I'm an engineer and have been reading about PET scanners and how they rely on the fact that a positron-electron annihilation will cause two photons to be emitted at 180 degrees from each other. After a bit of research I am left wondering on a couple of points.
Conservation of energy and linear momentum forbid the creation of only one photon.
Is this simply because emitting one photon would produce a force in one direction without an equal and opposite force?
In the most common case, two photons are created, each with energy equal to the rest energy of the electron or positron.
Does it follow then that there is a (lesser) probability that 3 photons can be emitted at equal energy and equal angle of 120 degrees?