# gjm comments on A Roadmap: How to Survive the End of the Universe - Less Wrong

7 02 July 2015 11:01AM

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Comment author: 02 July 2015 08:13:02PM 1 point [-]

I'm, oh, let's say 99% certain you're wrong about the CPT theorem suggesting any kind of sign-reversal in gravity.

If antimatter is attracted by ordinary matter, then CPT symmetry tells you (since CPT reversal swaps ordinary matter and antimatter and leaves "attracted" as it is) that ordinary matter is likewise attracted by antimatter. And, of course, if ordinary matter is attracted by ordinary matter then CPT symmetry tells you that antimatter is attracted by ordinary matter.

I suppose CPT symmetry is kinda compatible with there being (let's say) gravitons and antigravitons that somehow do different things, except that (1) in every sketch of quantum gravity I know of gravitons are their own antiparticles, and (2) in general relativity gravity is a consequence of the curvature of spacetime and I can't imagine how a separate antigravity could fit into that picture.

(I am not an actual proper physicist and it's not impossible that I'm confused; hence 99% rather than 99.999%.)

Comment author: 02 July 2015 08:39:21PM 0 points [-]

The full CPT swap also involves reversing the flow of time. So one could attract, and one could repel, and this relationship is CPT-symmetric. (Antimatter chases matte, CPT swap, antimatter (previously matter) chases matter - in the other direction.)

And in terms of curvature, it just means the curve can have positive/negative amplitude. Antimatter would be matter with an inverse curvature. (Predicted by CPT symmetry, as I understand it.)

Note that what we're talking about now is more-or-less mainstream physics, albeit filtered through my probably-a-decade-and-a-half-outdated understanding of it.

Comment author: 02 July 2015 09:18:45PM 1 point [-]

Reversing time doesn't swap attraction and repulsion. (One way of seeing that: attraction/repulsion is a matter of the sign of a second derivative, and d^2/dt^2 f(-t) = (d^2f/dt^2)(-t). No sign change.)

The thing I was saying I couldn't see how to make sense of in the GR picture was having "gravity" and "antigravity" be separate phenomena (which I thought you might be proposing), not "antigravity" as such. I don't think there's any fundamental conflict between GR and having things of negative mass.

Comment author: 02 July 2015 11:59:06PM *  0 points [-]

Wikipedia on the subject. We don't seem to have experimental evidence one way or the other, and reasons to expect either effect (with the consensus favoring normal attraction). In particular, the section on CPT suggests that CPT suggests that matter and antimatter are attracted to each other.