It has so far been assumed that the neutron is a complex particle consisting of a proton and an electron. This is the simplest assumption and it is supported by the evidence that the mass of the neutron is about 1.006, just a little less than the sum of the masses of a proton and an electron. Such a neutron would appear to be the first step in the combination of the elementary particles towards the formation of a nucleus. It is obvious that this neutron may help us to visualise the building up of more complex structures, but the...

On December 4, 1930, Wolfgang Pauli sent a letter to a group of colleagues who were attending a physics conference in Tübingen. He addressed them as “Dear Radioactive Ladies and Gentlemen.” The letter was sent from Zurich, and Pauli apologized for not being able to attend the conference personally, as it conflicted with a ball he wanted to attend in that Swiss city. (In his day Pauli was a very good dancer and had a fondness for women. Figure 2, taken several decades later, shows that he had acquired a substantial avoirdupois.) The letter is one of the most remarkable...

Some experiments in physics change paradigms. The experiments of 1956–1957 showing that in the weak interactions parity is not conserved are an example; physics was never the same again. This is also true, I think, of the experiments with cathode rays that J. J. Thomson described in an 1897 paper, which postulated the existence of what came to be known as electrons.

Thomson was born in 1856 in Manchester, England. His parents had decided on a career of engineering for him, but after his father died he went to Cambridge and never left. Ernest Rutherford was one of his...

By the mid-1930s the number of elementary particles that had actually been observed could be counted on the fingers of one hand. There were the proton, the neutron, the photon, the electron, and the positron. The last is the antiparticle to the electron. (I will later devote a chapter to antiparticles, but here I can note that the positron has the same mass as the electron and the opposite charge. Electrons and positrons can annihilate into a pair of gamma rays, which are very high-energy photons.) By the time World War II broke out, the muon had been added to...

In 1928 Paul Dirac produced an equation for the electron that united the quantum theory with Einstein’s relativity. Some people think that it is the most beautiful equation in theoretical physics:

$ \left( {ih{\gamma ^\mu }\frac{\partial }{{\partial {x^\mu }}} - mc} \right){\rm{\psi (}}{x^\mu }{\rm{) = 0}} $

I am not sure I would go that far, but it is a very beautiful equation.

However, Dirac recognized at once that there were problems with its solutions. For each value of the momentum of the electron there were four solutions, and only two of them made sense. The other two gave the electron a negative energy that was impossible. This led to a couple of years of...

In prior chapters I have noted that some particles were discovered in cosmic rays, the positron being an example. Someone unfamiliar with the subject might get the idea that there was a kind of backyard treasure hunt in which these particles were unearthed. Since the particles I will discuss in this chapter were also found initially in cosmic rays, I want to explain what this means, beginning with a discussion of what a cosmic ray is.

In 1896 the French physicist Henri Becquerel made the accidental discovery that a substance containing uranium emitted charged particles. This was the discovery of...

In March 1963 Murray Gell-Mann came to Columbia University to give a lecture on a scheme he had recently invented for classifying elementary particles and which he had somewhat playfully called the “Eightfold Way.” In the teachings of the Buddha a “noble eightfold path” was identified, which, if followed, would lead to a cessation of suffering. The parts of the path included “right speech,” “right intention,” and “right concentration.” In Gell-Mann’s scheme the cessation of suffering involved a way to classify the bewildering shower of elementary particles, which no one had predicted and which no one understood. In his scheme...

It was understood from the beginning that the Eightfold Way was only an approximate symmetry. This is evident from the masses of the particles. The pi-mesons are placed in an octet that also includes the strange K-mesons. The mass of the K-mesons is more than three times the mass of the pi-mesons. If the symmetry were exact, all the particles in the octet would have the same mass. But when you look at the properties of these particles it is not clear at all that they have anything to do with each other. That is why finding the symmetry that...

The reader may well wonder what the neutrino has to do with cosmology, which is the science of the evolution of the universe from the Big Bang. I have noted that neutrinos can go through light-years of lead hardly interacting. But there is a qualification that I did not state. The neutrinos I had in mind had the sorts of energies found in beta decay. The conditions in the very early universe were entirely different, however.

The Big Bang occurred about 13.7 billion years ago. One can readily get the idea from the name that one is discussing an explosion...

If we have learned anything from what has gone before, it is that theorists’ speculations should be taken seriously—or at least considered.

In 1916, after he had created his theory of gravitation, Einstein made the following suggestion. On analogy with the fact that accelerated charges radiate electromagnetic waves, accelerated masses should radiate gravitational waves. In Einstein’s theory these gravitational waves produce ripples in space-time. That such radiation exists has now received spectacular confirmation. In 1974 Joseph Hooton Taylor Jr. and Russell Hulse found a system of binary pulsars. These are massive stars that revolve around their common center of...