Is it possible for a ray of light, or even a single photon, to 'break' the vacuum of space?
If so, at what energies (in eV, e.g.) would this happen?
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Consider an electorn-positron collision, and the subsequent production if photons. It is known that there must be at least two photons produced. This is because momentum must be conserved. In the center-of-mass frame of the colliding particles, net momentum is zero. Production of a single photon will violate conservation of momentum, and thus we must have at least two photons produced.
Since all this is symmetric under time reversal, we can conclude that a single photon in free space cannot spontaneously decay into an electron-positron pair. However, if there is a nucleus present for the photon to interact with, pair production is indeed possible.