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As a particle and its antiparticle annihilate each other a huge amount of energy is released, and no mass is left. This energy always comes in the form of force mediating particles (photons, gluons). Can´t we say that this is the definition of pure energy because all the mass has disappeared? If a photon has energy, then according to the definition it is its ability to do work. But what´s the difference between the ability of a photon to to work and the ability to do work for a particle with electric charge? A photon doesn´t do work as an electron does in an electric field. The photon can´t move against a force, as the photon itself is a particle that makes up a force field. The electron cán move aginst a force thereby acquiring potential energy. The photon just disappears because in can be absorbed by the particles that produce the field.

Maybe I just want to say that a photon (or a gluon) ís energy while particles of matter possess energy.

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    $\begingroup$ You may say that, but what's the point? The photons are still photons, they don't care whether you call them "pure energy" or not. $\endgroup$ – ACuriousMind May 25 '16 at 22:13
  • $\begingroup$ Pure energy? There is lots of that in fiction, see e.g. en.wikipedia.org/wiki/Energy_being . $\endgroup$ – CuriousOne May 25 '16 at 22:25
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    $\begingroup$ I don't understand this fascination that people exhibit for that phrase. In any case, the annihilation of , say a positron and an electron results in photons, and those carry both linear and angular momentum as well as energy. $\endgroup$ – dmckee May 26 '16 at 0:57
  • $\begingroup$ More on pure energy: physics.stackexchange.com/q/9731/2451 , physics.stackexchange.com/q/15122/2451 and links therein. $\endgroup$ – Qmechanic May 26 '16 at 8:25
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The photons released are individually massless, but all of them together have an effective mass equal to the original masses of the particle and antiparticle; see my answer here. This isn't some mathematical abstraction either -- you can put the photons in a reflective box and weigh it, and it'll have extra weight.

It's safe to say that the phrase "annihilate into pure energy" is not true in any way.

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  • $\begingroup$ Photons don't have mass (at least not a whole lot if we can trust the arguments about the rest-mass limit of the electromagnetic field). They have mass-energy. Once you put them in a box, they are not photons anymore but quasiparticles. Sorry to be such a pain about it, but the phenomenology is pretty clear about these things. Energy is a property of systems. In classical mechanics we can treat it independently of the property matter that we call " mass, in relativity we can't, but mass-energy is still just a property. "mass is energy" is not precise physics language. $\endgroup$ – CuriousOne May 25 '16 at 23:00
  • $\begingroup$ @CuriousOne I agree on the language part. As for the 'photon/quasiparticle' thing, the box isn't really necessary. Would you agree that a system of photons going in opposite directions has mass? $\endgroup$ – knzhou May 25 '16 at 23:06
  • $\begingroup$ If we want to "weigh" the photons, the box is necessary. I think it's a good example to teach students the phenomenology of rest mass, mass-energy and energy and momentum, which one can, of course, measure without "boxing". The question is very relevant for e.g. quark confinement and nucleon mass. It is really at the center of the difference between matter and radiation. I think it warrants to be very explicit about what one means. Does a standing wave have rest-mass? Yes, because it has a localized energy density inside a resonator (box!). $\endgroup$ – CuriousOne May 25 '16 at 23:17
  • $\begingroup$ But if the pure energy of a photon is absorbed bu a charged particle, isn´t this energy now possessed by the charged particle )pot. and kin. energy'? $\endgroup$ – descheleschilder May 28 '16 at 3:54

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