Timescape Read Free

poles of a large magnet.
    "Superconducting setup, of course. We need the high field strength to get a nice, sharp line during transmission."
    Peterson studied the maze of wires and meters. Cabinets housing rank upon rank of electronics towered over them. He pointed out a particular object and asked its function.
    "Oh, I didn't think you'd be wanting to know much of the technical side," Renfrew said.
    "Try me."
    "Well, we've got a large indium antimonide sample in there, see."
    Renfrew pointed at the encased volume between the magnet poles. "We hit it with high-energy ions. When the ions strike the indium they give off tachyons. It's a complex, very sensitive ion-nuclei reaction." He glanced at Peterson.
    "Tachyons are particles that travel faster than light, you know. On the other side,–" he pointed around the magnets, leading Peterson to a long blue cylindrical tank that protruded ten meters away from the magnets.
    "–we draw out the tachyons and focus them into a beam. They have a particular energy and spin, so they resonate only with indium nuclei in a strong magnetic field."
    "And when they hit something in the way?"
    "That's the point," Renfrew said sharply. "Tachyons have to strike a nucleus in precisely the correct state of energy and spin before they lose any energy in the process. They pass right through ordinary matter. That's why we can shoot them across light years without having them scattered out of their path."
    Peterson said nothing. He scowled at the equipment.
    "But when one of our tachyons strikes an indium nucleus in precisely the right state– a situation that doesn't occur naturally very often–it will be absorbed. That tips the spin of the indium nucleus away from wherever it was pointing. Think of the indium nucleus as a little arrow that gets knocked to the side. If all the little arrows were pointing in one direction before the tachyons arrived, then they would get disordered. That would be noticeable and–"
    "I see, I see," Peterson said disdainfully. Renfrew wondered if he had overdone that bit about little arrows. It would be fatal if Peterson thought he was talking down to him–which of course he was.
    "That's some other fellow's indium, I suppose?"
    Renfrew held his breath. This was the tricky part. "Yes. An experiment operating in the year 1963," he said slowly.
    Peterson said drily, "I read the preliminary report. These prelims are often deceptive, but I understood that. The technical staff tell me it makes sense, but I can't believe some of the things you've written. This business of altering the past–"
    "Look, there's this fellow Markham coming–he'll put you straight on that."
    "If he can."
    "Right. See, the reason nobody's even tried to send messages back is an obvious one, once you think of it. We can build a transmitter, see, but there's no receiver. Nobody in the past ever built one."
    Peterson frowned. "Well, of course–"
    Renfrew went on enthusiastically. "We've built one, naturally, to do our preliminary experiments. But the people back in 1963 didn't know about tachyons. So the trick is to interfere with something they're already doing.
    That's the ticket."
    "Um."
    "We try to concentrate bursts of tachyons and aim them just so–"
    "Hold on," Peterson said, putting up a hand. "Aim for what? Where is 1963?"
    "Quite far away, as it works out. Since 1963, the earth's been going round the sun, while the sun itself is revolving around the hub of the galaxy, and so on. Add that up and you find 1963 is pretty distant."
    "Relative to what?"
    "Well, relative to the center of mass of the local group of galaxies, of course. Mind, the local group is moving, too, relative to the frame of reference provided by the microwave radiation background, and—"
    "Look, skip the jargon, can't you? You're saying 1963 is in the sky somewhere?"
    "Quite so. We send out a beam of tachyons to hit that spot. We sweep the volume of space occupied by the earth at that particular time."
    "Sounds