Here is a strange fact. Almost every basic law of physics works the same whether time runs forward or backward. Newton's laws of motion do. Maxwell's equations for electricity and magnetism do. Quantum mechanics does. General relativity does. Even the strong and weak nuclear forces have no built-in direction in time. Film two electrons bumping into each other, play it backward, and a physicist sees nothing impossible. Yet your coffee cools. Your body ages. A shattered cup never puts itself back together. Memory runs from past to future, never the other way. The universe started at the Big Bang and has been spreading out and cooling ever since. So something clearly points one way. But that one-way arrow is not in the laws themselves. It comes from how the universe started.
The physicist Arthur Eddington gave this one-way direction a name in 1927: the 'arrow of time.' He said the arrow comes from entropy. Entropy measures disorder, or more precisely, how many ways the pieces of a system can be arranged. The Second Law of Thermodynamics says that in a closed system, entropy never drops. Ice melts in warm water because a puddle can be arranged in far more ways than a neat crystal of ice. A cup shatters but never reassembles for the same reason: there are astronomically more ways for it to be broken than to be whole. Nothing forbids the universe from running backward. It is just wildly, wildly unlikely.
In 1908, the mathematician Hermann Minkowski rewrote Einstein's special relativity as a single four-dimensional thing he called spacetime. In this picture, time is not a river. It is a direction, like the directions of space, and in principle you could move along it. The whole history of the universe, from the Big Bang to its final cold death, sits there as one fixed four-dimensional block. Past events do not vanish when they finish; they still exist at their spot in spacetime. Future events are not missing; they already exist at their spot, waiting for 'now' to reach them. This view is called eternalism, or the block universe, and it fits both special and general relativity better than any other. The physicist Carlo Rovelli, in The Order of Time, calls it the 'view from nowhere.' From that vantage point every moment exists at once, and the flow of time is something your mind adds, not something physics contains.
The opposite view is called presentism. It says only the present moment exists. The past is gone, the future has not arrived, and only now is real. That matches how we feel, and almost everyone lives as if it were true. But it clashes hard with relativity. In special relativity there is no single, shared 'now.' Two people moving differently will disagree about whether two faraway events happened at the same time. There is no absolute present moment to mark the line between what exists and what does not. That makes presentism very hard to square with modern physics unless you bolt on extra ideas that cause their own trouble. The block universe is uncomfortable. It means your death already exists, fixed and real, somewhere in the four-dimensional block. But being uncomfortable is not the same as being wrong.
If all moments exist equally, why does time feel like it is flowing? Why does the past feel gone and the future feel unwritten? The physicist Julian Barbour, in his book The End of Time, argues that the sense of passing time comes from how one 'now' relates to the ones near it. Each moment holds records of earlier moments, and memory turns those records into a feeling of flow. You never touch the past directly. You experience a present brain state that carries encoded information about earlier states. The 'past' is always something you infer from present evidence: a fossil, a memory, a photograph. The flow of time is the story your mind tells about the direction in which information and disorder keep piling up. The story feels completely real, but it may not match anything in the outside world.