I am studying the phenomenon of pair production and I learned that it cannot occur in a vacuum. Rather it occurs near a nucleus which absorbs momentum to keep the momentum conserved.

Instead of a nucleus, can we have pair production with two photons? For example, the two photons collide to produce an electron-positron pair.

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    $\begingroup$ Related: physics.stackexchange.com/q/22916/2451 and links therein. $\endgroup$
    – Qmechanic
    Apr 29, 2023 at 13:31
  • $\begingroup$ Try a web search for "gamma-gamma collider". These kinds of processes exist, they are just very hard to produce and detect experimentally. $\endgroup$ Apr 29, 2023 at 13:32
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    $\begingroup$ @Qmechanic but that link doesn't address my question. $\endgroup$ Apr 29, 2023 at 14:26
  • $\begingroup$ There is a general confusion about this matter. I believe this related question answer may help: physics.stackexchange.com/a/790479/183646 $\endgroup$
    – Markoul11
    Nov 29, 2023 at 9:23

1 Answer 1


Yes the process you are saying is absolutely possible $$\gamma + \gamma \rightarrow e^+ +e^-$$ Is a lowest order process that can occur in QED, since it involves two vertex, and as you know the QED respects time reversal or t-parity the process

$$ e^+ +e^- → γ +γ$$ must be time reversal.

EDIT: Pair Annihilation the process may be interpreted as a incoming photon giving a positron and an electron and one of them absorbing another photon for momentum corrections in that case one of the fermion will become a propagator.

Just hold your screen upside down (reverse time direction) and you'll see the process just replace the $e^-$ and $e^+$ accordingly.

For more details refer to Chapter 9 of Lahiri & Pal A first book of Quantum Field Theory

  • $\begingroup$ It's a second order process and in QED the lowest order process that can occur are second order processes ig $\endgroup$
    – Pradyuman
    Apr 30, 2023 at 5:46
  • $\begingroup$ I stumbled upon this question trying to figure out if it was experimentally possible. Thanks for the answer. The experimental reality is that this requires 511keV photons, and free electron lasers are about a factor of 1000 away from those energies. This is why when we talk about light by light scattering, we mostly talk about the "box diagram" of two photons scattering into two photons. Because nobody is even imagining a beam of photons with energies of 511keV $\endgroup$
    – AXensen
    Feb 21 at 20:41
  • $\begingroup$ You should look into gamma-gamma collider. $\endgroup$
    – Pradyuman
    Apr 1 at 4:48

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