Tviolation

Time-reversal violation, or T-violation, is the phenomenon in which the fundamental laws of physics change when the direction of time is reversed. In quantum mechanics and particle physics, time reversal is an antiunitary operation that, roughly speaking, swaps motion to the opposite direction and takes complex conjugates of amplitudes. A process that does not occur with the same rate or in the same way when time is reversed is said to violate time-reversal symmetry.

In many theories of physics, including the Standard Model, the CPT theorem holds: the combined symmetry of charge conjugation (C), parity (P), and time reversal (T) is conserved in local, Lorentz-invariant quantum field theories. Under CPT invariance, violations of CP symmetry imply violations of T symmetry, and vice versa. Thus experimental evidence for CP violation often also signals T violation, though direct tests that compare a process with its true time-reversed counterpart are highly valuable.

Direct tests of T-violation have been pursued in particle decays and oscillations. The CPLEAR experiment demonstrated direct T violation in the neutral kaon system by comparing rates of transitions and their time-reversed processes. In the B-meson system, the BaBar collaboration reported direct observations of time-reversal violation through analyses of entangled B0 and B0bar decays, providing a model-independent confirmation of T-violation. In addition to these, searches for electric dipole moments in particles and atoms probe T-violating sources, since a nonzero permanent EDM would indicate T violation under CPT.

Within the Standard Model, T violation arises from a complex phase in the quark mixing (CKM) matrix and is typically small, making precise measurements challenging. Ongoing experiments aim to test T-violation across systems and to search for signs of new, beyond-Standard-Model sources of time-reversal symmetry breaking.