  The process of turning a proton into a neutron, positron and
electron-neutrino in a strong plane-wave electromagnetic field is studied. This
process is forbidden in vacuum and is seen to feature an exponential
suppression factor which is non-perturbative in the field amplitude. The
suppression is alleviated when the proton experiences a field strength of about
ten times the Schwinger critical field in its rest frame or larger. Around this
threshold the lifetime of the proton, in its rest frame, is comparable to the
conventional neutron decay lifetime. As the field strength is increased, the
proton lifetime becomes increasingly short. We investigate possible scenarios
where this process may be observed in the laboratory using an ultra-intense
laser and a high-energy proton beam with the conclusion, however, that it would
be very challenging to observe this effect in the near future.
