The Search for Superstrings, Symmetry, and the Theory of Everything

John Gribbin

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First annotation on Oct 8th, 2025. Last on Oct 13th, 2025.

Chapter 2

Chemistry depends on the number of electrons surrounding an atom, and that is always the same as the number of protons in the nucleus. So to change the weight of an atom without changing its chemistry you add, or subtract, electrically neutral particles (neutrons) to or from the nucleus. The atoms with the same chemistry but different atomic weights are called isotopes of the element concerned.
The first blow at the foundations came from a Japanese researcher, Hideki Yukawa. Yukawa had been born in 1907 (he died only in 1981), and after attending the universities of Kyoto and Osaka, in 1935 he was working for his Ph.D. (which he obtained in 1938) and teaching at Osaka University. In 1939, he returned to Kyoto as professor of physics. Like other physicists, Yukawa was puzzled about how the atomic nucleus held together. He reasoned that there must be another force, stronger than the electromagnetic force, which kept the protons in its grip even though the electric repulsion 'wanted' to separate them. But we don't see any evidence of such a strong force in the everyday world, so it must be a kind of force new to our experience, a force which only operates over a very short range, holding protons and neutrons together in the nucleus but permitting the individual particles (or, as we have seen, alpha particles) to fly free once they get beyond its range. Yukawa used an analogy with the electromagnetic force to describe his new force.
Paul Dirac, a British physicist who was born in 1902, was one of the pivotal figures in the quantum revolution of the 1920s. He fused the first version of quantum mechanics, developed by Werner Heisenberg, with Einstein's special theory of relativity, introducing the idea of quantum spin for the electron (an idea promptly taken over into other particles) in the process; he developed a very complete mathematical description of quantum theory, and wrote an influential textbook on the subject, still used by students and researchers today; and he played a major part in the development of QED, although to the end of his life (in 1984) he remained deeply unhappy with the business of renormalization, which he felt did no more than paper over the cracks in a flawed theory.
But in 1932 Anderson was studying cosmic rays, using a cloud chamber, a device in which the cosmic ray particles leave trails behind them, like the condensation trails produced by high-flying aircraft. These trails are photographed, and the patterns they make can then be analysed at leisure. One of the things Anderson did was to investigate how the trails changed under the influence of a magnetic field, and he found some trails that bent by exactly the same amount as the trail of an electron but in the opposite direction. This could only mean that the particles responsible had the same mass as an electron, but the opposite (positive) charge. The new particles—called anti-electrons or, more commonly, positrons—were soon identified with the particles predicted by Dirac's equations, and this was the work which earned Anderson his Nobel Prize. Dirac received the Prize, jointly with Erwin Schrödinger, in 1933.
The positron and neutron were discovered in the same year, 1932. The muon was discovered in 1936, the pion in 1946. By then, it was clear that matter came in two varieties: some particles which feel the strong force (protons and neutrons, and the pions which carry the force) and some which don't (the electron and, it turns out, the muon)
Things which feel the strong force are called hadrons, while things which don't feel the strong force are called leptons. All leptons are fermions, and have half-integer spin.
Later studies showed that a neutron in the nucleus is converted into a proton, while the electron is ejected, producing a new nucleus corresponding to an atom of a different element. In fact, this process happens only in a few, unstable nuclei. Most neutrons, in most atoms, are quite happy as they are.
The proton and electron, produced when a neutron decays, together have a total mass about 1.5 electron masses less than the mass of the neutron. So this much energy ought to be available, shared between the proton and electron, as kinetic energy. When the proton is left in an atomic nucleus, of course, it doesn't move much, so it seemed that almost all of the extra energy must go to the electron, giving it kinetic energy in addition to its rest mass, and that every electron produced in this way by a radioactive atom ought to run off with a large, and predictable, amount of kinetic energy. But experiments showed that the actual energy of a beta decay electron is always less than the energy available, and sometimes a lot less. Where had the extra energy gone?
Strangeness is just a property that fundamental particles seem to have (or, rather, it is a property that we need to put into our models if we want to think of the world as being made of particles). It is no more, and no less, mysterious than electric charge. Some particles carry charge, some do not, and charge comes in two flavours, which we call + and −. If we are being more precise, and include zero charge, we have three choices, +1, 0 and −1. Strangeness varies from particle to particle, and there are more options than with charge, but the principle is the same. Strangeness can be 0, −1, +1, +2, or even bigger. And strangeness has to be conserved in strong particle interactions. Just as a neutron can only turn into a proton by an interaction which produces an electron to balance the electric charges (and an antineutrino to preserve lepton number unchanged), so strangeness has to balance, by the creation of particles with the appropriate amount of strange 'charge', during strong interactions. This restricts the number of interactions allowed, in line with the 'strange' results physicists were obtaining in the 1950s—hence the name.
Looking back over a span of more than thirty years to the genesis of the quark model of matter, it is hard to tell just how seriously even the proponents of the model took it at first. The idea that protons and neutrons, and other particles, were actually made up of triplets of other particles, some with a charge of 1/3 of the electron's charge, some with a charge of 2/3, ran so much against the grain of everything that had been learned since the closing years of the nineteenth century that it could only be presented, at first, as a device, a mathematical trick for simplifying some of the calculations and giving an underlying structure to the patterns of the eightfold way.
Even Gell-Mann, who coined the name 'quark' from a line in Finnegans Wake, was almost coy about the concept in the paper in which he introduced it. He said: It is fun to speculate about the way quarks would behave if they were physical particles of finite mass (instead of purely mathematical entities as they would be in the limit of infinite mass)… a search for stable quarks of charge –1/3 or +2/3 and/or stable diquarks of charge –2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks.

Chapter 3

Actually, you can understand symmetry breaking quite easily in the context of the weakest field, gravity. To an astronaut in free fall in a spacelab, there is no special direction in space. If the astronaut lets go of a pen, it floats off in any direction the astronaut pushes it. All directions are equivalent; there is a basic symmetry. On the surface of the Earth, things are different. If you give a pen a slight push in any direction and let go of it, it always falls the same way, downwards. 'Downwards' means towards the centre of the Earth. Drop a pen at the North Pole and it falls downwards; drop a pen at the South Pole and it falls downwards. But the two 'downwards' are opposite to one another. The basic symmetry is hidden, or broken by the Earth's gravitational field.

Chapter 4

The open string theory of 1980 became known as the Type I theory, and the new, Type II theory introduced a key variation on the theme—closed loops of string. Type I theory only had open-ended strings; Type II theory only had closed loops of string.