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For a long time, physicists assumed that various symmetries characterized nature. In a kind of “mirror world” where right and left were reversed and matter was replaced by antimatter, the same physical laws would apply, they posited. The equality of these laws was questioned concerning the decay of certain elementary particles, however, and in 1956 Chen Ning Yang and Tsung Dao Lee formulated a theory that the left-right symmetry law is violated by the weak interaction. Measurements of electrons’ direction of motion during a cobalt isotope’s beta decay confirmed this.