Pair Production

… a particle-antiparticle pair, that is. A photon that has sufficient energy can transform into the pair of particles. The minimum energy of the photon must be 2m_oc², where m_o is the mass of one particle. If the photon has more than this energy, the excess will become kinetic energy of the particles.

This event cannot occur in empty space, because momentum cannot be conserved with the photon and particles alone, so it only occurs near a nucleus or some other massive particle which can take up some momentum. I am not sure if the exact nature of this mechanism is known, but it certainly occurs. How near to the nucleus does it need to be? Exactly how does the nucleus get the momentum?

The calculation below shows why momentum cannot be conserved in empty space.

Energy: hc/l = 2g m_oc²
Momentum: h/l = 2g m_ov_x. (defining the photon direction to be the x direction)
Eliminate h/l between these equations and show that v_x = c, impossible.

If a particle and its antiparticle meet, they annihilate each other and produce a burst of photons. To conserve both energy and momentum in empty space, it cannot be just one photon. Could they annihilate each other near a nucleus and produce one photon, pair production in reverse? I don't know why not, but it would be much less likely.

Back to the main quantum craze.

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mechanics
fluids, heat, electricity and magnetism
vibrations and waves
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