Would an object made of antimatter reflect light?

[Pasta]

@Mosier

I once read that most of what we see is matter, not antimatter.

How can we know this? Wouldn’t an antimatter star a thousand lightyears away look just like an ordinary star a thousand lightyears away? Isn’t it technically possible that our solar system is matter, and the rest of the universe is antimatter?

I’ll do the lazy thing and just quote my own posts from this and this other thread.

(1) We live in a region of matter. If there is a region of antimatter somewhere, the boundary between the matter and antimatter regions would glow with the products of annihilation. No such glow is seen anywhere, and from this non-observation one can set limits on the amount of antimatter, its proximity, and/or its spatial distribution.

(2) If there is an antimatter galaxy out there, it should have (among other things) antimatter stars making antihelium and antimatter supernova spewing antimatter everywhere. No such antimatter component of heavy cosmic rays has been seen.

Another important piece of information is the lack of light anti-isotopes (in particular, antihelium) in the primary cosmic ray flux. If there were antigalaxies, they would have antistars and antisupernovae blasting anti-isotopes outward. Antihelium is experimentally useful because it would be the most prolifically jettisoned isotope that is too hard to make in other ways. (Antiprotons, antideuterium, …, can occasionally show up through other means, but to make a whole antihelium pretty much requires an antistar.) All searches for antihelium have come up empty. The Alpha Magnetic Spectrometer, an experiment currently operating at the ISS, will push current limits down by another factor of 1000 if no antihelium is seen. If even a single antihelium nucleus is seen, that’ll be pretty solid evidence that a chunk of antimatter is out there somewhere.

There are other data pointing against a matter-antimatter symmetric universe that are somewhat less direct. For instance, the ratio of baryons to photons should be nine orders of magnitude smaller that what’s observed if things started out symmetric, at least according to our understanding of the early universe’s evolution (which actually isn’t half bad).