# 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

All the particle accelerator in the world are continuously (when they are operating) converting kinetic energy into matter. For example, when two protons collide at the LHC the kinetic energy of the protons is converted in to tens to hundreds of particles (matter) which the experimenters then try to characterize with their detectors. Most of these particles are very short-lived and either decay into multiple less massive particles or they may interact with the detector material and create even more particles. However there will always be a significant number of electrons, protons, positrons and antiprotons (that are all stable) as the eventual result of the collision. The positrons and antiprotons will eventually annihilate with electrons and protons but in principle if they could be kept separate from ordinary matter they would all last forever. So the original two protons can create multiple protons and electrons (and their antiparticles) and this additional matter is made from the kinetic energy of the original protons.

High energy cosmic rays that strike the earth from outer space do the same thing - the kinetic energy of the cosmic ray will be converted into additional electrons, protons and their antiparticles.

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.

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I wouldn't call pair production as an example of matter/mass creation from energy, because its only temporal. The pairs will disintegrate soon after, no additional particles remain. There is some asymmetry in disintegration, which is blamed for the creation (shortly after BB) of the mass we see in universe, but was that asymmetry ever seen in experiments? –  Georg Nov 10 '11 at 11:33