by Tommaso Dorigo

As I am in the process of proofreading the book I have written, “Anomaly!” (see here for what the book is about, and for coordinates to the World Scientific site where you can reserve your copy), I am dealing with clips of text that, for one reason or another, did not make it to the final version of the text. I hate that! I love each and every sentence I put together, so as a way of saving them from oblivion, I decided I would offer these clips erratically as blog posts. The one below was supposed to bethe start of a chapter on the history of particle physics. It deals with the two true fathers of experimental particle physics. Enjoy!

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The two scientists who should be credited the most for inventing from scratch the field of particle physics, demonstrating the use of the instruments and techniques that several generations of researchers would be fiddling with during the 20th century, are arguably Joseph John Thomson and Ernest Rutherford.

It was Thomson who first used the technology of electric and magnetic fields to command particle trajectories in a brilliant experiment, whose technical ideas were going to be used for much of the following century; and it was Rutherford who, in another groundbreaking experiment, showed how deep inferences could be drawn from the way particles bounced off one another.

          Thomson investigated the strange phenomenon of cathode rays using electric currents inside vacuum tubes at the Cavendish Laboratory of Cambridge University in 1897. Cathode rays had been first observed forty years earlier by Heinrich Geissler: when air was pumped out of a glass tube, and two electrodes implanted at its ends were subjected to a high potential difference, an eerie glow was seen filling the tube. Sometime later it was inferred that something radiated straight out of the negative electrode, causing a visible fluorescence in the glass opposite to it.

A long-standing controversy on the nature of those rays arose: had they a particle or a wave nature? This was strong motivation for a detailed study of the matter. Thomson was building on the work of many others: in particular, not long before Emil Wiechert had made a puzzling measurement which indicated that the rays, if interpreted as particles, had to have a much smaller mass-to-charge ratio than anything else known. This could be taken to mean that the corpuscles making up the rays were either smaller than anything known, or that they carried an enormous electric charge.

Thomson studied the phenomenon with care, reproduced Wiechert’s result, and demonstrated that an electric field did deflect the rays, if the vacuum in the tube was made good enough. He also understood the implications of the findings of another researcher, Philipp Lenard, who had determined the maximum length of propagation of cathode rays in various gases. Thomson concluded that if the rays were particles these had to have a very small mass. He could thus put forth the bold proposal that the rays were in fact bits of atoms, negatively charged:

We have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter… is of one and the same kind; this matter being the substance from which all the chemical elements are built up.

Thomson’s imagination leap was brilliant; and even more so was his experimental apparatus, which demonstrated how particles travelling inside vacuum tubes could be steered at will using electric and magnetic fields. That observation would one day be the basis of the design of particle accelerators as well as television sets, as soon as the required technology allowed those marvels to be built.

          Fifteen years later, as physicists were puzzling over the mystery of atoms, Lord Ernest Rutherford showed the path through which the understanding of the inner structure of matter could be furthered. In 1912, together with his assistants Geiger and Marsden, Rutherford performed a historic experiment by bombarding a gold foil with alpha particles. All what was known about alpha particles was that they were positively charged corpuscles which, as the French physicist Antoine Henri Becquerel had discovered in 1896, were spontaneously emitted by certain rare substances.

Rutherford thought that the observed pattern of deflections of the alpha particles resulting from their collision with a gold foil would provide a hint of the structure of the gold atoms. If atoms, as many believed,  were a pudding of positive goo with light-weight negative electrons embedded in it, the trajectories of alpha particles should only experience small deviations while passing through the thin gold foil. To his great surprise, Rutherford observed that some of the alpha particles bounced back off the foil at very large angles! Large-angle scatterings could be only produced if all the positive charge resided within a dense core within the gold atoms. This brought to the discovery of the atomic nucleus. In Lord Rutherford’s own words,

It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.

In retrospect, the most surprising thing about Rutherford’s experiment is perhaps not the bouncing of alpha particles off gold nuclei, but rather the fact that atomic nuclei were used to discover… atomic nuclei! Alpha particles are in fact just helium atoms stripped of their electrons. Of course, radioactive elements had been in the market for many years in 1912, but what exactly those substances emitted, or how they did it, was still completely unclear. All that was known about alpha particles back then was that they were electrically charged, point-like corpuscles which could be easily absorbed by thin sheets of matter.

          Rutherford’s findings are a landmark of twentieth century physics, but even more significant is the implicit message of his gold foil experiment, one which was ever since then printed deep in the sub-cortex of every physicist’s brain: the intimate structure of matter could be inferred from a scattering angle. The lesson was clear: if you need to know what is inside an object and you cannot open it, throw something at it!

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