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The first law of thermodynamics says energy cannot be created or destroyed. But we can collide photons to form electrons and positrons. Does this means that law does not apply in these microscopic scales?

And we can create mass from energy in the above process.but is it possible to make atoms that way? Like converting an electron to proton as a step in it? That would be like real energy- mass conversion.

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marked as duplicate by Kyle Kanos, Jim, Brandon Enright, jinawee, Prahar Apr 13 at 2:51

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possible duplicate of What is pure energy in matter antimatter annihilation? –  dj_mummy Apr 10 at 6:28
Just a hint: the phrase "pure energy" will only confuse you. It doesn't have a useful meaning. Just write and say "energy". –  dmckee Apr 10 at 15:30
@dmckee : I did not. Somebody changed my question wording. –  user44289 Apr 10 at 18:42
@ser44289: Oh-oh, that somebody was me. To be perfectly honest (and I hope you won't take this the wrong way), the first version (v1) did not make much sense unless you used the term 'energy' in a non-standard way to mean only 'pure energy' (which, in turn, scientifically is ill-defined, but at least informally usually refers to 'a collection of photons'.) The specific problem with v1 is that matter also contains energy (where the word energy in the last sentence is used in its standard scientific meaning). The second title (v2) is not a great question either, but makes semantic sense. –  Qmechanic Apr 11 at 20:21
That is OK. I studied Physics only upto high school formally so it was a little vague. :) –  user44289 Apr 12 at 2:15
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2 Answers 2

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Photons aren't pure energy - they are a particle like all other particles. Admittedly photons are massless but then so are gluons, and indeed above the electroweak phase transition temperature so are all particles.

So pair production from photons and annihilation into photons is just a scattering process like any other particle interaction.

However if is possible to convert kinetic energy to matter, and indeed particle colliders do this every day. This is how the Higgs boson was produced at the LHC. See the question What keeps mass from turning into energy? for more on this.

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Thank you for your answer. So protons and neutrons can also be created from electrons and photons? Like hydrogen atoms from Xrays or similar? I am a little naive in this so another question are all types of energy like mechanical chemical sound etc are electromagnetic force governed and electromagnetic energy in its core? –  user44289 Apr 10 at 7:06
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Energy conservation may be better stated as,

The total energy, i.e. energy in the form of mass + all other forms of energy is conserved for an isolated system.

This would mean that annihilation is simply an example of interconversion of energy from 'mass-energy' to 'light(electromagnetic) energy'.

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