MANY people who follow science will have heard of Ernest Rutherford鈥檚 experiment in which he discovered the atomic nucleus. Not so many, however, will know that it was his colleague, the German physicist Hans Geiger and their student Ernest Marsden who actually measured the scattering of alpha particles from gold foil and discovered that while most alphas penetrate the foil, some are scattered back towards the source. The great Rutherford, a New Zealander by birth, didn鈥檛 even have his name on the paper in which the results were published. But it was he who eventually interpreted the results in terms of a core of positive charge concentrated at the centre of the atom, and he who calculated what generations of physics students have come to know as 鈥淩utherford scattering鈥�. So perhaps it is only natural that most people have assumed that it was Rutherford who did the original experiment.
I suspect that the main reason for such misunderstandings is that textbooks (and lecturers) have enough ground to cover without getting tangled up in historical detail. So the history is summarised as briefly as possible, and as references become increasingly remote from the original source, an inaccurate picture begins to take shape. It鈥檚 rather like Chinese whispers, where the message is gradually altered from its original form to something quite unrelated by the time it reaches the end of the chain.
Another example of rewritten history came to my attention when I was preparing this week鈥檚 Inside Science, 鈥淗eart of the atom鈥�. For some reason I decided to check when it was that the famous Danish physicist Niels Bohr began to think of the atomic nucleus as a liquid drop. To my surprise, I discovered that the idea did not originate with him, but with George Gamow, a Russian who is probably best known as one of the originators of the big bang theory of the early Universe and the author of the splendid Mr Tompkins stories. Yet most of us who have studied physics, link Bohr with the liquid drop model, just as we link Rutherford with the gold-foil experiment.
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The book that set the record straight for me is Niels Bohr: A Centenary Volume, edited by A. P. French and P. J. Kennedy (Harvard University Press, 1985). In the chapter on 鈥淣iels Bohr and nuclear physics鈥�, Roger Stuewer, nuclear physicist and historian at the University of Minnesota, describes how the young Gamow left Leningrad in June 1928, and having impressed Bohr, was awarded a year-long fellowship at Bohr鈥檚 Institute of Theoretical Physics in Copenhagen. It was near the end of 1928 that the liquid drop model first took shape in Gamow鈥檚 fertile mind, and in February 1929, during a visit to Cambridge, Rutherford invited him to present his ideas at the Royal Society in London, during a 鈥淒iscussion on the structure of atomic nuclei鈥�, opened by Rutherford himself. The following year Gamow 鈥� who by now was a visiting fellow at the University of Cambridge 鈥� published a more detailed quantitative analysis in the Proceedings of the Royal Society.
Gamow was ahead of his time. In 1930, the neutron 鈥� a basic constituent of nuclei 鈥� had yet to be discovered. Gamow鈥檚 original model of the nucleus contained mainly alpha particles, plus some protons and electrons, which he assumed were bound together by forces rather like those at work in a drop of liquid. The basic idea was not unreasonable, since both alpha particles and electrons emerge from the nucleus in different forms of radioactivity. However, the discovery of the neutron in 1932 was to lead eventually to our present picture of the nucleus as being built from protons and neutrons, although for a year or so various hypotheses appeared based on different combinations of protons, neutrons, electrons and alpha particles.
In October 1933, at the Solvay Conference in Brussels 鈥� then a leading international scientific meeting 鈥� the German physicist Werner Heisenberg discussed the various hypotheses for the nucleus, beginning with Gamow鈥檚 liquid drop model. A major difficulty with this model concerned the presence of electrons. Moreover, Bohr, who took part in the discussion following Heisenberg鈥檚 talk, worried that even a heavy nucleus would not contain many alpha particles. Indeed, he did not seem very impressed. Three years later, however, the German physicist Carl Friedrich von Weizs盲cker developed a formula for nuclear masses based on Gamow鈥檚 work, which he acknowledged in his paper in the journal Zeitschrift f眉r Physik.
So how is it that we now usually associate the liquid drop model with Bohr rather than Gamow? Stuewer points out that when the Strasbourg-born Hans Bethe published the second part of his authoritative review of nuclear physics in Reviews of Modern Physics in 1937, he based his discussion of the liquid drop model on a paper by Bohr and his colleague Fritz Kalckar, which made no reference to Gamow. Bethe鈥檚 review became a 鈥渂ible鈥� of basic nuclear physics, and since then, asserts Stuewer, 鈥渧irtually everyone has assumed, incorrectly, that Bohr conceived the liquid drop model鈥�.
Indeed, says Stuewer, it seems that in 1937 Bohr probably inclined more towards the idea of the nucleus as an elastic solid (although at the time of the 1933 Solvay Conference he didn鈥檛 seem to think much of this either). It was the discovery of nuclear fission late in 1938 that was subsequently to focus his ideas on the nucleus as a liquid drop. In a classic paper submitted to Physical Review in June 1939, Bohr and the American theorist John Wheeler developed a detailed analysis of the fission process, based on an analogy with the break-up of a large drop of liquid into two smaller droplets. This completed the association of the liquid drop model with Bohr.
Interestingly, although Bohr and Wheeler refer to 鈥渢he liquid drop model of atomic nuclei鈥� without crediting it to anyone, their discussion of the energy released by nuclear fission begins with an acknowledgment to Gamow for a related idea 鈥� the relationship between the mass and binding energy of a nucleus, which can be calculated from the liquid drop model.