The passage of time is so familiar we don’t give it a second thought, but the concept of time is one of the things that make us human. Our experience and understanding of past, present, and future is essential to our navigation of the world around us. We experience time as a linear concept, flowing in just one direction, and indeed basic physical laws would indicate that it is the case. But will time ever reach an end?
The second law of thermodynamics was developed to understand steam engines in the early industrial revolution, but it has remarkable implications to the basic structure of the world. In its plainest form, the law states “heat cannot flow from cold regions to hot regions without external work being performed on the system”, which is to say “the entropy of any isolated system never decreases.”
There is no physical reason why the rubble from a demolished building couldn’t reassemble itself into the original building. It’s just very very very unlikely. Rubble is at a higher state of entropy than an office block. As one goes forward in time, entropy in an isolated system will always increase; therefore time’s arrow can only move in one direction.
This fact provides a solid theory for the long-term future of the universe: If the universe is a closed system, and there’s no Flying Spaghetti Monster or the like directing events from a heavenly cloud, then all energy (heat, etc.) will become evenly distributed. Stars will collapse upon themselves or explode, and dissipate their matter and heat across space. Eventually the universe will cease expanding and become a constant, largely empty expanse with matter approaching a uniform temperature.
At this point, when the universe is at its highest possible state of disorder, entropy will remain the same and time can be said to have stopped.
But that’s not going to happen for a long time. A very long time. It’s as many years away as there are atoms in the universe. So until then you need a watch.