In the classical realm, time moves smoothly and inexorably forward, uniformly no matter where we sit or how we ourselves move. This simplicity that time exhibits makes the laws of Nature easier to write down and manipulate. Forces act on masses and accelerate them; doubling the force doubles the acceleration. Two observers watching the same experiment agree on what happens when, and in what order events succeed each other. But this simplicity begins to unravel when we look more closely at fast-moving objects.
What is "fast"? Fast, from the viewpoint of spacetime, means moving at a fair fraction of the speed of light. From an everyday perspective, that's really quite fast, much faster than the motions of animals or vehicles near the Earth, and even faster than the movements of visible objects in the Solar System. When we move fast, time and space get mixed up — rotated into one another, just as directions can get mixed with each other when we turn.
If we face North, then East is on our right. We don't think it extraordinary, when we turn a few degrees leftward, to find that our right hand now points not due East but rather mostly East and a little North. Similarly, when we move rapidly through space, our time dimension begins to point a bit into the direction that we are heading, and the direction towards which we are moving points a bit into our time dimension.
What does that mean? It means that, when we (and anyone traveling along at the same speed and direction as we are) look at things and see them, although they seem normal to us, we will not agree with some of the observations of people who are moving relative to us. The differences in what we perceive and measure aren't mere artifacts caused by the time it takes light or other signals to reach us — the changes are much more fundamental than that.
We're free to consider ourselves to be at rest. But when we try to compare distances that we measure with those measured by moving observers, we find that they see lengths contracted along the direction in which they move. Worse, their clocks and sense of time passing go slower than for us. And we cannot agree about even the order in which some events happen, or about the simultaneity of events. We aren't special, either — every other observer has a similar disagreement with us (and with each other). Everybody sees everyone else's clocks run slow if they're moving.
Things get even more fun when gravity is involved — since observers who are near a large mass also seem to unavoidably have their clocks ticking slower than ours. And their distance measurements disagree with ours, even if they are not moving relative to us. Remember the thought experiment facing North with East on our right hand? Take a giant step forward, then left, then back, then right. On a curved planet, if our steps are big enough we won't get exactly back to where we started, and we won't be facing precisely North any more. Curved space near gravitating masses works likewise.
That's what it means for time and space to get twisted into one another. These changes don't just apply to people, of course — they happen to even the simplest subatomic particles, which age slower when they move faster precisely according to these rules. There's no escape. Nature isn't classical; our prejudices, formed from moving leisurely around in weak gravitational fields, aren't universally valid.
Sunday, June 20, 1999 at 07:51:48 (EDT) = 1999-06-20
(correlates: LeverAge, The Meaning of Life, OnSolitude, ...)