If a particle and antiparticle collide (in annihilation) to produce 2 photons, each with energy x, would you need a different photon of energy 2x (in pair production) to recreate this particle and antiparticle?


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Look at diagram (b). Read it left-to-right. A real electron and a positron annihilate. In the process, they produce two virtual photons. These two photons go on to produce another real electron and positron. You do not need “a different photon” or any additional energy.

You cannot speak about recreating this particle and antiparticle. Particles of the same type are indistinguishable. There is no way to be sure whether the electron and positron annihilated and created a “new” pair , or simply scattered as in (a). Both processes happen with a certain probability amplitude.


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