Entropy
The entropy is a physical measure, a function of state, generally noted S. A reversible reaction is a reaction for the one the entropy is kept. An irreversible reaction sees its entropy increasing. It appears in the second principal of the thermodynamics.
This is via the calcul of the entropy of the universe that we will be able to know if the universe will be in constant expansion or if it will 'fall back' on itself.
See Big Bang.
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