In thermodynamics, quantities such as energy, volume, quantity of matter and the infamous entropy are so-called extensive quantities: if the system size is doubled their value is doubled too, if the system size is tripled so is their value and so on.

That entropy is extensive ensures that if two identical — initially separated — systems are brought together to form a larger system of double the size without affecting “its nature” (in other words its size-independent properties), then the entropy of the universe should not change; meaning that if one were to perform the reverse operation (splitting a system into two identical parts), there would not be any way to discriminate the past from the future.

This property is part of the implicit postulates of the standalone theory of classical thermodynamics which was developed during the 19th century.

Later-on was developed a theory whose role was to rationalise thermodynamics through…