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?
Planet Pailly 
The simplest questions are often the hardest to answer!
In Einstein’s theories of relativity, time is one of the four dimensions of the universe, and can only be defined locally. Physicists often say that time is what a clock measures, and it is no coincidence that Einstein made extensive use of clocks in formulating his theories.
While talking about clocks might seem to be avoiding the definition, it is actually a neat way to introduce some objectivity. A clock is any physical system that proceeds in a regular fashion. It might be the period of a particular wavelength of electromagnetic radiation, or it could be the period of an electron transition in an atom. In both cases, you are looking at something cyclical.
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Yes, cyclical changes (astronomical, seasonal, atomic, or subatomic) are important for how we humans perceive time. But if time is the fourth dimension, then I feel like there must be something more to it that just human perception. I’m not sure if I’m making myself clear here.
I guess what I’m pondering is this: is time a fundamental property of the universe, or is it an emergent property?
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Einstein would have said it was a fundamental property, not tied in any way to human perception. That is the role of the atomic clock – to remove human subjectivity and to frame the measurement of time in terms of something that is beyond the ability of humans to influence.
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That’s my understanding as well, which is why Aristotle’s definition feels incomplete to me. Aristotle makes time an emergent property in a universe of change. If nothing ever changes, time wouldn’t exist. At least that’s what I get from Aristotle’s definition.
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I would say that the existence of time enables the possibility of change, but change is not inevitable, and an absence of change does not imply that time has ceased to exist.
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I tend to think of time as the one-way dimension of change. As Steve noted, it doesn’t guarantee change, but enables it. We can only measure it as ratios of other changes we observe in nature, changes we at least perceive happen consistently. Of course, the fact that the rate of change varies by speed or the curvature of spacetime complicates the picture.
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Well said! I definitely feel like Aristotle’s definition comes very close. But as with most things Aristotle said, there’s more to the universe than he could have known.
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I tend to think of time as a measure of a planet’s movement around a star, but there is a whole rabbit hole to delve into here I hadn’t considered before.
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That’s basically how I felt after reading James Gleick’s book. There’s so much more to time than I ever thought about before.
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Maybe there’s no such thing as time. It could just be academic.
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It could be. There are a lot of scientists who say time is an illusion.
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