käänteenvastaisuus

Käänteenvastaisuus refers to the principle of time-reversal symmetry in physics. It is the idea that the fundamental laws of physics are the same whether time runs forward or backward. If a physical process can occur, then its time-reversed counterpart should also be a valid physical process, assuming that only the direction of time is reversed.

In classical mechanics, most laws, such as Newton's laws of motion, are inherently time-reversal symmetric. For example, if you throw a ball upwards, the equations of motion describe its trajectory perfectly whether you watch it go up or see a film of it falling down played in reverse.

However, this symmetry is not always observed in all physical phenomena. The most notable exception is the second law of thermodynamics, which states that the entropy of an isolated system never decreases over time. This law implies a directionality to time, often referred to as the "arrow of time." While individual particle interactions might be time-reversible, the macroscopic behavior of systems, characterized by increasing entropy, is not.

In particle physics, some fundamental interactions, like the strong and electromagnetic forces, exhibit time-reversal symmetry. However, the weak nuclear force violates time-reversal symmetry, though this violation is extremely small. The CPT theorem, a fundamental theorem in quantum field theory, states that physics must be invariant under the combined transformations of charge conjugation (C), parity (P), and time reversal (T). This suggests that while T symmetry might be violated individually, the combination CPT is conserved. The concept of käänteenvastaisuus is crucial for understanding the fundamental nature of physical laws and the asymmetry of observable phenomena.