time-reversal invariance

Time reversal invariance is a principle in physics stating that if a sequence of events is possible, the same sequence in reverse order is also physically possible. This means that the fundamental laws of physics apply equally whether time is running forward or backward. In classical mechanics, laws are invariant under time reversal, meaning they remain unchanged when time is reversed. This symmetry is also present in relativistic mechanics.123

However, this symmetry is not exact for all interactions. While the second law of thermodynamics shows time asymmetry, it's considered a statistical derivative. In particle interactions, it was once believed that all fundamental laws were time-symmetrical. Evidence emerged in 1964 showing a violation of this symmetry in weak nuclear interactions, known as CP violation. Despite this, a more comprehensive operation combining time reversal with charge conjugation and parity reversal (CPT theorem) does leave physical laws invariant.24