whilst we are successful in colliding matter to identify yet further fundamental particles, and being aware there is a limit on speed due to mass end energy problems, and given that matter and mass are interchangeable, might it be possible to collide a single photon with another (this must rely on the particle nature of light). How could this be done? Ultimately every fundamental particle has a wavefunction and asscoiated energy. So if we remove the problems of mass can collision of photons show any new discoveries? Oh, and please do not go on about c being infinite and independant of direction etc, the question simply targets collision of photons.