How Do You Measure Time?
Last year, Alan Alda posed a challenge to science communicators, to explain a flame in terms that an 11-year old could understand. this drew a lot of responses, and some very good winners. This year’s contest, though still called the “Flame Challenge, ” asked for an answer to the question “What Is Time?”
This is a little closer to my corner of science, so I considered entering, but as previously noted, I’m crushingly busy at present. And either scripting/ shooting/ editing a video, or doing the necessary work to hack a written response down to the prescribed 300 characters was more time than I could really afford. Alas.
The finalists for 2013 have been announced, and as is sadly typical of competitions I don’t enter, I don’t really like any of them. The third of the video entries, the zero-production-value one, is probably the best, as far as I’m concerned, but I’m not very fond of any of them. So the following is an after-the-fact crack at the question, banged together very quickly at the end of a long weekend, but serving at least to illustrate the tack I would’ve taken had I had time to enter.
What Is Time?
Time, very simply, is what you measure with a clock.
That sounds like a complete punt, I know, the “because I said so” of science answers, but it’s true. You may have heard all sort of bizarre stuff about time– time passes at different rates if you move at high speed, time and space are the same thing, time slows down near a black hole, or that time travel into the past might actually be possible. All of that weird stuff is true, coming out of Albert Einstein’s theory of relativity, but the reason we know all that stuff comes down to this: time is what you measure with a clock.
The starting point of Einstein’s theory– and similar work by other scientists (Henri Poincaré, Hendrik Lorentz) at about the same time– is the realization that there isn’t a giant master clock at the center of the universe that everybody sets their watches by. They realized that if you want to talk about when things happen, you need to specify how you know that. If you want to know whether two events in different places happen at the same time, you need clocks at both of those places showing the same time, which turns out to be trickier than you might think.
When think carefully about how to get clocks to show the same time, weird things pop out. Two people who are moving relative to each other will each think that the other’s clock is ticking too slow. They’ll disagree about how much time passes between two events, and may even disagree about which event happened first. Our most exotic theories about time start by thinking about clocks.
So what’s a clock? A clock is anything that does a regular, repeated action. You count the number of times the action repeats, and that tells you how much time has passed. The sun rising and setting, a pendulum swinging back and forth, a light wave jiggling electrons in an atom: all of these have been used as clocks. Some clocks are more accurate than others, but they all do the same thing, which is to measure time.
And the very best clocks we know of confirm all the strange predictions of relativity. Scientists in Colorado have clocks so good they can measure the change in time from moving at walking speed, or from moving one foot higher in elevation. The Global Positioning System people use to navigate with their phones uses atomic clocks in space, and wouldn’t work without a correction for the change in time. There are even spinning stars thousands of light years away that act like clocks, whose slowing down confirms that gravity bends space and time, sending out waves that stretch and compress everything by a tiny amount.
So what is time? Time is what you measure with a clock. And when you think carefully about what that means and how to do it, you discover some pretty amazing things.
That’s more than the 300 word limit for a written answer, so we’ll pretend that I would’ve read this over some awesome video, with cool pictures and animations. Or maybe just staring into a webcam like the guy in the third of the video finalists (really, if I’d known that was an option, I might’ve entered for real…). Also, I cheated by throwing in a couple of hyperlinks. Sue me.
This also serves to illustrate my very-much-an-experimentalist, atomic-molecular-and-optical-physicist take on the whole question. General relativity is cool and mind-bending and all that, but at some point, you need to ground things in terms of actual physical measurements, which means time is ultimately about clocks. And I think it’s useful to remember that all the unification of space and time stuff ultimately began with scientists thinking very carefully about the extremely practical problem of synchronizing separated clocks.