Can two or more neutrons be bound together by the strong nuclear force, with no protons? I presume I would have heard of such a thing if it were possible, but can anyone confirm that there is no such thing?
Right, there’s no such thing. But recently there was some controversy about evidence found for FOUR neutrons binding together and being stable.
Search google for “tetraneutron.” Also try “dineutron” and you should see something.
The problem with neutrons is that they’re not stable when alone. After a few seconds a lone neutron will fly apart into a proton, an electron, and a neutrino.
The half life of a neutron is 11 minutes. Hardly a “few seconds”.
Half life is merely a statisical average. You can’t predict with any certainty how long a particular particle will take to decay, just that if you have a group of them after 11 minutes, on average, half will still be around, and after 22 minutes 1/4th will still be around and so on.