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This article started with such a bafflingly breathless tone about 80-year-old physics that I couldn't even finish it. Pair production is not exciting or controv

This article started with such a bafflingly breathless tone about 80-year-old physics that I couldn't even finish it. Pair production is not exciting or controversial.

whatshisface 9y ago
Pair production usually can't happen unless an atomic nucleus is nearby. This is necessary for the conservation of momentum. I don't think photon-photon pair production has been achieved yet.
- aidenn0 9y ago
  I believe you, but can you explain in more detail? The classic diagram in undergrad physics is of a single particle going in one direction converting to two particle/antiparticle pairs going off at angles with a total momentum equal to that of the first particle. Why can that situation not happen?
  - ridiculous_fish 9y ago
    That can't happen if the incoming particle is a single photon. One way to convince yourself is to realize that a closed system consisting of a single photon is massless, therefore it cannot later contain a massive particle without violating conservation of mass (in the sense of E^2 - p^2). Another way to think about it is that a photon contains the maximum amount of momentum possible for a given amount of energy. (E = p). If you transform any of its kinetic energy into rest energy, there's nothing that can carry the excess momentum. However if the photon is in the presence of a massive particle, that particle can soak up the excess momentum, and pair production is possible. Alternatively if you have two photons, their momenta can cancel and pair production is again possible. A fun physics problem is a photon striking an electron and producing an electron-positron pair; what was the minimum energy of the photon, in terms of electron rest energies? It's not two, for the reasons given above!
    - aidenn0 9y ago
      Thanks, that makes sense.
- QAPereo 9y ago
  *By humans. It's almost certainly happening in particularly large stars, and around black holes, not to mention just... in the vacuum, although in that latter case it's annihilation as well.

This article started with such a bafflingly breathless tone about 80-year-old physics that I couldn't even finish it. Pair production is not exciting or controversial.

whatshisface 9y ago

Pair production usually can't happen unless an atomic nucleus is nearby. This is necessary for the conservation of momentum. I don't think photon-photon pair production has been achieved yet.

aidenn0 9y ago
I believe you, but can you explain in more detail? The classic diagram in undergrad physics is of a single particle going in one direction converting to two particle/antiparticle pairs going off at angles with a total momentum equal to that of the first particle. Why can that situation not happen?
- ridiculous_fish 9y ago
  That can't happen if the incoming particle is a single photon. One way to convince yourself is to realize that a closed system consisting of a single photon is massless, therefore it cannot later contain a massive particle without violating conservation of mass (in the sense of E^2 - p^2). Another way to think about it is that a photon contains the maximum amount of momentum possible for a given amount of energy. (E = p). If you transform any of its kinetic energy into rest energy, there's nothing that can carry the excess momentum. However if the photon is in the presence of a massive particle, that particle can soak up the excess momentum, and pair production is possible. Alternatively if you have two photons, their momenta can cancel and pair production is again possible. A fun physics problem is a photon striking an electron and producing an electron-positron pair; what was the minimum energy of the photon, in terms of electron rest energies? It's not two, for the reasons given above!
  - aidenn0 9y ago
    Thanks, that makes sense.
QAPereo 9y ago
*By humans. It's almost certainly happening in particularly large stars, and around black holes, not to mention just... in the vacuum, although in that latter case it's annihilation as well.