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Arrow of time
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The second law gives time its direction.
The microscopic laws are time-symmetric, but the second law gives a direction to macroscopic processes because the early universe had very low entropy.
Broken eggs don't reassemble.
The explanation lies in initial conditions.
In more detail
The arrow of time describes why processes occur in one direction, such as eggs breaking but not reassembling. It is linked to the increase of entropy in the universe.
Real-world uses
- Entropy is used to explain irreversibility in everyday processes.
- Cosmologists study why the early universe had low entropy.
- Physicists explain why we remember the past but not the future by pointing to the increase of entropy.
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Frequently asked questions
Are physical laws time-symmetric?
Most fundamental laws are, so the arrow comes from the conditions and statistics of large systems.
Why does the arrow point to the future?
Because the universe began in a low-entropy state and has been increasing entropy since.
Why does entropy define the direction of time?
Physical processes tend to increase entropy, so the direction in which entropy grows is the one we experience as forward in time.
Related topics in Heat & Thermodynamics
Clausius–Clapeyron equation · Joule–Thomson effect · Heat death of the universe · Humidity and dew point · Greenhouse effect · Thermal equilibrium in the Sun-Earth system