Full Idea
The proton (938.3 MeV) is lighter than the neutron (939.6 MeV) and does not decay, but the heavier neutron can change into a proton by emitting an electron. (If you gather a bucketful of neutrons, after ten minutes only half of them would be left).
Gist of Idea
Neutrons are slightly heavier than protons, and decay into them by emitting an electron
Source
New Scientist writers (Why the Universe Exists [2017], 01)
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.5
A Reaction
Protons are more or less eternal, but some theories have them decaying after billions of years. Smashing protons together is a popular pastime for physicists.
Related Idea
Idea 21141 Neutrinos were proposed as the missing energy in neutron beta decay [New Sci.]