What Time Really Means
Time feels obvious until we try to explain what it is. In everyday life, it helps us plan a meeting, boil water, or catch a train. But in physics, time plays several roles at once, and separating them helps make sense of the puzzle.
First, time works like an address. Just as a place needs coordinates in space, an event also needs a when. That idea leads to spacetime, where the universe is described not as a changing three-dimensional stage but as a four-dimensional whole, with every object tracing a path from one moment to another.
Time also measures duration. A clock does this by repeating something regular, like the swing of a pendulum or the vibration of atoms inside a modern clock. What matters is not a mysterious substance called time, but the reliable comparison between one process and another.
The hardest part is the feeling that time flows. Physics does not really need that image. It can describe the universe as a complete spacetime structure, while our minds still experience moments one after another. That tension between the world as described by physics and the world as felt by human beings runs through everything that follows.
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