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Why is Room Temperature what it is?

Water at 310 K feels neutral. A shower a degree or two above this feels pleasantly hot. We can tolerate tens of degrees different from this in special circumstances: colder with warm clothes, hotter in a very dry atmosphere. But basically ~300 K is what we like.

Where does this temperature scale come from? Is it a consequence of basic physics and chemistry in some way? Or is it an accident of natural history? If the former, the concept of a habitable zone is very important. If the latter, extraterrestrials could have vastly different ideas of what habitable means.

An interesting answer is given this paper by Brandon Carter. Carter points out that biological processes depend on hydrogen bonds, and argues that hence life on Earth needs temperatures a few times below the binding energies of hydrogen bonds: too hot, and those bonds will all break; too cold, and there is no energy available to reform them. Hydrogen bonds are two orders of magnitude weaker than covalent bonds, so one expects biological temperatures to be similarly below the temperatures of the most energetic chemical processes. A bond energy of 8.3 kJ/mol, typical of hydrogen bonds, is equivalent to a temperature of 1000 K.

By this argument, room temperature is a universal concept, as long as life depends of hydrogen bonds. Carbon and water are not crucial.

We can try refining Carter's argument a little bit. Imagine a toy model for a hydrogen bond, as follows. We first consider a dipole, consisting of an equal and opposite charge-pair with unit separation. Then we consider another similar dipole, colinear but anti-aligned with the first, at distance 1/ε away. The internal potential energy of the two dipoles we regard as the `ionic' binding energy, and the interaction part as the `hydrogen bond' energy, and ε is of course the ratio of bond lengths. A simple calculation shows that the ratio of the two energies is (to leading order) ε3/4. Thus, a bond length ratio of three reduces the effective temperature by two orders of magnitude.