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 important 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 reason 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 only 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.
[Illustration: _Hideki Yukawa_]
[Illustration: _Victor F. Hess_]
[Illustration: _C. D. Anderson_]
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.
[Illustration: _C. F. Powell_]
(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 shifts of the particles inside the nucleus, involves
the much stronger nuclear interaction.