1. Particle Physics to 1932

“Imagine that you could cut a grain of sand perfectly in two, and that you could take one of the halves and cut it again, and so on. Could this process be repeated for ever ?” – Democritus or Leucippus

Particle physics, sometimes termed ‘elementary particle physics’, is the study of fundamental particles : which should mean particles which cannot be sub-divided. As far back as 1874, the Irish physicist George Johnstone Stoney introduced the concept of a unit of electricity, and by 1891 he had coined the term ‘electron’, laying the foundations for the discovery of the first sub-atomic particle, though it would be a further 70 years or so before particle physics became generally recognised as a field of its own.

Rewind to a scene some 2,300 years earlier still, around 440 BC. Two men are walking beside a Mediterranean shore. The sand is strewn with rocks and pebbles of various sizes. Then one of them asks that remarkable question. The two men stop and look at each other before walking on. After some minutes, one replies :

“If you could cut the grain of sand an infinite number of times, then eventually you would create an infinitely small space, too small to contain any matter.”

“Then the answer is ‘No’.”

“Yes.”

Again the men pause and scrutinize one another before walking on.

“So resulting from the last cut there is a fundamental piece of matter which cannot be cut any further.”

“Yes.”

“What shall we call this indivisible piece of matter ?”

“Uncuttable (atomos) ?”

“Okay.”

A little later, one continued :

“That still leaves empty spaces.”

“There’s nothing to say that things are as small as we have imagined, either the atoms or the spaces.”

“Indeed…where shall we go for lunch ?”

The men were the Greek philosophers, Democritus and Leucippus, and without access to a cyclotron, photographic emulsion, or any kind of laboratory facilities, they had just established atomic theory. It is not clear who asked the question, nor who answered “No”, although Leucippus, the mentor, deserves credit in either case.

It should be mentioned that there is documentation of some ideas of atomic theory even before this, from the 6th century BC. A philosopher called Kanada, in ancient India, proposed a hierarchy of matter, with the smallest, indivisible particles called paramanus.

So what happened to atomic theory between the 5th century BC and the start of the 19th century ? The answer is not much, for various reasons. The first is that Democritus and Leucippus had done such a good job. Then there was subsequently a tendency for philosophers to become more absorbed by teleological questions – the purposes served by the designs of nature, rather than their causes. Of course, for half of the intervening time there were the ‘Middle Ages’, when the sciences and arts alike were stifled, but even after religion had worn itself out for a while, atomic theory was off the critical path : it had to wait for developments in other science before it could proceed.

Maybe we should use the slack time to explain a few anomalies in terminology. One obvious thing is that the atom, as we now term each nucleus with its orbiting electrons, most certainly is not a fundamental particle and may under some conditions be sub-divided. Were Democritus and Leucippus alive today they might have cause for exasperation :

“What are they playing at, calling these big bits ‘atoms’ ?”

“Why are they referring to uncuttables as ‘quarks’ ?”

“What does the term ‘quark’ indicate anyway ?”

The particle identified as ‘uncuttable’ has been redefined twice within less than a hundred years. Yet if, each time a further underlying component of matter is discovered, we change the terminology for everything that has been previously identified, nobody would be able to understand anything anyone else says. Nothing new there, you might think.

Another area of confusion is that particle physics (or elementary particle physics) is now the study of some sub-atomic particles which may be sub-divided, and others (e.g. quarks) which are regarded as fundamental. Contrastingly (?), nuclear physics was concerned with the study of the atomic nucleus, which is composed of sub-atomic particles, which are composed of quarks. The distinction does make sense when you consider the evolution of atomic theory.

It was through the study of chemistry that the next significant advances were made. At the start of the 19th century John Dalton, working in Manchester, proposed the following : that all matter is composed of atoms, which are extremely small, indivisible and indestructible, and that the atoms of any one chemical element are all exactly alike in every respect, including weight, but are different from the atoms of every other chemical element. It was the emphasis Dalton gave to the differing weights for atoms of different elements which most stimulated further research and in 1869 the Russian chemist Dmitri Mendeléev formally presented a periodic table of elements sequenced by atomic weight, and proposed that the properties of the elements were a periodic function of their atomic weights.

Now we get back to George Stoney’s concept of an electron. In 1897, Manchester physicist (of Scottish parentage) J.J. Thomson demonstrated that cathode rays, which had been discovered in 1869 by German scientist Johann Hittorf, were composed of negatively charged particles, electrons. The first sub-atomic particle had been found.

Meanwhile there were rapid advances in the subject of radiation. In 1895 German physicist William Conrad Röntgen had discovered X-rays, work which was to earn him the 1901 (and first) Nobel Prize for Physics. The first ‘medical’ X-ray shows the bones of Mrs. Röntgen’s hand, complete with wedding ring. Among those inspired by Röntgen’s work were French physicist Antoine Henri Becquerel, who in 1896 discovered natural (spontaneous) radioactivity in uranium salts.

Polish-born physicist and chemist (she was awarded Nobel prizes in both fields) Marie Curie, and her husband Pierre, made a number of important contributions. In 1898, studying the uranium mineral pitchblende, the Curies observed considerably more radioactivity than they had expected from just uranium and concluded that there were other radioactive substances present. From this they discovered polonium and radium, the latter being several million times more radioactive than uranium. The Curies’ most significant work was the observation that the amount of radiation emitted by a radioactive source depended only on the amount of the source present. This work established beyond doubt that radiation was an atomic property, and not the product of a chemical reaction.

In 1900 Max Planck presented his ‘quantum hypothesis’, a piece of work that was to profoundly influence physics. His supposition was that electromagnetic energy could only be emitted in quanta. Planck considered that quantization was “a purely formal assumption…actually I did not think much about it…” This was far from formal : it was incompatible with classical physics and became acknowledged as the beginning of quantum physics. Initially his work was largely ignored, that is until 1905 when Albert Einstein published a paper which gave evidence for the existence of light quanta (photons).

The emergence of another giant of modern physics was more apparent. Ernest Rutherford, a New Zealander, had studied at the Cavendish laboratory (under the directorship of J.J. Thomson) where he had conducted a detailed study of radioactivity. He had categorised two types of radiation, the alpha particle and beta particle, the former being a heavy, slow object equivalent to a Helium nucleus; the latter being the small and very fast electron. In 1903 Rutherford, now working with Frederick Soddy, proposed that radioactivity was the product of nuclear disintegration and suggested that the disintegrated nucleus would form the nucleus of a different, more stable, element. By study of the periodic table they predicted that radioactive decay of the uranium and thorium series would result in different isotopes of lead (²⁰⁶Pb and ²⁰⁸Pb respectively), a result confirmed by experiment. In demonstrating that radioactive decay is predictable, this work stimulated much further research to identify new radioactive elements.

Rutherford made another key contribution to the understanding of radioactivity by proposing that each radioactive element has a different rate of decay. Radioactive decay decreases exponentially with time (as less and less of the unstable radioactive source remains), so it is not very useful to measure the decay-time of an element by the time taken for it all to disintegrate. Instead, the concept of ‘half-life’ was introduced (the time taken for half the atoms of the original radioactive atoms to disintegrate, or alternatively, the time taken for the level of radiation to fall to half its original value). Each radioactive element has its own signature half-life.

In 1911, having moved to the University of Manchester, Rutherford fired a stream of alpha particles at a thin sheet of gold foil. Many of the particles passed straight through – indicating that the gold foil had empty spaces in it, some of the particles bounced back – indicating that they had collided with something substantial (a nucleus). From these results he proposed a model of the atom as primarily an empty space but with a small, dense nucleus of heavy material.

Niels Bohr had travelled from Copenhagen in 1911 in order to pursue post-doctoral studies at Trinity College, Cambridge (under J.J. Thomson) and from there had gone on to Manchester, to work under Rutherford. In 1913, extending Rutherford’s model of the atom, Bohr published his own model of atomic structure. Bohr suggested that electrons orbited the atomic nucleus in specific, pre-determined, circular paths. Using Planck’s earlier idea of energy quanta, he also proposed that an electron might move from one path to another. Upon absorbing energy, an electron might move to an orbit further from the nucleus, and if dropping to an orbit nearer the nucleus, the electron would emit energy in the form of light quanta. Such changes of orbit became referred to as ’quantum leaps’. According to the specific quantum leap of the electron, light of different frequency is emitted.

The idea that electrons had only specific ‘shells’ of orbits available to them, each shell with a maximum complement of electrons available, more easily explained why elements within each group of the periodic table share similar characteristics. But Bohr’s model didn’t quite work. It could only predict the spectral lines of hydrogen (the hydrogen atom has only one electron) and even for hydrogen there were some unexplained spectral lines. The difficulty was eased a couple of years later when Arnold Sommerfeld (at Munich) introduced a generalization of Bohr’s atomic model, using elliptical instead of circular electron orbits.

DL, patiently observing :

“There has been a lot of progress.”

“Did you see that work about light quanta ?”

“That was quite brilliant.”

“When do you think they’ll find the large, charged nuclear particles ?”

“I think that they may have found them, but didn’t realise.”

Eugen Goldstein, at the Berlin Observatory, had in 1886 discovered that discharge tubes with a perforated cathode also emit a glow at the cathode end. He concluded that in addition to cathode rays, there is an anode ray that travels in the opposite direction. Because these latter rays passed through channels in the cathode, Goldstein called them canal rays. They are composed of positive ions whose identity depends on the residual gas inside the tube. If the gas is hydrogen, the anode ray comprises H⁺ ions, which are protons.

Identification of the proton came in 1919, again a breakthrough made by Rutherford. Firing a stream of alpha particles through nitrogen gas, his scintillation detector (a photoelectric device to monitor luminescence) revealed the emission of hydrogen nuclei. He deduced that the hydrogen nuclei had been emitted by nitrogen nuclei :

¹⁴N + ⁴He -> ¹⁷O + ¹H

The resulting oxygen isotope was confirmed by later cloud chamber photographs. As the atomic weight of every other element is approximately a whole number multiple of hydrogen’s, his further conclusion was that the hydrogen nucleus is an elementary particle and component of every atomic nucleus. Additionally, Rutherford had become the first person to artificially ‘split’ an atom.

Still Rutherford was not satisfied. Assuming that the atom is electrically balanced, the number of protons in each atomic nucleus should match the number of electrons which orbit. But for every atom except hydrogen (which has a nucleus comprising just a proton) there was a discrepancy of atomic weight. He proposed that the discrepancy might be accounted for by a third, as yet undiscovered sub-atomic particle with no electrical charge, the neutron.

Rutherford, who had succeeded J.J. Thomson as director of the Cavendish Laboratory, was now joined by James Chadwick. In 1930 German physicist Walter Bothe bombarded beryllium, boron and lithium with alpha particles and observed a new form of radiation. The radiation was electrically neutral and extraordinarily penetrating. Initial conclusions were that this was a high-energy form of gamma radiation. Chadwick repeated these experiments and in 1932 showed that the radiation consisted of particles with a mass approximately that of a proton. These were the elusive neutrons :

⁹Be + ⁴He -> ¹²C + ¹n

Now the model was electrically balanced and atomic weights approximately explained. Job done ?

DL :

“They’ve discovered that their ‘atom’ isn’t uncuttable.”

“Their model has a big flaw, too.”

“How do you mean ?”

“Well, they explain that electrons remain in orbit around the nucleus by the attraction of opposite electrical charge. But their model shows same charge particles staying very close to each other in the nucleus. This shouldn’t happen – they should repel each other.”

“They need another particle.”

“Yes, something large enough to do the job of binding the protons, but not so large that it upsets their calculations of atomic weight.”

“Something intermediate (mesos) ?”

“Yes, that might do it.”

“They may have difficulty finding it.”

“That’s the least of their worries.”

“How do you mean ?”

“Then they’ve still to find the uncuttable.”

“Ah, yes…but maybe their study of the intermediate will lead them to it.”

Bibliography.

Greenaway, Frank, John Dalton and the Atom, Cornell University Press 1966
Hicks, John, Comprehensive Chemistry, Macmillan 1963
Reid, Robert William, Marie Curie, Collins 1974
Rozental, Stefan, Niels Bohr, North-Holland 1967
Russell, Bertrand, A History of Western Philosophy, Simon & Schuster 1946