If we established a radio contact with an extraterrestrial civilization, assuming the signal latency permits, what question would we ask them to know if they are made of matter or anti-matter? I realize that there may not be anti-matter regions in the universe. However, my question is not about what the universe consists of. My question is about conceptual differences between matter and anti-matter that could be tested and communicated. My thinking is that we would need to ask two questions. One would establish the chirality of weekly interacting leptons. However, this alone would not be enough, because we would not have a common reference to the concepts of left and right. Then we would ask for the results of an experiment with a CP symmetry violation. This result in relation to the chirality data should create a distinction between left and right and also between matter and antimatter. Am I on the right track or completely derailed?
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$\begingroup$ how would they know if they are made of antimatter? $\endgroup$– ZeroTheHeroCommented Sep 15, 2017 at 18:54
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$\begingroup$ @ZeroTheHero, That's the question! There's potentially two kinds of matter in the universe. Our names for them are "matter" and "anti-matter." The ETs probably don't speak English, so they'll have different names for them. The question is, after we've explained what we mean by "matter" and "anti-matter", and after we've told them which kind constitutes our world; how do we match up our names to their names so that we can ask them which kind constitutes their world. $\endgroup$– Solomon SlowCommented Sep 15, 2017 at 19:15
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$\begingroup$ @ZeroTheHero: This is what I'm asking in my question. However, my thinking is this, right or wrong, you decide. There are no C violations that I'm aware of, but there are P and CP violations, both in weak interactions. Having the results of both, we can derive the actual value of C. Makes sense? Also, matter and antimatter are just labels in this sense. If we disregard the prevalence of matter in the universe and the CP violation, then it's all symmetrical. $\endgroup$– safesphereCommented Sep 15, 2017 at 19:23
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$\begingroup$ to each his own I suppose. I'd ask first how they put up with their in-laws... $\endgroup$– ZeroTheHeroCommented Sep 15, 2017 at 19:30
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1$\begingroup$ @Samuel Weir: The question is about fundamental symmetries of nature, not about me having secretly established a radio connection in my basement with ET ;) Are these symmetries "academic"? I guess, but then again there would be no nuclear reactors or lasers or atomic clocks or GPS satellites without understanding of the fundamental science. $\endgroup$– safesphereCommented Sep 15, 2017 at 19:58
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2 Answers
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The question would be fairly simple. Anti-matter aliens would have nuclear physics with anti-protons and anti-neutrons. Weak interactions would produce then produce what would appear to be right handed CP violations. The question to ask is what is the parity violation in the beta decay of cobalt-60 nuclei found by by Chien-Shiung Wu. If these aliens given an answer to the parity violation that is the mirror image of what we know then they are anti-matter.
answered Sep 15, 2017 at 20:06
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2$\begingroup$ How would they communicate to us that the CP violation is "right-hamded"? At this point we would not have a common reference of left and right, would we? In my question I argue that the CP violation alone would not be sufficient, but you would need two experiments, one with the P-violation to establish right and left and the other with the CP violation to derive C from CP and P. Is my argument incorrect? $\endgroup$ Commented Sep 15, 2017 at 20:14
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$\begingroup$ Communicating with aliens will be tough with respect to encryption and decryption. However, handedness might be communicated with circular polarized states of light. $\endgroup$ Commented Sep 15, 2017 at 21:45
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$\begingroup$ Here Richard Feunmann shows why your solution would not work. He makes a hypothetical phone call to a Martian and explains that the parity violation would not reveal whether the Martian is made of matter or antimatter: feynmanlectures.caltech.edu/I_52.html $\endgroup$ Commented Sep 16, 2017 at 18:56
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$\begingroup$ @LawrenceB.Crowell Your solution in terms of establishing a shared handedness reference via transmitting circularly-polarized is clearly against the intent of the question. $\endgroup$ Commented Sep 18, 2017 at 14:32
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I cannot see why the parity violation would establish right and left with respect to our right and left, as the whole forces setup will be reversed for them.
If such a situation arose, i.e. we got electromagnetic signals from an alien civilization, we would know the direction and should search that direction for signals of e+e- annihilation and proton antiproton , the specific peaks in the spectrum ( correcting for infrared spread), in the interface between our two regions.
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$\begingroup$ Anna, the second part of your answer is creative, but is out of the scope of the question, which is, what would we ask them, not what we could measure ourselves (the region is only accessible by radio). In other words, the question is about the symmetries of nature, not about the aliens :) You are correct, the P violation alone would not work, but the idea proposed is to compare the results of the P and CP violations. Sort of like solving a system of two equations with two unknowns. Can this be done? $\endgroup$ Commented Sep 16, 2017 at 18:17
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$\begingroup$ Well, we could ask them if they see in the photons the lines of e+e- and proton antiproton annihilation . Then both of us would know that we have antimatter to each other. CP violation is too esoteric and would require graduate school communications, hard to reach over light years. $\endgroup$– anna vCommented Sep 16, 2017 at 18:26
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$\begingroup$ Right, but still it's a hypothetical question. So we assume the possibility of a large anti-matter region in the universe with no signs of annihilation and also that the aliens have a graduate degree :) $\endgroup$ Commented Sep 16, 2017 at 18:36