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 Question 697

 Is time constant? — RH, Boise, Idaho
That's a complicated and interesting question. To begin with, consider how we measure time: we generally use repetitive mechanical systems to tick off short intervals of time and then count as those intervals pass by. Thus we measure time in terms of the swinging of a clock's pendulum or the vibration of a quartz crystal or the motion of an atom's electrons around its nucleus. If time were to speed up or slow down, it would affect the mechanical motions in our bodies just as much as it would affect the mechanical motions of our clocks, so we wouldn't notice any change in the ticking of our clocks. If time were somehow to begin passing half as fast as normal and you were to look at your watch, your watch would still appear to tick off seconds at the same rate. So the first answer to your question is that we can't tell if time is constant, so long as any changes in time occur uniformly and instantly throughout the entire universe.

The reason for including the bit about "uniformly and instantly throughout the entire universe" is that we can tell if time changes at one location but not another. For example, if time were to slow down near you but not near me, I would be able to look at your watch and see that it's running slow just as you would be able to look at my watch and see that it's running fast. Alternatively, we could synchronize our watches, wait a while, and then compare our watches again. Since your time is running more slowly than mine, our watches would no longer be synchronized. While this situation sounds unlikely, it does occur. The rate at which time passes depends on where you are and on how fast you are moving, a result described by the Special and General Theories of Relativity. Our universe mingles space and time in a complicated way and also permits gravity to influence the passage of time. In short, the faster you are moving or the nearer you are to a large gravitating object, the more slowly time passes for you.