- Quote: Considering all this complexity, you can understand why the proton phib has survived; it’s a quick sound bite, whereas the truth requires several pages.
- Endnote: There’s a second reason: it works quite well in predicting certain experimental results, for surprising reasons involving sophisticated math. There may perhaps be a third reason regarding the proton’s internal structure.
The proton is inherently complicated (despite physicists’ initial efforts to imagine it as simple) and its properties are only known through a combination of measurements of its properties and computer simulations of its internal details. The former are somewhat difficult to interpret, and the latter are still limited by computer power, so we still lack a truly precise understanding of the proton’s interior. However, this has been improving steadily, decade by decade.
Surprisingly, the reason that the proton phib was so popular among physicists turns out to be grounded in mathematics, though this wasn’t realized until 1994 in a wonderful paper of Dashen, Jenkins and Manohar. Their idea is quite subtle, far too much so for me to explain here. But it explains why thinking of protons and their cousins as if they were made of just three quarks (or more generally, are controlled by certain abstract symmetries that follow from that imagined idea) leads to a number of correct predictions about the relations between these particles’ masses, as well as certain other properties — even though it gives completely the wrong picture about their interiors! In short, there was an underlying math reason why the proton phib, incorrect as it is, gives reasonable predictions in certain cases.
One more point: Although it seems unlikely to me, based on existing data and simulations, it is still possible that it will turn out that (in the language of this post, where I am careful to distinguish quarks, the truly elementary particles, from kuarqs, the imagined objects of the proton phib) that kuarqs actually exist — that each of three extra quarks in the proton surrounds itself with gluons and quark/anti-quark pairs in such a way as to behave like almost like a single object with more mass than a quark has. I mention this only to emphasize that the case is not entirely closed. But it seems to me that the above-mentioned paper of Dashen, Jenkins and Manohar probably settled the issue, showing that kuarqs are a consequence of a remarkable mathematical accident, and that the original story of a proton’s interior really is a phib.
