The nervous neutron

I studied physics at university many, many years ago and have been interested in it - qualitatively (my maths was not a strength!) - ever since.  I have devoured articles and books about quantum mechanics, relativity, cosmology, thermodynamics and all manner of other branches of physics, but until a couple of weeks ago, one rather key fact had somehow completely escaped my attention.  I must have been asleep in the particular nuclear physics lecture in which this was announced all those years ago!

The neutron, in free space, is unstable!

The neutron - the electrically neutral, beige particle that helps build the nuclei of all but one of the elements that make up the natural world, the one component of an atom that doesn't respond to the strong electromagnetic force.  Bound up in most nuclei with other protons and neutrons, it will remain predictably stable, closely packed up against its near neighbours.  Set it free from the nucleus and it has a half life of around ten minutes!  In a little quantum kerfuffle, it suddenly and unpredictably emits an electron and an anti-neutrino and becomes a proton - the nucleus of hydrogen.  It is the proton that is stable!

Why does that happen?

The superficial explanation - for I am sure that there must be an infinitely more complex and subtle explanation which probably embraces a full understanding of the weak nuclear force, which I do not possess - is that nature always tries to revert back to its lowest energy state.  If it is not in that state a particle or system is likely to be unstable - wanting to find a way to shed energy and settle back down.  It is the same basic tendency in nature that makes water flow downhill, make hot things cool down and keeps the Moon in orbit around the Earth and the Earth around the Sun.

It's usually assumed that the proton and the neutron are the same except for the fact that the former has an electric charge.  But it turns out that the proton and the neutron are not identical.  The neutron is heavier than the proton, not by much, but just enough to induce it to want to shed some weight when it is able.

The neutron is not so beige after all!