In thermodynamics, ectropy is a measure of the tendency of a dynamical system to do useful work and grow more organized. Ectropy, in a loose sense, can be thought of as the opposite of entropy. Ectropy is minus entropy. That is, instead of saying "lose entropy" you can say "gain ectropy", instead of saying "gain entropy" you can say "lose ectropy".

The term was introduced in the late 20th century by mathematician and philosopher Willard Van Orman Quine and is often more intuitive than its counterpart. The term's merit is that in order to understand a concept, it can be useful to look at it from the other side. Sloppily speaking, ectropy signifies order; slightly more exactly, usable energy. Actually, what we call energy is often ectropy.

The Earth, for example, gets electromagnetic waves from the sun and sends electromagnetic waves back into space, but the incoming waves have shorter wavelengths (higher frequencies) and therefore more ectropy. So the Earth's ectropy is increased by the sun. When we eat, we take in ectropy from the food.

The Second Law of Thermodynamics says that in a closed system, ectropy will decrease. An organism which is isolated from the outside world will die and deteriorate because its ectropy decreases. It needs ectropy coming from the environment to keep living.


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