aikakäänteisyys

Aikakäänteisyys, often translated as time reversal or time symmetry, is a concept in physics referring to the hypothetical possibility that the laws of physics would be the same if time were running backward. Many fundamental laws, such as Newton's laws of motion and Maxwell's equations, are invariant under time reversal, meaning they describe the same physical processes whether time is moving forward or backward.

However, this symmetry is not observed in macroscopic phenomena. The most prominent example of this is the second law of thermodynamics, which states that entropy, a measure of disorder, always increases over time in a closed system. This natural tendency towards increasing disorder creates an irreversible arrow of time, making it impossible to rewind macroscopic events like a broken egg reassembling itself.

The distinction between microscopic time symmetry and macroscopic irreversibility is a significant topic in physics. While the fundamental interactions between particles may be time-reversible, the collective behavior of a vast number of particles leads to emergent properties that exhibit a clear directionality in time. Philosophers and physicists have debated the nature of time and the arrow of time for centuries, exploring its implications for causality, free will, and the very structure of the universe. Despite the theoretical possibility of time reversal at a fundamental level, the practical experience of time is one of unidirectional flow.