Editing Energy (section)

=== Reversible and non-reversible transformations ===
Thermodynamics divides energy transformation into two kinds: [[Reversible process (thermodynamics)|reversible processes]] and [[irreversible process]]es. An irreversible process is one in which energy is dissipated (spread) into empty energy states available in a volume, from which it cannot be recovered into more concentrated forms (fewer quantum states), without degradation of even more energy. A reversible process is one in which this sort of dissipation does not happen. For example, conversion of energy from one type of potential field to another is reversible, as in the pendulum system described above.

In processes where heat is generated, quantum states of lower energy, present as possible excitations in fields between atoms, act as a reservoir for part of the energy, from which it cannot be recovered, in order to be converted with 100% efficiency into other forms of energy. In this case, the energy must partly stay as thermal energy and cannot be completely recovered as usable energy, except at the price of an increase in some other kind of heat-like increase in disorder in quantum states, in the universe (such as an expansion of matter, or a randomization in a crystal).

As the universe evolves with time, more and more of its energy becomes trapped in irreversible states (i.e., as heat or as other kinds of increases in disorder). This has led to the hypothesis of the inevitable thermodynamic [[heat death of the universe]]. In this heat death the energy of the universe does not change, but the fraction of energy which is available to do work through a [[heat engine]], or be transformed to other usable forms of energy (through the use of generators attached to heat engines), continues to decrease.