Pair Production: 2 Photos or Electron/Positron?

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Pair production typically requires two photons to conserve momentum, but a high-energy photon can produce an electron-positron pair when it interacts with a nearby atomic nucleus. The nucleus absorbs the recoil momentum, allowing for this process to occur without a second photon. Heavier nuclei are more effective in this role because they can better accommodate the recoil momentum. This phenomenon highlights the importance of nuclear interactions in pair production. Understanding these dynamics is crucial for comprehending the conditions under which pair production occurs.
Claire84
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I'm pretty much okay with pair production but in the eqt we have that a photo gives an electron and a positron. If we do it the other way round we have an electron and a positron giving 2 photos in order to conserve momentum. Why do we not have 2 photos of energy for pair production ir order to conserve momentum?
 
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You are correct (up to a point). Pair production in a vacuum does require two photons to interact. However, when a high energy photon passes near a nucleus of an atom (the heavier the better), it can give rise to pair production alone, with the nucleus taking care of the momentum balance.
 
Is that to do with the recoil momentum of the nucleus? I've heard it mentioned before so just wondering if that's what is going on here.
 
Your description is correct. It also explains why heavier nuclei are better. It is easier for them to absorb the recoil momentum.
 

Thread 'Two equivalent statements of time reversal symmetric Hamiltonian'

Time reversal invariant Hamiltonians must satisfy ##[H,\Theta]=0## where ##\Theta## is time reversal operator. However, in some texts (for example see Many-body Quantum Theory in Condensed Matter Physics an introduction, HENRIK BRUUS and KARSTEN FLENSBERG, Corrected version: 14 January 2016, section 7.1.4) the time reversal invariant condition is introduced as ##H=H^*##. How these two conditions are identical?
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