For example, at high enough frequencies, a single photon has enough energy to create a positron-electron pair in free space. Photons at that frequency don't travel very far before they are destroyed by such a spontaneous generation of particles.
So what kind of energies are we talking about here, and what distances?
Photons with over 1 Million electron volts of energy can create a positron-electron pair, but only when near another massive particle (like the nucleus of an atom). The other massive particle is moved in the interaction but is otherwise not-necessarily changed. https://en.wikipedia.org/wiki/Pair_production. This process has been demonstrated experimentally. The mean free path of the energetic photon near an atomic nucleus is something down on the atomic scale, the experiment I read about used a piece of gold foil and generated lots of positron-electron...
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.