The Central Mystery of the Universe Is TimePhysicists assume everything is reversible in principle — yet time isn’t. Why not?
Physicist Paul Davies unpacks it, so far as anyone can: Physicists first got to grips with the problem of the arrow of time in the middle of the nineteenth century by considering the behavior of gas molecules rushing around and colliding. Imagine a box of gas with a barrier down the middle. Suppose the gas on the left is hotter than on the right. If the barrier is removed, the faster-moving molecules on the left collide with the slower ones on the right, redistributing the energy. Soon the gas reaches a uniform temperature, a condition known as thermodynamic equilibrium. This process is irreversible. You never see the opposite happening. Without external interference, heat always flows from hot to cold. It’s Read More ›