The past is a frozen country

摘要

If you see a half-melted ice cube sitting in puddle of water, it's a safe bet that it will be more melted in a few minutes' time. You can also be fairly sure that the ice cube was frozen solid not so long ago. One of the triumphs of 19th-century physics, mainly due to Ludwig Boltzmann and J. Willard Gibbs, was linking elementary, macroscopic observations such as these to the microscopic point of view, the behaviour of atoms and molecules. It's impossible to know the exact disposition of every molecule in a partly melted ice cube. But Boltzmann and Gibbs showed how to obtain a statistical description of the most likely microscopic arrangements for a given macroscopic system. Newtonian mechanics then gives a probabilistic prediction of what happens next.