What happened to nuclear electrons?

By Hamish Johnston at the 2013 CAP Congress in Montreal

Sometimes I think that physicists can dwell too much in the past. Scientific papers, for example, often begin with a potted history of the field and it’s only in the second page that something new is mentioned.

But the Italian–Canadian physicist Francesco Barletta of CGEP Rimouski here in Quebec thinks that the physics community can often completely forget very important aspects of its past. And after seeing his fascinating talk today about the rise and fall of the “nuclear electrons” model of the nucleus, I have to agree.

I like to think I know a thing or two about physics. But just like most others in the audience, I had never heard of the theory. However, it was a leading concept of nuclear physics before the discovery of the neutron in 1932.

The theory sought to explain why the masses of nuclei did not scale with their charge. The basic idea was that a nucleus could contain one or more electrons, which offset the positive charge of one or more protons. This seemed reasonable because physicists knew about electrons and protons at the time.

Barletta explained how the theory came about and, much more interestingly, how it was dismantled piece-by-piece by subsequent experimental and theoretical breakthroughs in nuclear physics.

I’m sure I was not the only one in the audience wondering which leading theories of today will be getting similar treatment in the history lectures of tomorrow.

A great example of the debate is this paper that appeared in Physical Review and discusses the spin (or lack of spin) of nuclear electrons.