T-symmetry
T-symmetry is the symmetry of physical laws under a time-reversal transformation (replacing t by −t in all the equations). Almost all physical laws that describe the interaction of fundamental particles exhibit t-symmetry, the only exceptions concern the weak nuclear force. At the same time most physical descriptions the bulk behavior of material at the macroscopic level, such as the second law of thermodynamics, do not exhibit T-symmetry.
Weak nuclear force
Many physicists believe T-symmetry is violated on all scales, including the microscopic, by the weak nuclear force. This is because there are strong theoretical reasons for believing that CPT-symmetry holds, but it is experimentally observed that that CP-symmetry is sometimes broken. It can be shown that if CP-symmetry is broken, there must be a balancing T-symmetry violation in order to preserve CPT-symmetry.
CP-symmetry violations have been observed in neutral kaons in 1964 in the Brookhaven National Laboratory. In 1998, a small T-symmetry violation has been observed in decays of neutral kaons in 1998 in the KTeV experiment of the Fermilab.
Macroscopic Laws
While T-symmetry appears to hold for most microscopic laws, it often does not hold for laws describing the behavior of bulk materials. Any equation which includes dissipation of energy through friction or viscosity or converts usable energy into energy that cannot be used for work, such as the [[second law of themodynamics]], will not exhibit T-symmetry.
The difference between the behavior of physical law at the microscopic and macroscopic scales results in the arrow of time and has long been of great interest to physicists and philosophers.