Entropy and Extropy
a short orientation for the rest of the site
You have seen a movie played backwards. A short clip on social media run in reverse for a joke, a film using reverse footage as a stylistic gesture, just the rewind on a video player. Whatever the source, you knew immediately that something was wrong. You did not have to think about it. The motions are mistaken in a way the brain identifies in less than a second. Coffee unspills. Glass unshatters. People stand up from chairs without using their muscles. The world looks like itself, and yet it is recognizably wrong.
The thing you are detecting, when you do that, is entropy — or more precisely, its arrow. We do not know what time is in any deep philosophical sense; the question has been open for thousands of years and physicists who try to nail it down still run into puzzles. But we know, at the level of our nervous systems, what direction time runs in. We know because almost everything that happens is irreversible, and the irreversibilities all point the same way. Eggs scramble; they do not unscramble. Coffee cups fall and shatter; they do not reassemble. Heat flows from hot to cold and never the other way without help. Memory works in one direction. The future is unknowable; the past is fixed.
The name physicists give to this asymmetry is the second law of thermodynamics, and the quantity it tracks is called entropy. Entropy is what makes movies look wrong played in reverse. Entropy creates time, in a sense — at least the arrow of time. The future is whichever direction entropy increases. The past is whichever direction it decreases. If the second law somehow went away tomorrow, we would lose, along with most of the machinery of macroscopic physics, our ability to tell the difference between what already happened and what is about to.
In casual terms, entropy is a measure of how disordered a system is. The shattered coffee cup has more entropy than the intact one, because there are far more ways to arrange the pieces in disorder than there are ways to arrange them in any particular ordered shape. The scrambled egg has more entropy than the unscrambled one, for the same reason. There are millions of ways to be a mess. There is only one way to be a particular intact thing. Disorder is the default. Order is rare.
The second law says, plainly, that left to itself, a system gets more disordered. Order does not just appear. If you walk into an empty room and there is a sand castle on the table, you assume someone built it; you do not assume the wind happened to blow the grains into a castle by chance. The assumption is correct, and the second law is what makes it correct.
What the law actually forbids is something subtler than “order is impossible.” What it forbids is order arising for free. Order can be built — but only if the building exports more disorder somewhere else than the order it creates. The books always balance, slightly in favor of disorder, in any closed account.
A sand castle gets built with sweat: a calorie’s worth of human metabolism is dissipated as heat for every grain shaped into the structure. A flower grows by pulling sunlight in and exhaling waste heat into the cold sky. Your body maintains its remarkable internal organization by burning food and breathing out warm exhaust. Every piece of order you can point to in the universe was paid for in disorder somewhere else, and the disorder somewhere else, in total, is always larger than the order produced. This is what the second law actually says when you look at it carefully. It does not say order can’t exist. It says order can’t exist for free.
This is also what makes life possible without breaking any laws. Life is order — staggering amounts of it, organized at every scale from the molecular folds of a single protein to the social structure of a city of ten million. Life is paid for, mostly by the sun. Earth gets a stream of low-entropy energy in from one direction (the small bright disk in the sky) and radiates higher-entropy energy out in all directions (warm infrared photons disappearing into the cold dark). The arithmetic always favors net disorder; the local result, here on Earth, is patches of stunning order — flowers, brains, cities, galaxies of culture — maintained against the gradient by the constant inflow of energy from outside.
What creationists are pointing at when they argue that evolution violates the second law is something real. The second law is real. It does impose a directionality on physical processes. They are simply wrong about whether it forbids life. The error is in the closed-system condition: Earth is not closed; the sun is pouring energy in; local order on Earth is paid for by exporting greater disorder back to the sky. The second law accounts for everything — including life, including evolution, including everything this site celebrates. Anyone who tells you otherwise is either confused or hoping you will be. How Sally Gets Away With It walks through this argument in more detail; this page is the background you need for it.
The other word, extropy, is younger and less well-known. It was coined in the 1980s by the philosopher Max More, originally as an organizing concept for the transhumanist project: the idea that intelligence, complexity, and life can spread and increase, and that we ought to want them to. As a technical term it has not won wide adoption in physics, which is fine. The concept is what matters.
Extropy, as we use it on this site, is the local upward motion that survives against the entropy gradient. It is the eddies in the river of dissolution. A flock of starlings is extropy. A heart is extropy. A city is extropy. A book is extropy. A conversation is extropy. The patterns the site celebrates are all instances of local order maintained by an energy gradient flowing through them. They are paid for, but they are real.
The relationship between entropy and extropy is not a fight; that framing is misleading. Extropy is not opposed to entropy. Extropy is what entropy looks like at fine enough resolution while energy is flowing through. The river is going downhill, and somewhere in the middle of the current there is a small whirlpool that holds its shape for a few minutes. The whirlpool is not breaking the river. The whirlpool is what the river does locally given the conditions. When the conditions change — when the gradient runs out — the whirlpool dissolves. Entropy was never the enemy. Entropy was always the medium in which extropy briefly lived.
A small demonstration
Below is a box with two hundred particles. A hundred yellow ones start on the left of a partition. A hundred blue ones start on the right. The colors are labels — each particle keeps its color forever, regardless of where it goes. When you press Open partition, the divider goes away, and the particles begin to mix.
The point of the demonstration is the asymmetry. Particles spread; they do not unspread. If you watched this same simulation in reverse, you would see something that looks like a violation of physics — the colors gathering themselves back into separate halves, no force visible, no work done. Forward, it looks like physics doing what physics does. Backward, it looks impossible. That asymmetry is the second law in microscopic form, and it is one of the deepest things about the universe we live in.
From here, the rest of the site makes more sense. Every page is in some way about a local pocket of low entropy that holds its shape against the spread, paid for by an energy flow you might or might not be able to see at the page’s scale. Sally is one. Harry is one. The starlings are one. You are one. Notice them while they last.