Scatter and Seep

Seep shows the how. She sets a hot stone against a cold one, touching. Slowly the cold stone warms and the hot stone cools. "That's touch," she says, "heat crossing straight from the hot into the cold where they press together." She holds a hand in the warm air rising off the hot stone — "current, heat carried by moving air" — and a hand beside it in the empty gap — "ray, heat crossing as invisible light, needing nothing between." Three roads. "By one road or all three at once, heat finds its way from the hot thing to the cold thing. That's my half — the travel. Heat is restless; it always crosses to wherever is colder." She glances at Scatter. "But I've never once seen it cross the other way on its own. And that part isn't mine to explain."

Scatter shows the why-only-one-way. She drops a bright bead of dye into clear water and it spreads to a pale even tint. "Watch it un-spread," she says. It doesn't. "Never does. That's the arrow — bunched-up things drift to spread-out, and spread-out never gathers back for free." She points at Seep's now-evenly-warm stones. "Same arrow. Heat bunched up in the hot stone spread out into the cold one, until it was even — and it will never un-spread, never gather back into a hot stone and a cold stone on its own. Not because it can't travel that way — Seep's roads run both directions — but because spread-out is so much more likely than bunched-up that the universe simply never drifts backward." She taps the even stones. "That's my half — the direction. Heat could, in theory, crawl back to the hot stone. It just never, ever does. The arrow only points one way."

Then the two of them showed, together, the thing neither could show alone: why a warm room full of energy still can't warm your cold cocoa back up. A student was sad her hot cocoa had gone cold. "The heat's not gone," she protested. "Keep says energy is conserved! It's still in the room somewhere! Why can't it come back?" And this took both of them. Seep spoke first. "You're right — the heat isn't gone. It seeped out of your cocoa — by touch into the mug and the table, by current into the air, by ray across the room — and now it's in the room, spread through all that air and all those walls. That's my half: it traveled out, by the three roads." Then Scatter. "And here's why it won't come back," she said gently. "The heat that left your cocoa is now spread out — a tiny bit of warmth in a huge cold room, scattered through miles of air. To warm your cocoa again, all that spread-out warmth would have to gather back — every scattered bit crossing from the cool room into the warmer cocoa — which is the arrow running backward. And the arrow never runs backward for free." The student frowned. "So the energy's still there, but it's... too spread out to use?" "Exactly!" they said together. "Keep is right — the total is conserved, nothing's lost," Scatter said. "But it spread, and spread-out warmth is warmth you can't get back — unless you pay." "And you can pay," Seep added, brightening. "A microwave spends fresh energy to push heat back into your cocoa — forcing warmth to cross into the already-warm cup, against the arrow. It costs real power to do it. That's what every heater, every stove, every warming thing is: a machine that pays to push heat the way it wouldn't go on its own." "One of us alone couldn't explain it," Seep said. "I could tell you the heat left, by three roads—" "—but only I can tell you why it won't come back for free," Scatter said. "The travel and the arrow. That's the whole law."