Time and Motion

If everything in the universe stopped moving, would time pass? You couldn't tell how long the stillness lasted: your clock wouldn't move [a]; you couldn't experience the stop: your neurons wouldn't be firing; you couldn't check via calendar: the Earth's rotation and orbit would pause; you couldn't age or check corrosion since no bacteria would live to eat away at your body and no chemical reactions could tarnish metal or rot at anything.

What we call "time" is just the procession of motion; today we can use atomic clocks and this measure time as the regular vibration of caesium atoms and there are other (less regular, less perfect) events to measure by such as the Earth's rotation, but these regular movements cannot be time themselves any more than a yardstick is itself space. It's all just commeasurable ratios of periods of motion.

So what are you actually measuring with a clock? It's the ratio of two speeds. If you take ten seconds to walk from one end of the room to the other, what that means is the Earth has completed one 8,640th of a rotation around its axis, a Caesium atom in your rest frame has vibrated 919,263,177 times, or a quartz crystal or pendulum has done something similar and so on. You can't really say something happened at this time or another then, which makes sense; relativistically speaking we can't even be sure two events happened simultaneously. You can only make statements about relative duration (and even then only in rest frames), not temporal positioning.

As such, to say we exist in "spacetime" or whatever is meaningless. We don't live "in time", time is not a thing in itself, we live in a physical world where objects (matter) are/is capable of motion. Anything that could be described as "outside of time" is really being described as "outside of motion" or really "outside of matter" (since matter - or energy, since they're the same thing - is always what is moving). Something that is eternal is really just something that never moves or changes - seems obvious but sometimes people think of this as being a consequence of something being eternal rather than the state of being eternal as a consequence of these properties.

Think about this: if you had a universe that consisted entirely of a ball in some otherwise empty space, and moved the ball somewhere else, all you would have to do to "roll back time" would be to simply move the ball back to where it was (assuming literally there is nothing else in the universe). The same is true for the real universe, which is obviously much more complex. The comparable example would be like moving something like 1080 balls back, but if you could do that, there is nothing to say that time would not completely have materially reversed in the universe.


a: That is, the quartz crystal or caesium atom that actually keeps time wouldn't move, not just the clock hands or LEDs