Which physical principle explains why you cannot unscramble an egg?

The second law of thermodynamics - and the striking thing is that nothing in the underlying physics forbids it. Every equation governing the molecules works identically forwards and backwards; a film of two atoms colliding looks fine in reverse. The arrow of time appears only in aggregate.

The reason is counting. There is one arrangement of molecules we would call an intact egg and an astronomical number we would call scrambled, so a system wandering at random overwhelmingly ends up in the second kind. It is not impossible for an egg to unscramble; it is improbable to a degree with no useful comparison. Boltzmann's insight was that the law is statistical, not absolute.

That leaves an unanswered question that physicists take seriously: if the laws are symmetric, why did the universe start in such an extraordinarily ordered state? Every arrow of time we experience - memory, cause before effect, ageing - appears to trace back to that initial condition, and nothing yet explains it. Egg proteins can in fact be partially unfolded in a laboratory, which is a chemistry result rather than a violation.