"What?"

He suddenly sobered. "I shouldn't have said that."

I said, "Maybe not, but you already have said it. Now you'd better tell me what you mean before I empty this here stein over your head."

"Forget it, Dan. I made a slip."

"Talk!"

"That's just what I can't do." He glanced around. No one was near us. "It's classified."

"Time travel classified? Good God, why?"

"Hell, boy, didn't you ever work for the government? They'd classify sex if they could. There doesn't have to be a reason; it's just their policy. But it is classified and I'm bound by it. So lay off."

"But-Quit fooling around about it, Chuck; this is important to me. Terribly important." When he didn't answer and looked stubborn I said, "You can tell me. Shucks, I used to have a 'Q' clearance myself. Never suspended, either. It's just that I'm no longer with the government."

"What's a `Q' clearance?"

I explained and presently he nodded. "You mean an `Alpha' status. You must have been hot stuff, boy; I only rated a `Beta.'"

"Then why can't you tell me?"

"Huh? You know why. Regardless of your rated status, you don't have the necessary `Need to Know' qualification."

"The hell I don't! `Need to Know' is what I've got most of."

But he wouldn't budge, so finally I said in disgust, "I don't think there is such a thing. I think you just had a belch back up on you."

He stared at me solemnly for a while, then he said, "Danny."

"Huh?"

"I'm going to tell you. Just remember your `Alpha' status, boy. I'm going to tell you because it can't hurt anything and I want you to realize that it couldn't possibly be of use to you in your problem. It's time travel, all right, but it's not practical. You can't use it."

"Why not?"

"Give me a chance, will you? They never smoothed the bugs out of it and it's not even theoretically possible that they ever will. It's of no practical value whatsoever, even for research. It's a mere by-product of NullGrav-that's why they classified it."

"But, hell, NullGrav is declassified."

"What's that got to do with it? If this was commercial, too, maybe they'd unwrap it. But shut up."

I'm afraid I didn't, but I'd rather tell this as if I had. During Chuck's senior year at the University of Colorado-Boulder, that is-he had earned extra money as a lab assistant. They had a big cryogenics lab there and at first he had worked in that. But the school had a juicy defense contract concerned with the Edinburgh field theory and they had built a big new physics laboratory in the mountains out of town. Chuck was reassigned there to Professor Twitchell-Dr. Hubert Twitchell, the man who just missed the Nobel Prize and got nasty about it.

"Twitch got the notion that if he polarized around another axis he could reverse the gravitational field instead of leveling it off. Nothing happened. So he fed what he had done back into the computer and got wild-eyed at the results. He never showed them to me, of course. He put two silver dollars into the test cageÄthey still used hard money around those parts then-after making me mark them. He punched the solenoid button and they disappeared.

"Now that is not much of a trick," Chuck went on. `Properly, he should have followed up by making them reappear out of the nose of a little boy who volunteers to come up on the stage. But he seemed satisfied, so I was-I was paid by the hour.

"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."