When does energy turn to matter?

I always hear about matter converting to energy - fusion, fission...

When does it go the other way around? What conditions lead to it? Are there reproducible experiments on this topic?

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I was going to write an answer about how endothermic nuclear reactions (in which energy turns into matter) are common in both fusion and fission reactors. But other people beat me to the punch. Suffice it to say they are, although they can't be the majority in a net-power reactor, obviously. –  AlanSE Nov 10 '11 at 5:01

It is annoying to talk about mass-energy conversion, because it is too often misinterpreted to mean that energy doesn't weigh on a scale before it "turns into matter". So I will preface the answer by saying that if you heat up a gas, the heat weighs on a scale, if you burn some paper and let the heat escape, the escaping heat makes the combustion products weigh less than if they stayed hot, and if you seal up a nuclear bomb, let it explode, and keep all the products and heat inside the box, the box has the same total mass before and after the explosion.

The conversion of energy, kinetic or electromagnetic, to particles with rest mass is experimentally observed only when the energy comes in big enough clumps to create a massive particle, when the energy is low, this is going to be the lightest charged particle, the electron. Electrons can only be created along with a positively charged particle, to conserve charge, and this is almost always a positron (in rare weak interactions you can make an electron, a proton, an antineutron and an electron-antineutrino). The production of electron positron pairs only happens for very hard X-rays, or particles moving with a comparable kinetic energy, and you don't have this much energy in a single particle even in an atomic explosion. You need to accelerate particles specially.

Because of this gap between the energy of particles and the energy of the lightest charged particle, you don't usually see conversion of energy to mass in day-to-day life.

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Sigh... yet another ignorant downvote. What can you do? –  Ron Maimon Nov 10 '11 at 7:50
+1, maybe the down vote is not due to ignorance but due to jealousy? I can even imagine who that person might be! ;-) –  user1355 Nov 10 '11 at 15:40
@sb1: No, that wasn't it--- I had a genuine error--- I said you make a proton a neutron, an electron, and an antineutrino, which is stupid. It's an electron, proton, antineutron, and antineutrino, I didn't cross the neutron! But the downvoter could have said "it's an antineutron, silly, not a neutron!". –  Ron Maimon Nov 10 '11 at 15:47
Exactly! It is just a silly mistake which one could have easily mention or even fix unless s/he is on a mission to lower your score. It's the most exact answer anyway spirit wise. –  user1355 Nov 10 '11 at 15:55

A photon is emitted when an electron jumps from a higher energy state to a lower energy state in an atom.

http://en.wikipedia.org/wiki/Atomic_spectral_line

In a process called pair production, a photon can turn into an electron and a positron.

http://en.wikipedia.org/wiki/Pair_production

These processes have been understood for about 50-100 years and are perfectly reproducible and measurable in a physics laboratory.

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On a much smaller scale, any chemical reaction that requires energy to run will convert the energy required by the reaction into a very slight additional mass. For example, photosynthesis which takes $$6\space CO_2 + 6\space H_2O +sunlight \rightarrow C_6H_{12}O_6 + 6\space O_2$$ will convert the energy of the sunlight into a very very slight additional total mass of the product chemicals when compared to the total mass of reactant chemicals. There was a question on this topic recently but I cannot find it right now.