"Let us suppose that we wish to tell an extraterrestrial being what we mean by matter and by antimatter. We do not know whether his world is made of the former or the latter. We can tell him: 'prepare a neutral K meson beam and go far enough from the production point to be sure to have been left only with the long-lifetime component. (...) Count the decays with a lepton of one or other charge and call positive the charge of the sample that is about three per thousand larger. Humans call matter the one that has positive nuclei.' If, after a while, our correspondent answers that his nuclei have the opposite charge, and comes to meet you, be careful, apologise, but do not shake his hand."("Introduction to elementary particle physics", A.Bettini, ed. Cambridge, pag.287)
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