Time reversal symmetry is a fundamental concept in physics that suggests the laws of physics remain valid even if the direction of time is reversed. In other words, if you took a video of a physical process—like two particles colliding—and played it backward, the reversed process would still obey the same physical laws.
This symmetry is particularly important in classical mechanics, electromagnetism, and quantum mechanics, where many equations do not inherently distinguish between forward and backward time. For instance, Newton’s laws and Maxwell’s equations work the same mathematically whether time moves forward or backward.
However, time reversal symmetry is not absolute. In some areas of physics, especially in the weak nuclear force (one of the four fundamental forces), this symmetry is violated. Experiments involving particles like kaons and B-mesons have shown processes that behave differently when time is reversed, indicating a violation of time-reversal symmetry (T-violation).
This violation is closely related to the matter-antimatter imbalance in the universe and plays a role in CPT symmetry (Charge, Parity, and Time reversal), a fundamental principle in quantum field theory.
Time reversal symmetry helps physicists test the consistency of physical laws and search for new physics beyond the Standard Model.