Protons are observed to be stable and their empirically observed half-life is at least 6.6×1035 yr.[5] Grand unified theories generally predict that proton decay should take place, although experiments so far have only resulted in a lower limit of 1035 years for the proton's lifetime. In other words, proton decay has never been witnessed and the experimental lower bound on the mean proton lifetime (2.1×1029 yr) is given by the Sudbury Neutrino Observatory.[6]
However, protons are known to transform into neutrons through the process of electron capture[7]:125 This process does not occur spontaneously but only when energy is supplied. The equation is: (also called inverse beta decay). "When a high energy-proton collides with an atom, it causes the ejection of an electron from the outer layer of the atom."
where p is a proton, e is an electron, n is a neutron, and νe is an electron neutrino.
The process is reversible; neutrons can convert back to protons through beta decay, a common form of radioactive decay. In fact, a free neutron decays this way with a mean lifetime of about 15 minutes.
