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I was reading the Wikipedia page on Mirror matter and I was puzzled by one of the statements in the article:

Mirror matter, if it exists, would need to use the weak force to interact with ordinary matter. This is because the forces between mirror particles are mediated by mirror bosons. With the exception of the graviton, none of the known bosons can be identical to their mirror partners.

I understand the idea that forces in general wouldn't interact as there would be mirror version of those forces, and the weak force would interact with both due to parity violation. What I don't understand is the motivation for why there wouldn't be "mirror gravity" in the way there would be "mirror electro-magnetism."

I'm not asking from the perspective of general relativity, but more from a graviton perspective. Is there something that prevents there from being a mirror-graviton, or is this just a going in assumption for the theory?

(The closest I got to an explanation in the article is in the external links section to TeV scale gravity, mirror universe, and ... dinosaurs. I haven't read that yet, planning on it however.)

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Weinberg's soft graviton theorem implies that there can only be one massless spin-2 particle and it must have the same coupling strength with every particle. This implies the equivalence principle; the graviton must couple to everything.

Incidentally, you can't disentangle gravitons from GR so easily; a set of results (for instance by Boulware and Deser) show that GR is the unique, consistent low-energy Lorentz-invariant theory of a massless spin-2 particle.

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