How exactly do photons create matter? I really have a hard time understanding this. I've seen an explanation where they use other, virtual particles to create electron and positrons, but even then, I don't understand how that process works, if that is the answer. I've searched the forums for this and can't seem to figure it out.
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$\begingroup$ Photons do not create matter. $\endgroup$– my2ctsCommented Dec 23, 2019 at 20:43
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2$\begingroup$ @my2cts $\gamma\to e^+e^-$ works (when near an atom) & could be considered as making matter, no? $\endgroup$– Kyle KanosCommented Dec 23, 2019 at 20:48
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2$\begingroup$ Allex, what kind of answer are you looking for? In Quantum ElectroDynamics (QED), there is a fundamental vertex involving a photon, electron, and positron. This vertex can represent the destruction of a photon accompanied byt the creation of the electron-positron pair at a spacetime event, and it is at the root of any explanation you might find. Are you asking exactly how this works (a photon disappears at the same time/place an electron-positron pair appears)? If so, I don't know if the answer is known (or knowable). $\endgroup$– Alfred CentauriCommented Dec 23, 2019 at 23:17
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$\begingroup$ Fundamental particle vertices are part of a series expansion, which made Feynman famous, that is equivalent to the field theory that came first. Any actual event potentially involves an infinite number of these vertices adding to the field theory solution. Schwinger refused to use these diagrams and did it all with field theory, in which the photon mode morphs smoothly into the electron-positron pair. $\endgroup$– Ponder StibbonsCommented Dec 24, 2019 at 1:02
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1 Answer
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You are basically asking about pair production, and it is based on fundamental laws of physics, including the mass energy equivalence (which among other things states that mass/matter and energy can be transformed vica versa).
In physics, mass–energy equivalence is the principle that anything having mass has an equivalent amount of energy and vice versa The mass–energy formula also serves to convert units of mass to units of energy (and vice versa), no matter what system of measurement units is used.
https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence
Thus, when a photon (near an atomic nucleus) transforms (meaning ceases to exist as photon and transform into other particles), its energy can we transformed into the energy and mass of two particles (particle antiparticle pair).
Now as per the comments, contrary to popular belief, pair production is possible from two photons too.
http://hitoshi.berkeley.edu/229A/final-sols.pdf
Energy is conserved this way, because all of the photons' energy is converted into the mass and energy of the particle antiparticle pair, and momentum needs to be conserved too, that is why you need a atomic nucleus nearby.
Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specifically to a photon creating an electron–positron pair near a nucleus. For pair production to occur, the incoming energy of the interaction must be above a threshold of at least the total rest mass energy of the two particles, and the situation must conserve both energy and momentum.[1]
answered Dec 23, 2019 at 21:08
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1$\begingroup$ Pair production is not "two photons annihilating"- it's a single photon being converted to an electron positron pair in the presence of other matter. If you did have two photons going to $\rm e^+e^-$ there'd be no need for an atomic nucleus, since that can conserve both energy and momentum perfectly fine without. $\endgroup$– Chris ♦Commented Dec 23, 2019 at 21:18
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$\begingroup$ @Chris correct, that is what I wrote, do you see the parenthesis? But I will make it more clear just to be sure. By the way, two photon can convert into a particle antiparticle pair too (and no nucleus is needed). that is pair production too. hitoshi.berkeley.edu/229A/final-sols.pdf $\endgroup$ Commented Dec 23, 2019 at 22:30