Because of
momentum conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum. Thus, matter can be created out of two photons. The
law of conservation of energy sets a minimum
photon energy required for the creation of a pair of fermions: this
threshold energy must be greater than the total
rest energy of the fermions created. To create an electron-positron pair, the total energy of the photons, in the rest frame, must be
at least 2
me
c2 = 2 × = (
me is the mass of one electron and
c is the
speed of light in vacuum), an energy value that corresponds to soft
gamma ray photons. The creation of a much more massive pair, like a
proton and
antiproton, requires photons with energy of more than (hard gamma ray photons). The first published calculations of the rate of e+–e− pair production in photon-photon collisions were done by
Lev Landau in 1934. It was predicted that the process of e+–e− pair creation (via collisions of photons) dominates in collision of
ultra-relativistic charged particles—because those photons are radiated in narrow cones along the direction of motion of the original particle, greatly increasing photon flux. In high-energy
particle colliders, matter creation events have yielded a wide variety of exotic heavy particles precipitating out of colliding photon jets (see
two-photon physics). Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using
particle accelerators,
air showers,
radioactive isotopes, etc.). It is possible to create all fundamental particles in the
Standard Model, including quarks, leptons and bosons using photons of varying energies above some minimum threshold, whether directly (by pair production), or by decay of the intermediate particle (such as a W− boson decaying to form an electron and an electron-antineutrino). As shown above, to produce ordinary
baryonic matter out of a
photon gas, this gas must not only have a very high
photon density, but also be very hot – the energy (
temperature) of photons must obviously exceed the rest mass energy of the given matter particle pair. The threshold temperature for production of electrons is about 1010
K, 1013 K for
protons and
neutrons, etc. According to the
Big Bang theory, in the early
universe, mass-less photons and massive fermions would inter-convert freely. As the photon gas expanded and cooled, some fermions would be left over (in extremely small amounts ~10−10) because low energy photons could no longer break them apart. Those left-over fermions would have become the matter that exists today in the universe. ==See also==