Robert Heinlein - Door Into Summer

"A week later one of those cartwheels reappeared. Just one. But before that, one afternoon while I was cleaning up after he had gone home, a guinea pig showed up in the cage. It didn't belong in the lab and I hadn't seen it around before, so I took it over to the bio lab on my way home. They counted and weren't short any pigs, although it's hard to be certain with guinea pigs, so I took it home and made a pet out of it.

"After that single silver dollar came back Twitch got so worked up he quit shaving. Next time he used two guinea pigs from the bio lab. One of them looked awfully familiar to me, but I didn't see it long because he pushed the panic button and they both disappeared.

"When one of them came back about ten days later-the one that didn't look like mine-Twitch knew for sure he had it. Then the resident 0-in-C for the department of defense came around-a chair-type colonel who used to be a professor himself, of botany. Very military type... Twitch had no use for him. This colonel swore us both to double-dyed secrecy, over and above our `status' oaths. He seemed to think that he had the greatest thing in military logistics since Caesar invented the carbon copy. His idea was that you could send divisions forward or back to a battle you had lost, or were going to lose, and save the day. The enemy would never figure out what had happened. He was crazy in hearts and spades, of course... and he didn't get the star he was bucking for. But the `Critically Secret' classification he stuck on it stayed, so far as I know, right up to the present. I've never seen a disclosure on it."

"It might have some military use," I argued, "it seems to me, if you could engineer it to take a division of soldiers at a time. No, wait a minute. I see the hitch. You always had `em paired. It would take two divisions, one to go forward, one to go back. One division you would lose entirely... I suppose it would be more practical to have a division at the right time in the first place."

"You're right, but your reasons are wrong. You don't have to use two divisions or two guinea pigs or two anything. You simply have to match the masses. You could use a division of men and a pile of rocks that weighed as much. It's an action-reaction situation, corollary with Newton's Third Law." He started drawing in the beer drippings again. "MV equals MV ... the basic rocket ship formula. The cognate time-travel formula is MT equals MT."

"I still don't see the hitch. Rocks are cheap."

"Use your head, Danny. With a rocket ship you can aim the kinkin' thing. But which direction is last week? Point to it. Just try. You haven't the slightest idea which mass is going back and which one is going forward. There's no way to orient the equipment."

I shut up. It would be embarrassing to a general to expect a division of fresh shock troops and get nothing but a pile of gravel. No wonder the ex-prof never made brigadier. But Chuck was still talking:

"You treat the two masses like the plates of a condenser, bringing them up to the same temporal potential. Then you discharge them on a damping curve that is effectively vertical. Smacko!-one of them heads for the middle of next year, the other one is history. But you never know which one. But that's not the worst of it; you can't come back."

"Look, what use is it for research if you can't come back? Or for commerce? Either way you jump, your money is no good and you can't possibly get in touch with where you started. No equipment-and believe me it takes equipment and power. We took power from the Arco reactors. Expensive... that's another drawback."

"You could get back," I pointed out, "with cold sleep."

"Huh? If you went to the past. You might go the other way; you never know. If you went a short enough time back so that they had cold sleep... no farther back than the war. But what's the point of that? You want to know something about 1980, say, you ask somebody or you look it up in old newspapers. Now if there was some way to photograph the Crucifixion...but there isn't. Not possible. Not only couldn't you get back, but there isn't that much power on the globe. There's an inverse-square law tied up in it too."

"Nevertheless, some people would try it just for the hell of it. Didn't anybody ever ride it?"

Chuck glanced around again. "I've talked too much already."

"A little more won't hurt."

"I think three people tried it. I think. One of them was an instructor. I was in the lab when Twitch and this bird, Leo Vincent, came in; Twitch told me I could go home. I hung around outside. After a while Twitch came Out and Vincent didn't. So far as I know, he's still in there. He certainly Wasn't teaching at Boulder after that."

"How about the other two?"

"Students. They all three went in together; only Twitch came Out. But one of them was in class the next day, whereas the other one was missing for a week. Figure it out yourself."

"Weren't you ever tempted?"

"Me? Does my head look fiat? Twitch suggested that it was almost my duty, in the interests of science, to volunteer. I said no, thanks; I'd take a short beer instead... but that I would gladly throw the switch for him. He didn't take me up on it."

"I'd take a chance on it. I could check up on what's worrying me... and then come back again by cold sleep. It would be worth it."

Chuck sighed deeply. "No more beer for you, my friend; you're drunk. You didn't listen to me. One,"-he started making tallies on the table top-"you have no way of knowing that you'd go back; you might go forward instead."

"I'd risk that. I like now a lot better than I liked then; I might like thirty years from now still better."

"Okay, so take the Long Sleep again; it's safer. Or just sit tight and wait for it to roll around; that's what I'm going to do. But quit interrupting me. Two, even if you did go back, you might miss 1970 by quite a margin. So far as I know, Twitch was shooting in the dark; I don't think he had it calibrated. But of course I was just the flunky. Three, that lab was in a stand of pine trees and it was built in 1980. Suppose you come out ten years before it was built in the middle of a western yellow pine? Ought to make quite an explosion, about like a cobalt bomb, huh? Only you wouldn't know it."

"But- As a matter of fact, I don't see why you would come out anywhere near the lab. Why not to the spot in outer space corresponding to where the lab used to be-I mean where it was.

or rather--"

"You don't mean anything. You stay on the world line you were on. Don't worry about the math; just remember what that guinea pig did. But if you go back before the lab was built, maybe you wind up in a tree. Four, how could you get back to now even with cold sleep, even if you did go the right way, arrive at the right time, and live through it?"

"Huh? I did once, why not twice?"

"Sure. But what are you going to use for money?"

I opened my mouth and closed it. That one made me feel foolish. I had had the money once; I had it no longer. Even what I had saved (not nearly enough) I could not take with me-shucks, even if I robbed a bank (an art I knew nothing about) and took a million of the best back with me, I couldn't spend it in 1970. I'd simply wind up in jail for trying to shove funny money. They had even changed the shape, not to mention serial numbers, dates, colors, and designs. "Maybe I'd just have to save it up."

"Good boy. And while you were saving it, you'd probably wind up here and now again without half trying... but minus your hair and your teeth."

"Okay, okay. But let's go back to that last point. Was there ever a big explosion on that spot? Where the lab was?"

"No, I don't think so."

"Then I wouldn't wind up in a tree-because I didn't. Follow me?"

"I'm three jumps ahead of you. The old time paradox again, only I won't buy it. I've thought about theory of time, too, maybe more than you have. You've got it just backward. There wasn't any explosion and you aren't going to wind up in a tree... because you aren't ever going to make the jump. Do you follow me?"