Sciency Words: Time

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:

TIME

How would you define the word time?

I recently read a book called Time Travel: A History by James Gleick.  This is one of the big questions raised by that book, and it’s a question that’s kept nagging at me.  What is time?  We all know what time is, don’t we?  We use the word all the… well, all the time.

But if you had to write a dictionary definition, what would you say?  Keep in mind the first rule of dictionaries: don’t use the word your defining in the definition of that word.  Gleick offers several interesting suggestions.  Time is the experience of duration.  Time is what keeps everything from happening all at once.  Time is the thing that clocks measure.

These are fun definitions, but I don’t find them fully satisfying.  Maybe we could turn to this classic explanation of time given in Doctor Who:

People assume time is a strict progression of cause to effect, but actually, from a non-linear, non-subjective viewpoint, it’s more like a big ball of wibbly-wobbly, timey-wimey… stuff.

In my own science fiction writing, time is often described as “a living thing,” something that’s constantly shifting, constantly changing.  History keeps rewriting itself, and time travelers speak of time in almost adversarial terms.  But while that might work for the kinds of Sci-Fi stories I want to tell, I don’t think this “living thing” notion is an actual, practical way to define what time is.

The closest I’ve come to finding a satisfying definition of time is an idea that goes back to Aristotle: time is a measure of change.  The sun changes its position in the sky.  So do the moon and all the stars.  The seasons change, one into the next into the next, until the cycle repeats.  All these cyclical changes set the standard by which we measure non-cyclical changes.  That’s what time is!

Or is it?  I said this is the closest I’ve come to finding a satisfying definition, but it still feels incomplete.  Thanks to Einstein and the theory of general relativity, we now know that time itself changes relative to acceleration and/or gravity.  So how can the measure of change be changeable?  There must be more to it than that, right?