Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:
RETROPY
In any serious conversation about time travel—and I mean any serious, scientific discussion of time travel, as in the kind of discussion actual real-life physicists might have—there’s a term that is virtually guaranteed to come up: entropy.
I’ve tried to define entropy before for Sciency Words, but I’ve never felt like I’ve done the term justice. It’s a big concept, and kind of a weird concept, and sometimes a depressing concept. It’s also a concept that most of us sort of grasp intuitively, even if we can’t quite put it into words.
The simplest definition is that entropy is the amount of disorder in a system, or perhaps the degree to which a system has decayed. Another good definition is that entropy is the measure of the amount of energy in a system that cannot or can no longer be used for work.
According to the second law of thermodynamics, the total entropy of any closed system will tend to increase over time. You can depend upon that! This makes entropy relevant to time travelers, because it’s one of the very few physical properties that is dependent on which direction time is flowing.
As we move forward in time, entropy will increase. And if entropy is increasing, you (as a time traveler) can be sure that you are traveling forward in time. And if you observe that the entropy of a closed system is decreasing, you can be sure you’re traveling into the past.
In the vocabulary of professional time travelers, there should probably be a special term for when entropy goes into reverse. I don’t know what that word is, but fellow blogger and poet James Ph. Kotsybar (also known as the Bard of Mars) recently proposed a pretty good option: retropy, short for retro-entropy.
He even wrote a haiku about it. It’s worth checking out, along with many of the Martian Bard’s other science-themed poems.
Planet Pailly 
Of course, local entropy may decrease. It is only global entropy that must increase.
I wonder how you’d include that in a plot? Or is it just the kind of buzzword a time traveller might use to pep up their conversation?
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Of course you are right. You can make entropy decrease, so long as it increases somewhere else, and there’s an entropy equation of some kind showing that the total increase must always be greater than the decrease.
But as I said, I never feel like I can do justice to the word entropy. There’s so much complexity and so much nuance wrapped up in this one little word.
As for retropy, I don’t know how or if I would incorporate it into a story. But given how much I write about time travel, it’s definitely a word I’m glad to add to my vocabulary.
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I’ve always found the “disorder” definition of entropy unsatisfying and confusing. It seems inherently value-laden. One person’s disorder might be another person’s ideal state.
The measure of energy no longer available for work definition is better. I think of entropy as the inverse of the remaining amount of possible change a system can go through in the future, or how close a system is to its final resting state.
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Those sound like good definitions to me. I once heard entropy defined as the “degree of freedom” in a system. The more entropy you have, the more “free” particles are to do as they please. I’m not sure if that’s the clearest way to introduce the concept, but it was an interesting twist on the usual disorder thing.
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