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The theory of general relativity tells us that non-massive entities, and their behaviors, are possible sources of gravity. Mass isn't needed, the theory says.

What's the real-world evidence that non-massive entities, and their behaviors, are sources of gravity? I'm guessing that even if something responds to curved spacetime, that does not necessarily mean it's a source of curvature, itself--?

Also, I do not see how a correspondence between shapes of present-day gravitating structures and CMB radiation inhomogeneities would be an answer to my question. Even during the universe's radiation-dominated era, there was still some matter, which I imagine wasn't perfectly homogeneous (and weren't production/annihilation events happening anyway, generating mass in random spots even as transparency started to take effect?)? So I wonder if mass inhomogeneities were really the only early gravity wells. Not radiation inhomogeneities, per se.

Likewise, what's the real-world evidence that even massive entities, and their behaviors, are sources of gravity when those entities are antiparticles...forgive me for not yet making this an entirely separate Physics StackExchange question...

Thanks very much for your time.

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  • $\begingroup$ In GR, anything with energy can source gravity, even if the particle has zero rest mass. Anti-particles are made of the same type of matter as particles and in particular have energy (No! anti-particles do NOT have negative mass/energy). Also, the evidence that these particles source gravity is obtained from the fact that any probe sent near these objects behaves exactly as we expect them to under the influence of the gravity of GR. $\endgroup$ – Prahar Mar 19 '15 at 11:29
  • $\begingroup$ By "real-world", do you mean experimental? $\endgroup$ – Danu Mar 19 '15 at 11:29
  • $\begingroup$ Yes, I absolutely mean "experimental," i.e. "observed." Thanks for possibly helping me clarify, Danu. $\endgroup$ – user50489 Mar 19 '15 at 11:34
  • $\begingroup$ Prahar, if I understand correctly, in your comment you mentioned some "probe" that we have "sent near" a massless object (or an antiparticle? not sure). Can you please tell me what exactly this probe was, who sent it, and what was it sent near? Thank you. $\endgroup$ – user50489 Mar 19 '15 at 11:39
  • $\begingroup$ possible duplicate of Has the gravitational interaction of antimatter ever been examined experimentally? $\endgroup$ – Rob Jeffries Mar 19 '15 at 13:27
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To deal with your last point first, the coupling of anti-matter to the gravitational field of the earth is discussed in Has the gravitational interaction of antimatter ever been examined experimentally? It's exceedingly unlikely we'll ever be able to amass enough anti-matter to measure the gravitational field created by the anti-matter, but if the anti-matter couples to the Earth's field in the same way that matter does this will be strong indirect evidence.

Back to the main point. The obvious example is just the mass you see around you. The gravitational mass of the matter around us is due mostly to protons and neutrons, and they are made up from quarks. However if you add up the masses of the quarks in a proton or neutron they come to only a percent or so of the measured proton mass. The rest is binding energy. So we conclude that binding energy produces spacetime curvature.

I guess that what you're really getting at is whether we have ever measured the gravitational field due to photons, and the answer is that we haven't. Indeed it seems very unlikely that we will ever be able to since the energy densities required would be extraordinarily high.

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  • $\begingroup$ Thanks, John Rennie, for your always-great writing style and concision. With regard to what you called "the mass you see around you"...are you saying we know that such masses NOT ONLY respond to spacetime curvature as theory predicts based on their masses including QCD binding energy, BUT ALSO generate their own spacetime curvature as theory predicts based on their masses including QCD binding energy (as opposed to: ignoring their QCD binding energy when they generate their own spacetime curvature)? I ask because I feel it's easier to accurately test the former than the latter. MORE FOLLOWS $\endgroup$ – user50489 Mar 19 '15 at 12:45
  • $\begingroup$ CONTINUED FROM PRIOR But, can you tell me what tests or observation-based arguments tend to prove the latter? In other words, how did we come to believe general relativity's principle, that the amounts/types of energy which are affected by gravity are indeed the same as the amounts/types of energy which generate gravity? $\endgroup$ – user50489 Mar 19 '15 at 12:46

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