The thermodynamic arrow
The Second Law of Thermodynamics implies an arrow of time in the sense that the entropy (or disorder) of an isolated system such as the whole universe never decreases - it either increases or remains constant.
The tendency for entropy to increase is reflected in a wide range of physical transformations that are are seen going one way but almost never in reverse such as:
  1. Temperature equalization of bodies at different initial temperatures.
  2. Escape of gas from an enclosed pressurized container into a larger environment.
  3. Mixing coffee and milk
  4. Scrambling eggs.
These are all manifestations of the general tendency for entropy to increase. The Second Law is not contradicted by localized entropy decreases - such as the evolution of life - since these are more than offset by entropy increases in the larger system.
Immediately related elementsHow this works
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The Arrow of Time  »The Arrow of Time 
The physics of time »The physics of time
The thermodynamic arrow
Why do we see an entropy gradient? »Why do we see an entropy gradient?
Thermodynamic arrow is basic »Thermodynamic arrow is basic
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