‘Time’s nothing, memory’s what matters…’ –Louis-Ferdinand Céline
Humanity has long struggled over the nature of time. In the last century, physicists were shocked to discover that the arrow of time cannot be derived from the laws of physics which appear perfectly symmetric. For every solution for t, there seems to be an equally valid solution for -t (except in a few cases involving the weak force in which case the symmetry is more complex, involving charge, parity and time).
At first glance that looks puzzling. But after a few years reflection, most physicists agreed that it’s perfectly possible for symmetric laws to give rise to asymmetric phenomena. Physicists have identified a number of such asymmetric phenomena that represent “arrows of time”, says Claus Kiefer at the Institut fur Theoretische Physik in Cologne, Germany.
Perhaps the most famous is the thermodynamic arrow of time in which the entropy of a closed system must always increase. But there is also a quantum mechanical arrow of time in which a preferred direction of time is determined by decoherence and a gravitational arrow of time in which the preferred direction is determined by gravitational collapse.
“What is peculiar is the fact that the time direction of the phenomena is always the same,” says Kiefer. It’s almost as if the arrow of time were predetermined in some way. “The question raised by the presence of all these arrows is whether a common master arrow of time is behind all of them,” he asks.
What master law might be responsible? Kiefer’s conjecture is that the direction of time arises when quantum mechanics is applied to the universe as a whole, a branch of science known as quantum cosmology.
{ The Physics arXiv Blog | Continue reading }
In the natural sciences, arrow of time, or time’s arrow, is a term coined in 1927 by British astronomer Arthur Eddington used to distinguish a direction of time on a four-dimensional relativistic map of the world, which, according to Eddington, can be determined by a study of organizations of atoms, molecules, and bodies.
Physical processes at the microscopic level are believed to be either entirely or mostly time symmetric, meaning that the theoretical statements that describe them remain true if the direction of time is reversed; yet when we describe things at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time. An arrow of time is anything that exhibits such time-asymmetry.
The symmetry of time (T-symmetry) can be understood by a simple analogy: if time were perfectly symmetric then it would be possible to watch a movie taken of real events and everything that happens in the movie would seem realistic whether it was played forwards or backwards.
An obvious objection to this assertion is gravity: after all, things fall down, not up.
