Timescape

“Just tell him that. And bring my friend here another stout.”

When the waiter had backed away, obviously unwilling to confront either Peterson or the manager, Markham murmured, “Great. How’d you know I’d prefer another stout?”

“Intuition,” he said with weary camaraderie.

•  •  •

They had both had more drinks when Peterson said, “Look, it’s Sir Martin who’s really the technical type on the British delegation. I’m a nonspecialist, as they call it. What I want to know is, how in hell do you get around this grandfather paradox bit? That fellow Davies explained about the discovery of tachyons right enough, and I accept that they can travel into our past, but I still can’t see how one can logically change the past.”

Markham sighed. “Until tachyons were discovered, everybody thought communication with the past was impossible. The incredible thing is that the physics of time communication had been worked out earlier, almost by accident, as far back as the 1940s. Two physicists named John Wheeler and Richard Feynmann worked out the correct description of light itself, and showed that there were two waves launched whenever you tried to make a radio wave, say.”

“Two?”

“Right. One of them we receive on our radio sets. The other travels backward in time—the ‘advanced wave,’ as Wheeler and Feynmann called it.”

“But we don’t receive any message before it’s sent.”

Markham nodded. “True—but the advanced wave is there, in the mathematics. There’s no way around it. The equations of physics are all time-symmetric. That’s one of the riddles of modern physics. How is it that we perceive time passing, and yet all the equations of physics say that time can run either way, forward or backward?”

“The equations are wrong, then?”

“No, they’re not. They can predict anything we can measure—but only as long as we use the ‘retarded wave,’ as Wheeler and Feynmann called it. That’s the one that you hear through your radio set.”

“Well, look, surely there’s a way to change the equation round until you get only the retarded part.”

“No, there isn’t. If you do that to the equations, there’s no way to keep the retarded wave the same. You must have the advanced wave.”

“All right, where are those backward-in-time radio shows? How come I can’t tune into the news from the next century?”

“Wheeler and Feynmann showed that it can’t get here.”

“Can’t get into this year? I mean, into our present time?”

“Right. See, the advanced wave can interact with the whole universe—it’s moving back, into our past, so it eventually hits all the matter that’s ever born. Thing is, the advanced wave strikes all that matter before the signal was sent.”

“Yes, surely.” Peterson reflected on the fact that he was now, for the sake of argument, accepting the “advanced wave” he would have rejected only a few moments before.

“So the wave hits all that matter, and the electrons inside it jiggle around in anticipation of what the radio station will send.”

“Effect preceding a cause?”

“Exactly. Seems contrary to experience, doesn’t it?”

“Definitely.”

“But the vibration of those electrons in the whole rest of the universe has to be taken into account. They in turn send out both advanced and retarded waves. It’s like dropping two rocks into a pond. They both send out waves. But the two waves don’t just add up in a simple way.”

“They don’t? Why not?”

“They interfere with each other. They make a crisscross network of local peaks and troughs. Where the peaks and troughs from the separate patterns coincide, they reinforce each other. But where the peaks of the first stone meet the troughs of the second, they cancel. The water doesn’t move.”

“Oh. All right, then.”

“What Wheeler and Feynmann showed was that the rest of the universe, when it’s hit by an advanced wave, acts like a whole lot of rocks dropped into that pond. The advanced wave goes back in time, makes all these other waves. They interfere with each other and the result is zero. Nothing.”

“Ah. In the end the advanced wave cancels itself out.”

Suddenly music blared over the Whim’s stereo: “An’ de Devil, he do de dance whump whump with Joan de Arc—”

Peterson shouted, “Turn that down, will you?”

The music faded. He leaned forward. “Very well. You’ve shown me why the advanced wave doesn’t work. Time communication is impossible.”

Markham grinned. “Every theory has a hidden assumption. The trouble with the Wheeler and Feynmann model was that all those jiggling electrons in the universe in the past might not send back just the right waives. For radio signals, they do. For tachyons they don’t. Wheeler and Feynmann didn’t know about tachyons; they weren’t even thought of until the middle ‘60s. Tachyons aren’t absorbed the right way. They don’t interact with matter the way radio waves do.”

“Why not?”

“There different kinds of particles. Some guys named Feinberg and Sudarshan imagined tachyons decades ago, but nobody could find them. Seemed too unlikely. They have imaginary mass, for one thing.”

“Imaginary mass?”

“Yes, but don’t take it too seriously.”

“Seems a serious difficulty.”

“Not really. The mass of these particles isn’t what we’d call an observable. That means we can’t bring a tachyon to rest, since it must always travel faster than light. So, if we can’t bring it to a stop in our lab, we can’t measure its mass at rest. The only definition of mass is what you can put on the scales and weigh—which you can’t do, if it’s moving. With tachyons, all you can measure is momentum—that is, impact.”

“You have a complaint about the food, sir? I am the manager.”

Peterson looked up to find a tall man in a conservative gray suit standing over their table, hands clasped behind him military style. “Yes, I did. Mostly I preferred not to eat it, in view of what it did to that lady a short while ago.”

“I do not know what the lady was eating, sir, but I should think your—”

“Well, I do, you see. It was certainly close enough to what my friend here ordered to make him uncomfortable.”

The manager bridled slightly at Peterson’s manner. He was sweating slightly and had a harried look. “I fail to see why a similar type of food should—”

“I can see it quite plainly. A pity you can’t.”

“I am afraid we shall have to charge you for—”

“Have you read the recent Home Office directives on imported meats? I had a hand in writing them.” Peterson gave the manager the full benefit of his assessing gaze. “I would say you probably get much of your imported meat from a local supplier, correct?”

“Well, of course, but—”

“Then you presumably know that there is a severe restriction on how long it can be kept before use?”

“Yes, I’m sure…” the manager began, but then hesitated when he saw the look on Peterson’s face. “Well, actually I haven’t read much of those lately because—”

“I think I would take more care in future.”

“I am not sure the lady actually ate any imported meat whatever—”

“I would look into it, if I were you.”

Abruptly the man lost some of his military bearing. Peterson looked at him with assurance.

“Well, I think we can forget the misunderstanding, sir, in light of—”