The Faber Book of Science (52 page)

There are also antineutrinos, and the difference between the two is that they spin in opposite directions about their direction of flight, like bullets fired respectively from barrels with right-handed and
left-handed
rifling. You may object that this is not a valid distinction, that to an observer overtaking a neutrino its direction of flight would appear the opposite, and hence it would look like an antineutrino to him. But a neutrino always travels with the speed of light and thus cannot be overtaken since no object can move faster. It looks like a neutrino to any observer, whichever way and however fast he moves.

You may feel that I have spent too much time on an almost unobservable particle, but in the first place I think the physicists can be proud of having traced and pinned down something so elusive, and secondly, the effects of neutrinos in the large are not negligible. The energy of the sun and stars comes from nuclear transformations in their interiors. Some of the transformations are of the beta kind, and it has been computed that no less than about one-sixth of the energy is taken away by neutrinos and lost in the depths of space.

How did the photon, the quantum of electromagnetic radiation, come to be discovered? Our eyes being sensitive to a particular kind of radiation which we call light, once the laws of the quantum theory were understood, the existence of light quanta or photons followed of necessity. But would a race of blind physicists have discovered the photon? I think they would. They could have discovered electric and magnetic phenomena and found out their laws, and from these laws they would have deduced the existence of electromagnetic waves (just as Maxwell did in 1864, though the fact that he knew light might have helped him!) and hence of photons. Once the electromagnetic forces were known the rest could be done by mathematics alone.

Do other forces also give rise to waves and hence to associated quanta? What about gravity? One can show that gravitational waves (if they exist; the mathematicians are still arguing) would have exceedingly small effects, and so the ‘graviton’ – the quantum of gravitational waves – is hardly even a matter of speculation. It is certainly not identical with the neutrino though some people suspect the neutrino may in some way be connected with gravity.

A force that is much more important on the subatomic scale is the so-called nuclear force which holds the protons and neutrons together
in nuclei. Some twenty-five years ago a young Japanese
mathematician
, H. Yukawa, began to wonder whether the nuclear forces might be capable of wave motion, and what the associated quanta would be like. His task was difficult, for very little was known about the nuclear force, except that it fell off much more rapidly with distance than the electric force,
i.e.,
much more rapidly than the inverse square of the distance. But just how it fell off was not known. Yukawa did what mathematicians do in such cases: he made the simplest assumption that was compatible with the scanty experimental data, and went ahead. His result was startling: the quanta would be heavy, with a mass about 300 times that of the electron, and they might carry a positive or negative electric charge, equal to that of a proton. They would be very different from the photon which has no charge and no intrinsic mass.

Yukawa’s heavy quanta were eventually (1947) discovered in cosmic rays and they are now usually called pions (brief for pi-mesons; the term meson denoting the fact that their mass is intermediate between that of the electron and the proton). They are rare in nature, at least at sea level, but high up in the stratosphere they are quite common because there they are constantly produced by the impact of the fast cosmic ray particles entering the atmosphere from outside. As Yukawa had guessed, they are unstable and live on an average for only one forty-millionth of a second. Yukawa expected that they would break up into an electron and a neutrino, and indeed occasionally they do so, but mostly they break up into a neutrino and a particle that was quite unexpected: the muon (brief for mu-meson). Muons are much tougher than the original pions. They live about 80 times longer and can go clean through atomic nuclei: they can travel right down through the atmosphere, and every square inch of ground is struck by several muons a minute. Muons are lighter than the pions – only 207 times as heavy as electrons, while pions are 273 times as heavy. Being much more common near sea level, they were the first to be discovered (1937) and at first were thought to be Yukawa’s ‘heavy quanta.’ The war interfered with their study, and it was not until 1947 – when pions were discovered – that the confusion was straightened out.

In some ways the muon is the most mysterious of the new particles – precisely because it is so commonplace. Apart from its instability (it breaks up into an electron, a neutrino, and an antineutrino) it is just an overweight electron. It resembles the electron completely in every
respect, that is, spin, magnetic properties, and indifference to nuclear forces. Why the electron should exist in two sizes we do not know. None of the other particles do.

A few years ago there was a brief flutter of excitement when, for a short time, it looked as if the muon might be tremendously important as the key that would unlock the energy of fusion reactions. By a fusion reaction is meant a collision between two light nuclei that results in the formation of a heavier nucleus from them. This process, on a grand scale, keeps the sun hot. Explosively it is the energy source in the hydrogen bomb. In each case, a temperature of millions of degrees is needed to make the process start, but the muon, it was found, could cause ‘cold fusion.’ In an ordinary hydrogen molecule, two hydrogen nuclei are held together by the electrons that circle around them; the distance – about one 400-millionth of an inch – is too large for fusion to happen. But when a muon is present it will tend to take the place of one of the electrons and will describe an orbit 200 times smaller; the two nuclei will be pulled 200 times closer together, and fusion will quickly occur. (One nucleus must be ‘heavy hydrogen,’ that is, the hydrogen isotope of mass number 2; two protons will not fuse.) The muon merely acts as a catalyst; when fusion occurs it finds itself loose again and immediately starts to round up another pair of nuclei. Unfortunately each fusion requires about a millionth of a second, and so the muon in its short life cannot do the trick more than once or twice, and the energy it liberates is much less than that needed to produce the muon in the first place. The excitement passed, but the episode showed again how the most academic type of research can perhaps lead to important industrial applications. If the muon had happened to have a lifetime a few thousand times longer, the process would have worked.

Both the pion and the muon exist with either a positive or a negative charge, one the antiparticle of the other. In addition, there is a neutral pion; this has an extremely short life, millions of times shorter even than that of the charged pion, and breaks up into two photons. A photon must be considered to be its own antiparticle, for if all the electromagnetic fields that make up a photon are reversed, the photon is still the same as it was before. The neutral pion must also be considered its own antiparticle since it breaks up into two photons almost at once.

We have now dealt with 14 particles: the electron, proton, neutron,
muon, and neutrino, each with its antiparticle; and the photon and the three pions – positive, neutral, and negative.

There are 16 others which are called the ‘strange particles.’ Some are heavier than protons and are called hyperons; there are six of these – each with its antiparticle, not all of which have as yet been observed. The others are about half the weight of a proton and are called kaons (brief for
K
-mesons). They were called strange particles because when they were first discovered (from 1948 onward) their behavior was very puzzling. Even now, though we have a scheme that accounts for most of their properties, they are still pretty mysterious.

Consider, for example, the particle that comes next in mass after the proton. It is called the Lambda particle (written with a capital L because its symbol is A, Greek capital lambda). It is uncharged and after a short life of about one 10,000-millionth of a second it breaks up into a proton and a negative pion. It can also be made by what looks like the inverse process, namely by hitting a proton with a negative pion; but the pion needs much more energy than one would compute – by Einstein’s formula – from the masses of the particles concerned. It turns out that together with a Lambda, a kaon is always produced; neither a kaon nor a Lambda can be produced alone. The present explanation is that the kaon and the Lambda are saddled with opposite amounts of something which has been given the slightly facetious name ‘strangeness’ and which cannot be created in the brief space of a collision. In this it resembles an electric charge, but it has no other physical effect, and it is not permanent. It takes some time to disappear and therefore delays the break-up of the Lambda which otherwise ought to occur much more rapidly. The same scheme applies also to the three Sigma particles (positive, neutral, and negative), which come next in mass. But beyond these are two particles – the Xi particles (negative and neutral) which have twice as much ‘strangeness.’ They are therefore produced with two kaons (not one) and they break up in two steps with the Lambda as an intermediate stage. The theory accounts for a great many complexities of behavior, but what ‘strangeness’ is we do not know.

There are two kaons (positive and neutral) and six hyperons. If we add the antiparticles, we have 16 strange particles. Together with the 14 mentioned earlier that makes 30. Are they all fundamental? We are fairly certain that none of them is ‘simply’ a compound of two or more. But perhaps some are more fundamental than others. There is still a great deal we do not know.

For a long time it was thought that among the particles accompanying the electron on the list of fundamental fermions [a class of elementary particles] would be the neutron and proton, the constituents of atomic nuclei. However, that turned out to be false; the neutron and proton are not elementary. Physicists have learned on other occasions as well that objects originally thought to be fundamental turn out to be made of smaller things. Molecules are composed of atoms. Atoms, although named from the Greek for uncuttable, are made of nuclei with electrons around them. Nuclei in turn are composed of neutrons and protons, as physicists began to understand around 1932, when the neutron was discovered. Now we know that the neutron and proton are themselves composite: they are made of quarks. Theorists are now quite sure that it is the quarks that are analogues of the electron. (If, as seems unlikely today, the quarks should turn out to be composite, then the electron would have to be composite as well.)

In 1963, when I assigned the name ‘quark’ to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been ‘kwork’. Then, in one of my occasional perusals of
Finnegans
Wake,
by James Joyce, I came across the word ‘quark’ in the phrase ‘Three quarks for Muster Mark.’ Since ‘quark’ (meaning, for one thing, the cry of a gull) was clearly intended to rhyme with ‘Mark’, as well as ‘bark’ and other such words, I had to find an excuse to pronounce it as ‘kwork’. But the book represents the
dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the ‘portmanteau words’ in
Through
the
Looking
Glass.
From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry ‘Three quarks for Muster Mark’ might be ‘Three quarts for Mister Mark,’ in which case the pronunciation ‘kwork’ would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.

The recipe for making a neutron or proton out of quarks is, roughly speaking, ‘Take three quarks.’ The proton is composed of two ‘
u
quarks’ and one
‘d
quark’, while the neutron contains two
‘d
quarks’ and one ‘
u
quark’. The
u
and
d
quarks have different values of the electric charge. In the same units in which the electron has an electric charge of – 1, the proton has a charge of +1, while the neutron has charge 0. The charge of the
u
quark in those same units is ⅔ and that of the
d
quark –⅓. Sure enough, if we add ⅔, ⅔, and –⅓, we get 1 for the charge of the proton; and if we add –⅓, –⅓, and ⅔, we get o for the charge of the neutron.

The
u
and
d
are said to be different ‘flavors’ of quark. Besides flavor, the quarks have another, even more important property that is called ‘color’, although it has no more to do with real color than flavor in this context has to do with the flavors of frozen yoghurt. While the name color is mostly a joke, it also serves as a kind of metaphor. There are three colors, labeled red, green, and blue after the three basic colors of light in a simple theory of human color vision. (In the case of paints, the three primary colors are often taken to be red, yellow, and blue, but for mixing lights instead of paints for their effect on human observers, yellow is replaced by green.) The recipe for a neutron or proton is to take one quark of each color, that is, a red quark, a green quark, and a blue quark, in such a way that color averages out. Since, in vision, white can be regarded as a mixture of red, green, and blue, we can use the metaphor to say that the neutron and proton are white.