muon be. This was discovered to be true on a purely experimental basis, long before the near identity of muon and electron was suspected; muons simply showed no tendency to interact with nuclei. For a while, Yukawa’s theory seemed to be tottering.
PIONS AND MESONS
In 1947, however, the British physicist Cecil Frank Powell discovered another type of meson in cosmic-ray photographs. It was a little more massive than the muon and proved to possess about 273 times the mass of an electron. The new meson was named a pi meson or a pion.
The pion was found to react strongly with nuclei and to be just the particle predicted by Yukawa. (Yukawa was awarded the Nobel Prize in physics in 1949, and Powell received it in 1950.) Indeed, there was a positive pion that acted as the exchange
force between protons and neutrons, and there was a corresponding antiparticle, the negative pion, which performed a similar service for antiprotons and antineutrons. Both are even shorter-lived than muons; after an average lifetime of about 1/40 microsecond, they break up into muons plus neutrinos of the muon variety. (And, of course, the muon breaks down further to electrons and additional neutrinos.) There is also a neutral pion, which is its own antiparticle. (There is, in other words, only one variety of that particle.) It is extremely unstable, breaking down in less than a quintillionth of a second to form a pair of gamma rays.
Despite the fact that a pion “belongs” within the nucleus, it will fleetingly circle a nucleus before interacting with it, sometimes, to form a pionic atom as was detected in 1952. Indeed, any pair of negative and positive particles or particle systems can be made to circle each other; and in the 1960s, physicists studied a number of evanescent “exotic atoms” in order to gain some notion about the details of particle structure.
The pions were the first to be discovered of a whole class of particles, which are lumped together as mesons. These do not include the muon, although that was the first known particle to be given the name. Mesons interact strongly with protons and neutrons (figure 7.8), while muons do not and have thus lost the right to be included in the group.
Figure 7.8. Meson collision with a nucleus. A high-energy meson from secondary cosmic radiation struck a nucleus and produced a star made up of mesons and alpha particles (lower left); the energetic meson then traveled along the wavering path to the upper right, where it was finally stopped by collision with another nucleus.