Time, Asymmetry of

The directionality of time is a common, everyday experience. However, that direction, called the arrow of time, is simply a convention. Nothing in the principles of physics prevents processes from occurring in either time direction. In the human macroworld, however, many events have a much higher probability for occurring in one time direction than the reverse. That is, for all practical purposes, they are irreversible.

Air never flows back into a punctured tire to reinflate it. Heat never flows from lower temperature to higher temperature. People never grow younger. These events are never observed—yet they are not in principle impossible. The bodies of our normal experience are composed of a vast number of particles that move around randomly. The probability that particles of outside air just happen to move in the direction of a hole in a flat tire to inflate it is not zero; it's just so low that we are not likely to ever see it happen. A dead person could even come back to life spontaneously, but don't count on it.

An apparent asymmetry in time exists, but it is a statistical effect that is not present in the processes involving few particles, such as the processes observed in chemical, nuclear, and elementary particle reactions. These reactions all occur in either time direction. For example, hydrogen and oxygen can combine to form water, with some heat given off:

H2 + O → H20 + heat

The reverse process also occurs when heat added to water produces hydrogen and oxygen:

H20 + heat → H2 + O

While the rate for a reverse process will not always equal that of the original process because of various statistical factors, the basic probabilities are exactly equal. At least that is the case for all chemical and nuclear reactions and for most elementary particle reactions.

The operation that reverses the time direction of a process is referred to as T for time reversal. Two additional related operations are also defined. In one, designated C for charge conjugation, the particles in a reaction are all changed to their anti-particles. In the other, designated P for parity, the handedness of the process is changed—as if the process is viewed in a mirror. from very basic axioms in physics, it can be proved that all physical processes are invariant (unchanged) under the combined operation CPT. This is called the CPT theorem

Indeed, all chemical and most nuclear and elementary particle reactions are invariant under the operations C, P, and T when each is performed separately. However, in 1956, C. S. Wu and her collaborators showed that parity invariance was violated in the beta-decay of radioactive cobalt-60. In general, the weak force, one of the four basic forces in the universe, violates parity invariance.

In 1964, Val Fitch and James Cronin discovered that the decays of neutral K-mesons were not invariant to the combined operation CP Assuming that the CPT theorem is correct, this provided indirect evidence that time reversal invariance is also violated in these reactions.

Since then, direct evidence for T-violation has been found in the decays of neutral K-mesons. CP-violation has also been confirmed in the decay of B-mesons.

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