by Isaac Asimov
As an example of particles other than the pion that are members of the group, there are the K-mesons, or kayons. These were first detected in 1952 by two Polish physicists, Marian Danysz and Jerzy Pniewski. These are about 970 times as massive as an electron and, therefore, about half the mass of a proton or neutron. The kayon comes in two varieties, a positive kayon and a neutral kayon, and each has an antiparticle associated with it. They are unstable, of course, breaking down to pions in about a microsecond.
BARYONS
Above the meson are the baryons (a term I mentioned earlier), which include the proton and the neutron. Until the 1950s, the proton and the neutron were the only specimens known. Beginning in 1954, however, a series of still more massive particles (sometimes called hyperons) were discovered. It is the baryon particles that have particularly proliferated in recent years, in fact, and the proton and neutron are but the lightest of a large variety.
There is a law of conservation of baryon number, physicists have discovered, for in all particle breakdowns, the net number of baryons (that is, baryons minus antibaryons) remains the same. The breakdown is always from a more massive to a less massive particle and thus explains why the proton is stable and is the only baryon to be stable. It happens to be the lightest baryon. If it broke down, it would have to cease being a baryon and thus would break the law of conservation of baryon number. For the same reason, an antiproton is stable, because it is the lightest antibaryon. Of course, a proton and an antiproton can engage in mutual annihilation since, taken together, they make up one baryon plus one antibaryon for a net baryon number of zero.
(There is also a law of conservation of lepton number, which explains why the electron and positron are the only leptons to be stable. They are the least massive leptons and cannot break down into anything simpler without violating that conservation law. In fact, electrons and positrons have a second reason for not breaking down. They are the least massive particles that can possess an electric charge. If they break down to something simpler, they lose the electric charge—a loss forbidden by the law of conservation of electric charge. That is, indeed, a stronger conservation law than the conservation of baryon number, as we shall see, so that electrons and positrons are, in a way, more stable than protons and antiprotons—or, at least, they may be more stable.)
The first baryons beyond the proton and neutron to be discovered were given Greek names. There was the lambda particle, the sigma particle, and the xi particle. The first came in one variety, a neutral particle; the second in three varieties, positive, negative, and neutral; the third in two varieties, negative and neutral. Every one of these had an associated antiparticle, making a dozen particles altogether. All were exceedingly unstable; none could live for more than a hundredth of a microsecond or so; and some, such as the neutral sigma particle, broke down after a hundred trillionth of a microsecond.
The lambda particle, which is neutral, can replace a neutron in a nucleus to form a hypernucleus—an entity that endures less than a billionth of a second. The first to be discovered was a hypertritium nucleus made up of a proton, a neutron, and a lambda particle. This was located among the products of cosmic radiation by Danysz and Pniewski in 1952. In 1963, Danysz reported hypernuclei containing two lambda particles. What’s more, negative hyperons can be made to replace electrons in atomic structure, as was first reported in 1968. Such massive electron-replacements circle the nucleus at such close quarters as to spend their time actually within the nuclear outer regions.
But all these are the comparatively stable particles; they live long enough to be directly detected and to be easily awarded a lifetime and personality of their own. In the 1960s, the first of a whole series of particles was detected by Alvarez (who received the Nobel Prize in physics in 1968 as a result). These were so short-lived that their existence could only be deduced from the necessity of accounting for their breakdown products. Their half-lives are something of the order of a few trillionths of a trillionth of a second, and one might wonder whether they are really individual particles or merely a combination of two or more particles, pausing to nod at each other before flashing by.
These ultra-short-lived entities are called resonance particles; and, as physicists came to have at their disposal ever greater energies, they continued to produce ever more particles until 150 and more were known. These were all among the mesons and the baryons, and these two groups were lumped together as hadrons (from a Greek word for bulky). The leptons remained at a modest three flavors, each flavor containing particle, antiparticle, neutrino, and antineutrino.
Physicists became as unhappy with the multiplicity of hadrons as chemists had been with the multiplicity of elements a century earlier. The feeling grew that the hadrons had to be made up of simpler particles. Unlike the leptons, the hadrons were not points but had definite diameters—not very large ones, to be sure, only around a 10-trillionth of an inch, but that is not a point.
In the 1950s, the American physicist Robert Hofstadter investigated nuclei with extremely energetic electrons. The electrons did not interact with the nuclei but bounced off; and from the bouncing, Hofstadter came to conclusions about hadron structure that eventually proved to be inadequate but were a good start. As a result, he shared in the Nobel Prize in physics in 1961.
THE QUARK THEORY
One thing that seemed needed. was a sort of periodic table for subatomic particles—something that would group them into families consisting of a basic member or members with other particles that are excited states of that basic member or members (table 7.1).
Something of the sort was proposed in 1961 by the American physicist Murray Gell-Mann and the Israeli physicist Yuval Ne’ernen, who were working independently. Croups of particles were put together in a beautifully symmetric pattern that depended on their various properties—a pattern that Gell-Mann called the eightfold way but that is formally referred to as SU # 3. In particular, one such grouping needed one more particle for completion. That particle, if it was to fit into the group, had to have a particular mass and a particular set of other properties. The combination was not a likely one for a particle; yet, in 1964, a particle (the omega-minus) was detected with just the predicted set of properties; and in succeeding years, it was detected dozens of times. In 1971 its antiparticle, the antiomega-minus, was detected.
Even if baryons are divided into groups and a subatomic periodic table is set up, there would still be enough different particles to give physicists the urge to find something still simpler and more fundamental. In 1964, Gell-Mann—having endeavored to work out the simplest way of accounting for all the baryons with a minimum number of more fundamental sub-baryonic particles—came up with the notion of quarks. He got this name because he found that only three quarks in combination were necessary to make up a baryon, and that different combinations of the three quarks were needed to make up all the known baryons. This reminded him of a line from Finnegan’s Wake by James Joyce: “Three quarks for Musther Mark.”
In order to account for the known properties of baryons, the three different quarks had to have specific properties of their own. The most astonishing property was a fractional electric charge. All known particles had either no electric charge, an electric charge exactly equal to that of the electron (or positron), or an electric charge equal to some exact multiple of the electron (or positron). The known charges, in other words, were 0, +1, −1, +2, −2, and so on. To suggest fractional charges was so odd, that Gell-Mann’s notion met with strong initial resistance. It was only the fact that he managed to explain so much that got him a respectful hearing, then a strong following, then a Nobel Prize in physics in 1969.
Gell-Mann started with two quarks, for instance, which are now called up-quark and down-quark. Up and down have no real significance but are only a whimsical way of picturing them. (Scientists, particularly young ones, are not to be viewed as soulless and unemotional mental machines. They tend to be as joke-filled, and sometimes as silly, as the average novelis
t and truck driver.) It might be better to call these u-quark and d-quark.
The u-quark has a charge of +⅔ and the d-quark has one of –⅓. There would also be an anti-u-quark with a charge of –⅔ and an anti-d-quark with a charge of +⅓.
Two u-quarks and one d-quark would have a charge of +⅔, +⅔, and –⅓—a total of +1—and, in combination, would form a proton. On the other hand, two d-quarks and one u-quark would have a charge of –⅓, –⅓, and +⅔—a total of 0—and, in combination, would form a neutron.
Three quarks would always come together in such a way that the total charge would be an integer. Thus, two anti-u-quarks and one anti-d-quark would have a total charge of −1 and would form an antiproton, while two anti-d-quarks and one anti-u-quark would have a total charge of 0 and would form an antineutron.
What’s more, the quarks would stick together so firmly, thanks to nuclear interaction, that scientists have been totally unable so far to break protons and neutrons apart into separate quarks. In fact, there are suggestions that the attraction between quarks increases with distance so that there is no conceivable way of breaking up a proton or neutron into its constituent quarks. If so, fractional electric charges may exist, but they can never be detected, which makes Gell-Mann’s iconoclastic notion a little easier to take.
These two quarks are insufficient to account for all the baryons, however, or for all the mesons (which are made up of combinations of two quarks). Gell-Mann, for instance, originally suggested a third quark, which is now called the s-quark. The s can be said to stand for “sideways” (to match up and down) but is more often said to stand for “strangeness” because it had to be used to account for the structure of certain so-called strange particles—strange, because they existed for longer times before breaking down than would be expected.
Eventually, though, physicists investigating the quark hypothesis decided that quarks would have to exist in pairs. If there was an s-quark, there would have to be a companion quark, which they called a c-quark. (The c stands not for “companion” but for “charm.”) In 1974, an American physicist, Burton Richter, and another, Samuel Chao Chung Ting, working independently, with intense energies, isolated particles that had properties requiring the c-quark. (These were particles with “charm.”) The two shared the Nobel Prize for physics in 1976, as a result.
The pairs of quarks are flavors; and, in a way, they match the lepton flavors. Each flavor of quark has four members—for instance, the u-quark, the d-quark, the anti-u-quark, and the anti-d-quark—just as each flavor of leptons has four members—for instance, the electron, the neutrino, the antielectron and the antineutrino. In each case, there are three flavors known: electron, muon, and tauon among the leptons; u- and d-quarks, s- and c-quarks, and, finally, t- and b-quarks. The t-quark and the b-quark stand for “top” and “bottom” in the usual formulation; but among the whimsical, they stand for “truth” and “beauty.” The quarks, like the leptons, seem to be particles of point-size and to be fundamental and structureless (but, we can not be sure, for we have been fooled in this respect already, first by the atom, and then by the proton). And it may be that in both cases, there may be an indefinite number of flavors, if we had more and more energy to expend in order to detect them.
One enormous difference between leptons and quarks is that leptons have integral charges, or none at all, and do not combine; whereas quarks have fractional charges and apparently exist only in combination.
The quarks combine according to certain rules. Each different flavor of quark comes in three varieties of property—a property that leptons do not possess. This property is called (metaphorically only) color, and the three varieties are called red, blue, and green.
When quarks get together three at a time to form a baryon, one quark must be red, one blue, and one green, the combination being without color, or white. (This is the reason for red, blue, and green; for in the world about us, as on the television screen, that combination will give white.) When quarks get together two at a time to form a meson, one will be a particular color, and the other that particular anticolor, so that the combination is again white. (Leptons have no color, being white to begin with.)
The study of quark combinations in such a way that color is never detected in the final product, just as fractional electric charges are not, is referred to as quantum chromodynamics, chromo coming from the Greek word for “color.” (This term harks back to a successful modern theory of electromagnetic interactions which is called quantum electrodynamics.)
When quarks combine, they do so by means of an exchange particle which, in constantly shifting back and forth, serves to hold them together. This exchange particle is called a gluon, for obvious reasons. Gluons have color themselves, which adds complications, and can even stick together to form a product called glueballs.
Even though hadrons cannot be pulled apart to form isolated quarks (two in the case of mesons, three in the case of baryons), there are more indirect ways of demonstrating quark existence. Quarks might be formed from scratch if enough energy can be concentrated in a small volume, as by smashing together very energetic streams of electrons and positrons (as sufficed to form the tauon).
The quarks produced in this way would instantly combine into hadrons and antihadrons which would stream off in opposite directions. If there was enough energy, there would be three streams forming a three-leaf clover—hadrons, antihadrons, and gluons. The two-leaf clover has been formed; and, in 1979, there were announcements of experiments in which a rudimentary third leaf was just beginning to form. This is considered a strong confirmation of the quark theory.
Fields
Every particle possessing mass is the source of a gravitational field that stretches outward in all directions indefinitely, the intensity of the field decreasing in proportion to the square of the distance from the source.
The intensity of the field is incredibly small where individual particles are concerned, so small that to all intents and purposes the field can be ignored where particle interactions are studied. There is, however, only one kind of mass, and the gravitational interaction between two particles seems always to be an attraction.
What is more, where a system consists of many particles, the gravitational field, from a point outside the system, seems to be the sum of all the individual fields of all the particles. An object such as the sun or the earth behaves as though it has a field of the intensity one would expect if it consisted of a particle containing all the mass of the body located at the center of gravity of the body. (This is precisely true only if the body is perfectly spherical and of uniform density, or of varying density where the variations extend outward from the center in exact spherical symmetry; and all this is almost true for objects like the sun or the earth.)
The result is that the sun and, to a lesser extent, the earth have gravitational fields of enormous intensity, and the two can interact, attract each other, and remain firmly together even though separated by a distance of 93 million miles. Systems of galaxies can hold together though spread over distances of millions of light-years; and if the universe ever starts contracting again, it will do so because of the pull of gravity over the distance of billions of light-years.
Every particle possessing electric charge is the source of an electromagnetic field that stretches outward in all directions indefinitely, the intensity of the field decreasing in proportion to the square of the distance from the source. Every particle possessing both mass and electric charge (and there is no electric charge without mass) is the source of both fields.
ELECTROMAGNETIC INTERACTION
The electromagnetic field is many trillions of trillions of trillions of times as intense as the gravitational field in the case of any given single particle. However, there are two kinds of electric charge, positive and negative, and the electromagnetic field exhibits both attraction and repulsion. Where the two kinds of charge are present in equal numbers, the charges tend to neutralize each other and no electromagnetic fiel
d is present outside the system. Thus, normal intact atoms are made up of equal numbers of positive and negative charges and are therefore electrically neutral.
Where one charge or the other is present in excess, an electromagnetic field is present, but the mutual attraction of opposite charges makes it certain that any excess present in either direction is microscopically small so that electromagnetic fields where present cannot compare in intensity with the gravitational fields of bodies of the size of a large asteroid or beyond. Thus, Isaac Newton, who dealt with gravitational interactions alone, was able to build a satisfactory explanation of the motions of the bodies of the solar system, one that could be extended to include the motions of stars and galaxies.
Electromagnetic interactions cannot be ignored altogether and play a role in the formation of the solar system, in the transfer of angular momentum from the sun to the planets, and probably in some of the puzzling manifestations of the rings of small particles that circle Saturn, but these are comparatively small refinements.
Every hadron (mesons and baryons and their constituent quarks) is the source of a field that stretches outward in all directions indefinitely, the intensity of the field decreasing so rapidly with distance that it cannot make itself usefully felt at distances greater than the diameter of an atomic nucleus. Such a field, while overpoweringly important within a nucleus, or whenever two speeding particles skim by each other at nuclear distances, can be ignored at greater distances. Such a field plays no role in the general movements of astronomical bodies but is important in consideration of events in the cores of stars, for instance.
Leptons are also the source of a field that can only be felt at nuclear distances. Indeed the range of this field is even shorter than that of the hadron field. They are both nuclear fields, but they are very different, not only in the type of particle they are associated with, but in their intensities. The hadron field is, particle for particle, 137 times as strong as the electromagnetic field.
