The description I have given is fine as a broad-brush summary, but there are more exotic physical systems in which to ensure that reverse-run sequences are allowed by the laws of physics we must subject the system to two other manipulations beyond the reversal of time: we must also reverse the charges of all particles (so-called charge conjugation) and also reverse the roles of left- and right-handedness (so-called parity reversal). The laws of physics, as currently understood, necessarily respect the conjunction of all three of these reversals, something known as the CPT theorem (with C
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