Gregory Benford - Timescape

“Ah!” Peterson suddenly saw it. “So the receiver in turn gets a weaker signal from the future. The switch isn’t hit so hard because the backward-in-time signal is weaker. So it doesn’t move so quickly toward the ‘off’ mark.”

“That’s it. The closer it gets to ‘off,’ the slower it goes. There’s an information wave traveling forward into the future, and—like a reflection—the tachyon beam comes back into the past.”

“What does the experiment do then?”

“Well., say the switch gets near ‘off,’ and then the tachyon beam gets weak. The switch doesn’t make it all the way to ‘off’ and—like that toggle controlling the lights—it starts to fall back toward ‘on.’ But the nearer it gets to ‘on,’ the stronger the transmitter gets in the future.”

“So the tachyon beam gets stronger,” Peterson finished for him. “That in turn drives the switch away from ‘on’ and back towards ‘off.’ The switch is hung up in the middle.”

Markham leaned back and drained his stout. His tan, weakened by the dim Cambridge winter, crinkled with the lines of his wry smile. “It flutters around there in the middle.”

“No paradox.”

“Well…” Markham shrugged imperceptibly. “No logical contradictions, yes. But we still don’t actually know what that intermediate, hung-up state means. It does avoid the paradoxes, though. There’s a lot of quantum-mechanical formalism you can apply to it, but I’m not sure what a genuine experiment will give.”

“Why not?”

Markham shrugged again. “No experiments. Renfrew hasn’t had the time to do them, or the money.”

Peterson ignored the implied criticism; or was that his imagination? It was obvious that work in these fields had been cut back for years now. Markham was simply stating a fact. He had to remember that a scientist might be more prone simply to state things as they were, without calculating a statement’s impact. To change the subject Peterson asked, “Won’t that stuck-in-the-middle effect prevent your sending information back to 1963?”

“Look, the point here is that our distinctions between cause and effect are an illusion. This little experiment we’ve been discussing is a causal loop —no beginning, no end. That’s what Wheeler and Feynmann meant by requiring only that our description be logically consistent. Logic rules in physics, not the myth of cause and effect. Imposing an order to events is our point of view. A quaintly human view, I suppose. The laws of physics don’t care. That’s the new concept of time we have now—as a set of completely interrelated events, linked self-consistently. We think we’re moving along in time, but that’s just a bias.”

“But we know things happen now , not in the past or future.”

“When is ‘now’? Saying that ‘now’ is ‘this instant’ is going around in circles. Every instant is ‘now’ when it ‘happens.’ The point is, how do you measure the rate of moving from one instant to the next? And the answer is, you can’t. What’s the rate of the passage of time?”

“Well, it’s—” Peterson stopped, thinking.

“How can time move? The rate is one second of movement per second! There’s no conceivable coordinate system in physics from which we can measure time passing. So there isn’t any. Time is frozen, as far as the universe is concerned.”

“Then…” Peterson raised a finger to cover his confusion, frowning. The manager appeared as though out of nowhere.

“Yes sir?” the man said with extreme politeness.

“Ah, another round.”

“Yes sir.” He hustled off to fill the order himself. Peterson took a small pleasure in this little play. To get such a response with a minimum display of power was an old game with him, but still satisfying.

“But you still believe,” Peterson said, turning back to Markham, “that Renfrew’s experiment makes sense? All this talk of loops and not being able to close switches…”

“Sure it’ll work.” Markham accepted a glass dark with the thick stout. The manager placed Peterson’s ale carefully before him and began, “Sir, I want to apol—”

Peterson waved him into silence, impatient to hear Markham. “Perfectly all right,” he said quickly.

Markham eyed the manager’s retreating back. “Very effective. Do they teach that in the best schools?”

Peterson smiled. “Of course. There’s lecture, then field trips to representative restaurants. You have to get the wrist action just right.”

Markham saluted with the stout. After this silent toast he said, “Oh yes, Renfrew. What Wheeler and Feynmann didn’t notice was that if you send a message back which has nothing to do with shutting off the transmitter, there’s no problem. Say I want to place a bet on a horse race. I’ve resolved that I’ll send the results of the race back in time to a friend. I do. In the past, my friend places a bet and makes money. That doesn’t change the outcome of the race. Afterward, my friend gives me some of the winnings. His handing over the money won’t stop me from sending the information—in fact, I can easily arrange it so I only get the money after I’ve sent the message.”

“No paradox.”

“Right. So you can change the past, but only if you don’t try to make a paradox. If you try, the experiment hangs up in that stuck-in-between state.”

Peterson frowned. “But what’s it like? I mean, what does the world seem like if you can change it round?”

Markham said lightly, “Nobody knows. Nobody’s ever tried it before.”

“There were no tachyon transmitters until now.”

“And no reason to try to reach the past, either.”

“Let me get this straight. How’s Renfrew going to avoid creating a paradox? If he gives them a lot of information, they’ll solve the problem and there’ll be no reason for him to send the message.”

“That’s the trick. Avoid the paradox, or you’ll get a stuck switch. So Renfrew will send a piece of the vital information—enough to get research started, but not enough to solve the problem utterly.”

“But what’ll it be like for us? The world will change round us?”

Markham chewed at his lower lip. “I think so. We’ll be in a different state. The problem will be reduced, the oceans not so badly off.”

“But what is this state? I mean, us sitting here? We know the oceans are in trouble.”

“Do we? How do we know this isn’t the result of the experiment we’re about to do? That is, if Renfrew hadn’t existed and thought of this idea, maybe we’d be worse off. The problem with causal loops is that our notion of time doesn’t accept them. But think of that stuck switch again.”

Peterson shook his head as though to clear it. “It’s hard to think about.”

“Like tying time in knots,” Markham conceded. “What I’ve given you is an interpretation of the mathematics. We know tachyons are real; what we don’t know is what they imply.”

Petersen looked around at the Whim, now mostly deserted. “Strange, to think of this as being an outcome of what we haven’t done yet. All looped together, like a hooked rug.” He blinked, thinking of the past, when he had eaten here. “That coal stove—how long have they had that?”

“Years, I suppose. Seems like a sort of trademark. Keeps the place warm in winter, and it’s cheaper than gas or electricity Besides, they can cook at any time of day, not just the power hours. And it gives the customers something to watch while they’re waiting for their orders.”

“Yes, coal’s the long-term fuel for old England,” Peterson murmured, apparently more to himself than Markham. “Bulky though.”