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 pairs.
A single photon in otherwise empty space cannot create a pair of particles I was wrong when stating that. However, space with nothing but two photons in it can create matter. Two photons each with a bit over 511 million electron volts of energy can collide and result in the creation of a positron and an electron. https://en.wikipedia.org/wiki/Two-photon_physics Alternatively a single 80 Tera Electron Volt photon can collide with a very low energy photon to create an electron-positron pair. This effect actually makes our existing universe opaque to photons above 80 TeV because our universe is filled with approximately 0.0003 eV photons known as the Cosmic Microwave Background radiation. This background radiation is left-over radiation from the big bang which by now has cooled down to about 3 Kelvin in temperature. I don’t know any of the actual mean-free-paths associated with this, just that they are much shorter than interstellar 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 pairs.
A single photon in otherwise empty space cannot create a pair of particles I was wrong when stating that. However, space with nothing but two photons in it can create matter. Two photons each with a bit over 511 million electron volts of energy can collide and result in the creation of a positron and an electron. https://en.wikipedia.org/wiki/Two-photon_physics Alternatively a single 80 Tera Electron Volt photon can collide with a very low energy photon to create an electron-positron pair. This effect actually makes our existing universe opaque to photons above 80 TeV because our universe is filled with approximately 0.0003 eV photons known as the Cosmic Microwave Background radiation. This background radiation is left-over radiation from the big bang which by now has cooled down to about 3 Kelvin in temperature. I don’t know any of the actual mean-free-paths associated with this, just that they are much shorter than interstellar distances.