Ok I am understanding of how mass turns into light, but how does light turn into mass?
This is amazing : both processes are so close to each other. How could you accept one but bother with the converse !?
As you know, mass and energy are related : mass is one "trapped" form of energy, or "potential". The process are conversions from one form to another : light is pure energy stored in the EM, carrying no quantum number (except some angular mometum). So light turning into mass, is a transfer of energy from the EM field, to a matter field such as the electronic Dirac spinor field. This transfer must respect conservation of the quantum numbers, so only the total angular momentum must come out non-vanishing. The total electric charge must be zero.
To me, both transfers are exactly as amazing and funny, i.e. strange and unnacceptable at first sight. The pair creation is an exchange from the EM to the matter field : it can occur at any spot along the propagation of the energy. The matter field eventually "grasped" the propagating energy "in order to" store it in a trapped form. The pair annihilation requires the space-time meeting of an associated pair anti-particle / particle : one contemplates the "trapped" energy released in a purely kinematical form propagating in the EM. So one could consider pair annihilation more surprising !
For instance, a very higly virtual photon will constantly fluctuate in pairs while propagating.
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