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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.

user50489
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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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John Rennie
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