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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 left-right symmetry had already been proven violated when, in 1964, James Cronin and Val Fitch discovered that the matter-antimatter symmetry is violated when the neutral K-meson decays. Their experiment also proved that symmetry does not apply during time reversal: reactions going backward in time are not identical to those going forward.