Why does the proton spin? Physics holds a surprising answer
It’s tempting to add the spins of the quarks together, but that’s not what the experiments agree with!
“We must regard it rather as an accident that the Earth (and presumably the whole solar system) contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about.” -Paul Dirac
You can take any particle in the Universe and isolate it from everything else, yet there are some properties that can never be taken away. These are intrinsic, physical properties of the particle itself — properties like mass, charge, or angular momentum — and will always be the same for any single particle. Some particles are fundamental, like electrons, and their mass, charge and angular momentum are fundamental, too. But other particles are composite particles, like the proton. While the proton’s charge (of +1) is due to the sum of the three quarks that make it up (two up quarks of +2/3 and one down quark of -1/3), the story of its angular momentum is much more complicated. Even though it’s a spin = 1/2 particle, just like the electron, simply adding the spins of the three quarks that make it up together isn’t enough.
