Arrow of time

The arrow of time is a concept used because almost all of the processes of physics at the microscopic level are time symmetric, meaning that the equations used to describe them are the same if the direction of time were reversed, yet when we describe things at the macroscopic level, there is an obvious direction of time.

Related Topics:
Physics - Time symmetric

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At the macroscopic level there is the second law of thermodynamics, or law of entropy, which roughly speaking is the principle that the amount of disorder in a closed system will increase with time. To use an everyday example, a plate gains entropy if it breaks. So entropy can be used as an "arrow" to point in the direction time is moving. This is not unique. For example, at the macroscopic level, we see things like friction, viscosity, and dissipation of energy, which produce an arrow of time, while all of these things appear to be absent at the microscopic level.

Related Topics:
Second law of thermodynamics - Entropy - Friction - Viscosity - Dissipation

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