Part of Asimov's essay collection, this essay is the most accessible introduction to the standard model of atom. It was published on November 1980.
All the countless myriads of things, living and non-living, large and small, here and in the farthest galaxies, can't really be countless myriads. That would be too complex, too messy to suit our intuition which is that the Universe is basically simple, and that all we need is to be subtle enough to penetrate that simplicity.
The Greeks suggested the Universe was made up of a few "elements,*' and some supposed that each element was made up of invisibly small "atoms” (from a Greek word meaning "indivisible”) which, as the name implied, could not be divided into anything smaller.
Nineteenth-century chemists agreed in essence. But what nineteenth-century chemists found were elements by the dozens, each with its characteristic atoms. Again, too complex and too messy.
In 1869 Dimitri Mendelfcev arranged the elements into an orderly "periodic table,” and in the early twentieth century the rationale of that periodic table was worked out.
It seemed that the atoms were not indivisible after all. They were made up of still smaller "subatomic particles.” Each atom contained a tiny nucleus at the center, and this was in turn composed of comparatively massive "protons” and "neutrons,” the former with a positive electric charge and the latter uncharged. Outside the nucleus were "electrons” — much less massive than protons or neutrons — which carried a negative electric charge.
By altering the numbers of protons, neutrons, and electrons, the nature and properties of every different type of atom could be explained, and scientists could claim that out of these atoms everything in the Universe was built up. For a while, in the 1930s and 1940s, it seemed that the ultimate constitution of the Universe, the ultimate particles, had been deciphered, and the result was satisfactorily simple. Three different types of particles made up everything.
But there were some puzzles. The electrons were bound to the central nuclei by an "electromagnetic interaction.” The negatively charged electron and the positively charged nucleus attracted each other.
Within the nucleus, however, there was no electromagnetic attraction between positively charged protons and uncharged neutrons, and there was a strong electromagnetic repulsion between protons. So what held the nuclei together?
In 1935 Hideki Yukawa suggested the existence of what has come to be known as “strong interaction,” an attraction among protons and neutrons that is much stronger than any electromagnetic repulsion that existed, and one that decreased in intensity with distance so rapidly that it only made itself felt over subatomic distances.
This explained many subatomic events, but it didn't explain the way in which a free neutron spontaneously changed into a proton, liberating an electron in the process. For this and other such changes, the “weak interaction” force was proposed. This force was also very short-range, but it was considerably weaker than either the strong or the electromagnetic interactions.
A fourth interaction is the “gravitational interaction,” but this is so exceedingly weak that it plays no measurable role in the subatomic world, although it is the dominant force when large masses of matter are considered over astronomical distances.
No fifth interaction has ever been discovered, and at the moment it is not expected that any will be found. In terms of the forces that cause subatomic particles to interact, the Universe seems to be in good simple shape.
The subatomic particles can be divided into two groups. There are the massive “hadrons,” which are affected by the strong interaction, and the less massive “leptons,” which are not. The proton and neutron are each a hadron; the electron is a lepton.
As the twentieth century wore on, it became clear that the neutron, proton, and electron did not answer all questions. There had to be “anti-particles” that resembled the ordinary particles in every respect except that some key characteristic is in an opposite form. There is an anti-electron (or “positron”), just like an electron but positively charged; an anti-proton, just like a proton but negatively charged; an anti-neutron, just like a neutron but with a magnetic field in the opposite direction.
To explain certain subatomic events, a neutrino (and a companion, an anti-neutrino) had to be postulated, and they were indeed eventually detected. They had neither mass nor electric charge.
By 1960 scientists knew of eight leptons: the electron and anti-electron, the neutrino and anti-neutrino, the muon and anti-muon (a muon is just like an electron but is about two hundred times as massive), and a muon-neutrino and anti-muon-neutrino . (The muon-neutrino differs from the ordinary neutrino since both take part in different subatomic changes, but the exact nature of the difference has not yet been worked out.)
In addition there is the photon, the particle-like unit of light that composes radio waves, x-rays, gamma rays, and electromagnetic radiation generally. A photon is exchanged whenever two particles undergo an electromagnetic interaction, so that it is also known as an “exchange particle.” Physicists suppose that each of the four interactions has its own exchange particle.
Eight leptons present a not-so-simple picture, but not impossibly so. Physicists could live with it.
Not so in the case of the hadrons. Beginning in the late 1940s, physicists built particle accelerators that produced subatomic particles with greater and greater energies. The proton and neutron were not alone. These accelerators produced many hadrons which existed only at high energy levels and which quickly decayed. The higher the energies available, the more hadrons were formed, until physicists had found hundreds, with no end in sight.
This was unbearable. If there were that many different hadrons, then they had to be made of something still more fundamental, if our intuitive feeling of the simple Universe were to be correct.
In 1953 Murray Gell-Mann came up with the suggestion that all hadrons were made up of "quarks," whose charges were one-third that of an electron in some cases, and two-thirds in other cases. (The name “quark” is taken from James Joyce's Finnegan's Wake, where Joyce comes up with "three quarks” as a nonsense verison of "three quarts." "Quark" is thus an appropriate name because three at a time are required to make up protons and neutrons.)
Gell-Mann began by suggesting only two types of quarks, which he called “up” and "down” (or "u” and "d") for purposes of distinction, though the description can't be taken literally. Two d-quarks and a u-quark total up to a zero charge and make a neutron. Two u-quarks and a d-quark total up to a unit charge and make a proton. There are also anti-u-quarks and anti-d-quarks which, properly put together, make up the anti-neutron and anti-proton.
Many of the other hadrons could be satisfactorily built up out of quarks or antiquaries (or, in the case of hadrons known as "mesons,” out of one of each). To explain some of the hadrons, however, more massive quarks, "strange quarks” and "charmed quarks,” had to be postulated. (These are whimsical names, without real meaning— just physicists amusing themselves. They might be called s-quarks and c-quarks instead.) Particles containing the c-quark (charmed particles) were first detected only in 1974.
There seem to be analogies between leptons and quarks.
Among the leptons, for instance, there is at the least energetic, bottom level the electron/anti-electron and the neutrino/ anti-neutrino. At a higher energy level is the muon/anti-muon and the muon-neutrino/anti-muon-neutrino. There are indications now that at a still higher energy level there is a tau-electron/anti-tau-electron and a tau-neutrino / anti-tau-neutrino. Perhaps there are endless such levels of leptons if we could imagine ourselves going up the energy scale endlessly.
Similarly, among the quarks at the bottom level there are the u-quark/anti-u-quark and the d-quark/anti-d -quark; at a higher energy level, there are the s-quark/anti-s-quark and the c-quark/ anti-c-quark.
Physicists are searching for a still more energetic pair, the t-quark/anti-t-quark and the b-quark/anti-b-quark, where the "t” and "b” stand for "top” and "bottom”— or for "truth” and "beauty,” depending on how poetic a particular physicist feels.
Again, there may be endless such quark-pairs as we imagine ourselves going up the energy scale endlessly. These different quark-pairs are referred to as different “flavors” of quarks.
As one goes up the energy scale, the leptons increase in mass faster than the quarks do. At some very high energy level, leptons and quarks may become equally massive and may perhaps merge into a single type of particle.
The quark theory, unfortunately, is not firmly established. For one thing, quarks cannot be detected as independent particles. No matter how energetically we smash hadrons, none of the quarks that are supposed to compose them are ever shaken loose.
Does that mean that quarks are just mathematical abstractions that have no concrete reality? (After all, ten dimes make a dollar, but no matter how you tear a dollar bill, no dime will ever fall out of it.)
One theory is that the attractive forces holding individual quarks together within a hadron grow stronger as they are pulled apart. That would mean that any force serving to pull the quarks apart would quickly be overwhelmed as the attraction between quarks is increased in the process.
Quarks differ among themselves as leptons do. Leptons carry an electric charge which may be either positive, negative, or zero. Each different flavor of quark on the other hand has something called “color,” which can be either “red,” “green,” or “blue.” (This is just a metaphorical way of speaking, and is not to be taken literally.)
Apparently, when quarks get together three at a time, there must be one red quark, one green quark, and one blue quark, the combination being “white.” When quarks get together two at a time, then there must be a color and its anti-color, the combination again giving white.
The behavior of quarks with respect to combinations by charge and color is described in a theory called “quantum chromodynamics,” abbreviated “QCD.”
Quarks interact by means of the strong interaction, and an exchange particle should be involved, one that is analogous to the photon in the electromagnetic interaction. The exchange particle for the strong interaction is called the “gluon” (because it “glues” the quarks together).
The gluon is more complicated than the photon. Charged particles interact by way of the photon, but the photon has no charge of its own. Colored particles interact by way of the gluon, but the gluon itself has color. There are, in fact, eight different gluons, each with a different color combination. Nor can gluons be shaken out of hadrons any more than quarks can.
This is unfortunate. As the number of varieties of quarks and gluons, with all their flavors and colors, increases, and as quantum chromodynamics gets more and more complicated, the whole structure begins to seem less likely, and to need more experimental support. If there were some way in which quarks and gluons could be detected, physicists might have more confidence in quantum chromodynamics.
Even if quarks and gluons can't be shaken out of hadrons, might it not be possible for them to be formed out of energy? Physicists form new particles out of energy every day. The more energy they have to play with, the more massive the particles they form. If they can get enough energy, they could form quarks.
With enough energy, they would form groups of quarks of different flavors and colors. Naturally, quarks, so formed, would instantaneously combine in twos and threes to form hadrons. Such hadrons would stream out in two opposite directions (given enough energy), one stream representing the hadrons, the other the corresponding anti-hadrons.
Where would the necessary energy come from? The most energetic particle accelerators that now exist are particle-storage rings that pump enormous energies into electrons in one place and enormous energies into positrons in another place. When the electrons and positrons are both speeding along almost at the speed of light, they can be made to collide head-on and annihilate each other, converting all their mass into energy. The energy of motion plus the energy of annihilation comes up to about fifteen billion electron-volts, and this should be enough to form quarks.
And there have been such experiments, which did indeed produce streams of hadrons and anti-hadrons. The higher the energy, the tighter and narrower the streams are.
But what about gluons? If gluons come off too, we would expect to see three jets of hadrons coming off at angles of 120 degrees, like the three leaves in a three-leaf clover.
To get this, one ought to have higher energies still. A new particle-storage ring was built in Hamburg, Germany, capable of producing energies of up to thirty billion electron-volts.
Even this amount of energy is just barely above the requirement for producing the gluons, so that one would not expect to have a clear three-leaf clover effect. Using the Hamburg machine, one usually got the two jets, but every once in a while there seemed to be the beginning of a third jet, and this was enough to make some physicists feel that the gluon had been detected.
Even if it was, however, it is still disturbing that there are so many flavors and colors of quarks and gluons, and that there is a second group (though a simpler one) of leptons and photons. Can it be that once again the ultimate has receded and that we must ask ourselves if leptons and quarks alike are built up out of still more fundamental particles?
A physicist, Haim Harari, suggests that this more fundamental particle might be called the “rishon,” the Hebrew word meaning “first.” He points out that if one imagines a T-rishon with an electric charge one-third that of an electron and a V-rishon with no charge (together with an anti-T-rishon and an anti-V-rishon), then all the lower-energy leptons and quarks can be built up of rishons taken three at a time.
Still considering how difficult it is to get evidence for the existence of quarks and gluons, the thought of going beyond that to get evidence for still more fundamental particles would be enough to make the most hardened physicist quail.
Even if we succeed, will the “rishon" or something like that be the answer? Or is there no answer to the question of what the Universe is made of? Do the ultimate particles we search for recede endlessly and mockingly as our instruments and theories become more subtle, luring us always on to one more step ... then one more step ... then one more step ...?