In our Universe, all stable atomic nuclei have protons in them; there’s no stable “neutronium” at all. But what’s the reason why?
Here in our Universe, almost every combination of fundamental particles you can imagine will lead to an unstable state. Anything containing a strange, charm, bottom, or top quark will decay in extremely short order, as will anything containing a muon or tau lepton, as well as their antiparticles. In fact, the only quark-containing particles that are stable at all are the proton, and other atomic nuclei (protons and neutrons combined) that contain at least one proton. But if a proton and neutron, when they’re bound together into a deuteron, are more stable together than when they’re free, then why can’t two (or more) neutrons bind together to make a stable, bound state of neutronium?
It’s an idea that dates all the way back to 1926: six full years before the discovery of the neutron. It’s also the question of Scott McGregor, who wants to know:
“I had trouble understanding why a deuterium nucleus is stable, but neutron pairs are not observed. I believe that due to the nuclear force, even a pair of neutrons should be lower energy than separate neutrons… Can you elaborate on this a bit more to explain why neutron pairs don’t exist or are unstable?”
It’s a really fascinating question that reaches deep into the heart of nuclear physics, and carries some surprising insights for us along the way. Let’s dive in and find out!
Let’s start with the basics: a free proton and a free neutron, separately. Even though they’re composed of quarks and gluons on the inside — with two up quarks and one down quark for a proton and one up quark and two down quarks for a neutron — it’s the overall properties, rather than the internal properties, of protons and neutrons that are more important for understanding how they bind together. Overall:
- protons have a mass of 938.272 MeV/c², an electric charge of +1, and are fermions, with spins of ±½,