The science of energy, heat, and work, and the hard limits they place on every engine, body, and machine — built up backward from the practical problem of the steam engine rather than from pure theory, its laws say that energy is never created or destroyed yet always grows more useless, that disorder tends to increase in any isolated system, and in saying so they give time its one-way direction and the universe its slow run downhill.
There is a grim joke physicists tell about the three laws of thermodynamics: you cannot win, you cannot break even, and you cannot quit the game. It is not really a joke. The first law says energy is conserved, so you can never get more out of a system than you put in. The second says no engine can turn all its heat into work — some always escapes as waste — so you can never even break even. The third says you can never reach the absolute cold that would let you escape. These are among the most universal rules in all of science. They tax every bargain in the universe, and in forbidding heat from ever flowing back uphill on its own, they are also the reason time runs one way: why an egg scrambles but never unscrambles, and why the past is different from the future.