There stood Fred in the doorway of apartment 27B, with a pallor that said he hadn’t seen the Sun since November, reeking of recycled air and ozone. The stairs had not been kind to Fred today.
“The rent was due last week,” Ben said, filling his mouth from a bowl of soggy shredded wheat. “So, three months you owe me.”
Fred stepped inside and locked the deadbolt. He latched the chain, then jammed a chair under the handle. He turned but did not check the window. He sat instead on the floor with his back against the wall and exhaled a long and careful breath as though the room might break around him.
“So. You went to the storage unit again? Yeah?” Ben asked, chomping on the wet wheat. “I told you about how people are going to complain about the smell. They’re gonna cut the lock.”
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“No. I didn’t go to the unit,” Fred said flatly. “MIT went to the unit. Then a private contractor I’ve never heard of with Department of Energy clearance collected the contents of the unit. Then they collected me.”
Ben lowered his spoon. “Oh, shit. They arrested you?”
“They recruited me. They had already confirmed the maths, Ben. What did I tell you? About the axioms? The Universe-Quine is real. They ran the simulation on the discrete lattice, and, uh,” Fred rubbed his face with both hands. “And we finished it.”
Ben stared at him. The refrigerator compressor turned on. A 60-Hz hum filled the room, more than mundane.
“You and MIT built a time machine?”
“Yup.”
“You’re kidding.” Splash.
“Nope.”
“Does it work?”
“Huh?” Fred looked up, his eyes dull.
“Did you go back in time?”
Fred let out a laugh that never arrived. “How could I possibly know that?”
“What?”
“Ben. Think. You aren’t thinking about the axioms. I’m always telling you about the axioms and it’s like you don’t listen to me.” His voice was thin. “The Universe is a reversible cellular automaton. OK? And reversibility implies bijectivity. One to one both ways. So, if we invert the computation, we are not external observers. We don’t get shunted out of the timestream and deposited in the past. We are the data being processed.”
“Wait, so —”
“If I reverse the system state from Tuesday back to Monday, the physical configuration of my brain returns to Monday. The synaptic patterns encoding the memory of Tuesday are dismantled to reconstruct the prior state.”
Ben frowned.
“So it’s useless. You turn it on, the Universe rewinds, you forget you turned it on, you live the day again, you turn it on again, the Universe rewinds. It’s just a loop.”
“Yes, but not a closed loop. Just an incredibly tight one.” Fred pulled a loose thread on the rug. “Because, determinism is absolute only in a vacuum, but we don’t live in a vacuum. We live in a thermal bath full of quantum jitter and bit rot and decoherence.”
Fred waved his hands around in the air.
“When you re-run the exact same Tuesday 10 trillion times, eventually, eventually, the thermal noise in the substrate creates a branching path. An electron tunnels where it didn’t before or a decision changes. Eventually, the loop breaks.”
Ben felt a dread settle into his stomach. The hum was louder than ever now. “So, you aren’t travelling to the past to fix things? You’re just brute-forcing probability until you get the result you wanted?”
“Not me, exactly.”
“You’re savescumming reality? With MIT?”
“And the Department of Energy. Listen, no one’s looking for history. They’re looking for a specific future. The basic idea is, when the timeline hits a ‘Fail’ condition, some invariant they set is broken — like a nuclear exchange, ecological collapse or maybe just a market crash they didn’t like — the machine triggers a global reversal. We rewind ten years. We don’t remember the failure. We play it out again, maybe this time the dice roll better. If they don’t, we always try again. Because of the loop. Almost always.”
“Who are they?”
Fred didn’t respond. He looked at the door. The chair was sliding free.
“But, you’re here,” Ben acknowledged slowly. “You’re back. Which means you stopped. The machine is off.”
“I mean. Is it?” Fred whispered. “Or did the tape snap?”
Ben had never paid more attention to Fred before.
“What do you mean?”
“Computing is an act, it generates heat. Even in a reversible system. Every time we rewind the Universe, we generate waste heat. We increase the entropy of the system running the simulation, just looping the same decade over and over, grinding the gears until the metal gives.”
Fred looked at the ceiling now, his face slack and hollow.
“We tried to calculate the cumulative error. We think we’ve probably repeated the early twenty-first century enough times to burn out the Sun, if you just count the total duration of the loops. We kept waiting for the perfect timeline. We had to, you understand. The axioms. But, the noise is getting too loud.”
Fred gnawed at the bandages around his thumbs. His eyes narrowed and shone.
“Fred, listen. You’re home. You’re safe.”
“I don’t think I am, Ben. I think we hit the integer limit. The machine turned off because the pointer couldn’t find the address any more.” Fred turned his gaze to Ben, his eyes suddenly sharp, lucid, terrified. The refrigerator hummed and the wheat disintegrated in its bowl. “Ben. What happens when a program tries to read freed memory?”
“I don’t know.”
“Neither do I. But I think it probably looks a lot like —”
And then there was a great and overwhelming sound and the room was full of light.
