The Neutron: The Nuclear Interaction

In one place, and only one, did the proton-neutron theory seem a little weaker than the proton-electron theory. The electrons in the nucleus were thought to act as a kind of glue holding together the protons.

But the electrons were gone. There were no negative charges at all inside the nucleus, only the positive charges of the proton, plus the uncharged neutron. As many as 83 positive charges were to be found (in the bismuth-209 nucleus) squeezed together and yet not breaking apart.

In the absence of electrons, what kept the protons clinging together?

Was it possible that the electrical repulsion between 2 protons is replaced by an attraction if those protons were pushed together closely enough? Can there be both an attraction and a repulsion, with the former the more 102important at very short range? If this were so, that hypothetical attraction would have to have two properties. First, it would have to be extremely strong—strong enough to overcome the repulsion of two positive charges at very close quarters. Secondly, it would have to be short-range, for no attractive force between protons of any kind was ever detected outside the nucleus.

In addition, this short-range attraction would have to involve the neutron. The hydrogen-1 nucleus was made up of a single proton, but all nuclei containing more than 1 proton had to contain neutrons also to be stable, and only certain numbers of neutrons.

Until the discovery of the neutron, only two kinds of forces, or “interactions”, were known in the universe. These were the “gravitational interaction” and the “electromagnetic interaction”. The electromagnetic interaction was much the stronger of the two—trillions and trillions and trillions of times as strong as the gravitational attraction.

The electromagnetic attraction, however, includes both attraction (between opposite electric charges or between opposite magnetic poles) and repulsion (between like electric charges or magnetic poles). In ordinary bodies, the attractions and repulsions usually cancel each other entirely or nearly entirely, leaving very little of one or the other to be detected as surplus. The gravitational interaction, however, includes only attraction and this increases with mass. By the time you have gigantic masses such as the earth or the sun, the gravitational interaction between them and other bodies is also gigantic.

Both the gravitational and electromagnetic interactions are long-range. The intensity of each interaction declines with distance but only as the square of the distance. If the distance between earth and sun were doubled, the gravitational interaction would still be one-fourth what it is now. If the distance were increased ten times, the interaction would still be 1/(10 × 10) or 1/100 what it is now. It is for this 103reason that gravitational and electromagnetic interactions can make themselves felt over millions of miles of space.

But now, with the acceptance of the proton-neutron theory of nuclear structure, physicists began to suspect the existence of a third interaction—a “nuclear interaction”—much stronger than the electromagnetic interaction, perhaps 130 times as strong. Furthermore, the nuclear interaction had to decline very rapidly with distance much more rapidly than the electromagnetic interaction did.

In that case, protons in virtual contact, as within the nucleus, would attract each other, but if the distance between them was increased sufficiently to place one outside the nucleus, the nuclear interaction would decrease in intensity to less than the electromagnetic repulsion. The proton would now be repelled by the positive charge of the nucleus and would go flying away. That is why atomic nuclei have to be so small; it is only when they are so tiny that the nuclear interaction can hold them together.

In 1932 Heisenberg tried to work out how these interactions might come into being. He suggested that attractions and repulsions were the result of particles being constantly and rapidly exchanged by the bodies experiencing the attractions and repulsions. Under some conditions, these “exchange particles” moving back and forth very rapidly between 2 bodies might force those bodies apart; under other conditions they might pull those bodies together.

In the case of the electromagnetic interaction, the exchange particles seemed to be “photons”, wave packets that made up gamma rays, X rays, or even ordinary light (all of which are examples of “electromagnetic radiation”). The gravitational interaction would be the result of exchange particles called “gravitons”. (In 1969, there were reports that gravitons had actually been detected.)

Both the photon and the graviton have zero mass and there is a connection between that and the fact that electromagnetic interaction and gravitational interaction decline 104only slowly with distance. For a nuclear interaction, which declines very rapidly with distance, the exchange particle (if any) would have to have mass.

In 1935 the Japanese physicist Hideki Yukawa (1907- ) worked out in considerable detail the theory of such exchange particles in order to decide what kind of properties the one involved in the nuclear interaction would have. He decided it ought to have a mass about 250 times that of an electron, which would make it about ¹/₇ as massive as a proton. Since this mass is intermediate between that of an electron and proton, such particles eventually came to be called “mesons” from a Greek word meaning “intermediate”.

Once Yukawa published his theory, the search was on for the hypothetical mesons. Ideally, if they existed within the nucleus, shooting back and forth between protons and neutrons, there ought to be some way of knocking them out of the nucleus and studying them in isolation. Unfortunately, the bombarding particles at the disposal of physicists in the 1930s possessed far too little energy to knock mesons out of nuclei, assuming they were there in the first place.

There was one way out. In 1911 the Austrian physicist Victor Francis Hess (1883-1964) had discovered that earth was bombarded from every side by “cosmic rays”. These consisted of speeding atomic nuclei (“cosmic particles”) of enormous energies—in some cases, billions of times as intense as any energies available through particles produced by mankind. If a cosmic particle of sufficient energy struck an atomic nucleus in the atmosphere, it might knock mesons out of it.

In 1936 the American physicists Carl David Anderson (1905- ) and Seth Henry Neddermeyer (1907- ), studying the results of cosmic-particle bombardment of matter, detected the existence of particles of intermediate mass. This particle turned out to be lighter than Yukawa had predicted; it was only about 207 times as massive as an electron. Much worse, it lacked other properties that Yukawa had predicted. It did not interact with the nucleus in the manner expected.

In 1947, however, the English physicist Cecil Frank Powell (1903-1969) and his co-workers, also studying cosmic-particle bombardment, located another intermediate-sized body, which had the right mass and all the other appropriate properties to fit Yukawa’s theories.

Anderson’s particle was called a “mu-meson”, soon abbreviated to “muon”. Powell’s particle was called a “pi-meson”, soon abbreviated to “pion”. With the discovery of the pion, Yukawa’s theory was nailed down and any lingering doubt as to the validity of the proton-neutron theory vanished.

(Actually, it turns out that there are two forces. The one with the pion as exchange particle is the “strong nuclear interaction”. Another, involved in beta particle emission, for instance, is a “weak interaction”, much weaker than the electromagnetic but stronger than the gravitational.)

The working out of the details of the strong nuclear interaction explains further the vast energies to be found resulting from nuclear reactions. Ordinary chemical reactions, with the electron shifts that accompany them, involve the electromagnetic interaction only. Nuclear energy, with the 107shifts of the particles inside the nucleus, involves the much stronger nuclear interaction.