From the May 1974 issue

Our changing view of the nature of matter 1954–1974

The electromagnetic force

In quantum electrodynamics (QED) intermediary photons removed the enigma of “action at a distance” between charged particles. However, when trying to calculate physical quantities we finished up with infinite values. A charged particle sets up its photon cloud which changes the properties of the charged particle which changes the properties of the cloud and so on. In the late 1940s, R P Feynman, J Schwinger and S Tomonaga noticed that behind the infinities lie apparent infinities of particle mass and charge. When they “renormalized” the theory by feeding in physically observed values of mass and charge, finite values emerged for electromagnetic phenomena.

But why do we have to plug in the observed mass and charge for the calculations to work with such perfection? It would obviously be philosophically more satisfying if they emerged naturally from the theory itself.

Perhaps the electron is a black hole! One can plausibly argue that its Schwarzchild radius of about 10–55 cm would give it enough size to get rid of the QED infinities and give the correct electron mass. So perhaps gravitation is deeply involved with the electric properties of matter but so far no firm framework has been found for these ideas.

The strong force

In 1954 the picture was of protons and neutrons [baryons] bound in the nucleus through intermediary mesons, pions. But trouble was brewing. A heavier meson, the kaon, had been spotted in nuclear emulsion photographs of cosmic rays, and heavier baryons (such as the lambda and sigma) had been seen.

The floodgates really opened when the large bubble chambers of L Alvarez came into operation at the Berkeley 6 GeV Bevatron. A whole host of new mesons and baryons were identified and the list has grown to include over 200 distinct particles. Initially thought of as rather exotic entities, something new was needed to explain their behaviour.

The most important advance came in 1961 from Y Ne’eman and M Gell-Mann using SU(3), a simple 19th century unitary group of transformations in three complex dimensions for studying sets of three objects. They classified the particles in groups, multiplets, mathematically treating them as if in reality they were built up of three basic “building blocks” [quarks].

The most spectacular prediction was of the existence of the omega minus. Playing with their three mathematical blocks, they grouped baryons of spin 3/2 in a group of ten and by 1961 nine of these had been identified. During the CERN conference of 1962, Gell-Mann wrote on the blackboard the mass (1680 MeV), charge (–1) and strangeness (–3) of the missing tenth member. At the beginning of 1964, the omega minus, with precisely these properties, was seen for the first time at Brookhaven.

The weak force

The first big change in our view of the weak force came in 1956. T D Lee and C N Yang, attempting to explain some puzzling observations on kaon decay into two and three mesons, made the revolutionary suggestion that in weak interactions Nature might be particular about the direction in which things happen, violating parity. Immediately after, C S Wu looked at electrons emerging from the beta decay of cobalt 60. They were spinning clockwise and, to preserve the angular momentum balance, emerging neutrinos (which could not be detected) must always be spinning anticlockwise. By now we know that there are only left-hand spinning neutrinos; right-hand spinning neutrinos do not exist.

This was a profound philosophical change in our view of Nature. We believed that Nature was symmetric and “right” and “left” were human conventions to help us find our way around traffic islands. Parity violation in the weak interaction has overthrown this belief and left the suspicion that Nature always cares about the direction that things happen but that in strong and electromagnetic interactions this is hidden by more powerful effects. Now we could contact intelligent beings in a remote corner of the Universe and communicate to them in which direction we turn around traffic islands.

• Compiled from texts on pp156–166.

Compiler’s Note

Point-like black holes are still a matter of conjecture but big ones made headline news recently. In 2015, on 14 September and again on 26 December, jubilant teams at the LIGO observatory in the US watched the Earth being shaken by gravitational waves created when pairs of huge black holes collided, 1.3 and 1.4 billion years ago, respectively. Closer to home, the composite Event Horizon Telescope (EHT), located at nine sites, is zooming in on a picture of Sagittarius A*, the black hole at the centre of the Milky Way, a mere 26,000 light-years away.

As for traffic islands, with the number of potentially Earth-like exoplanets discovered by astronomers steadily increasing, so does the chance of finding aliens out there smart enough to appreciate the properties of terrestrial traffic islands, although it would be tricky to explain why the polarity flips from place to place across our planet!

About the author

Compiled by Peggie Rimmer.