'Is that so? I see. Any ideas, yourself, as to why he's gone, or where he's gone?'

'Nothing concrete,' he said pursing his lips, 'but everyone knows he is a sick man. He had a stroke two years ago that kept him out of classes for a semester. He never did get well. His left side was paralyzed for a while and he still limps. Another stroke would kill him. It could come any time.'

'You think he's dead, then?'

'It's not impossible.'

'But Where's the body, then?'

'Well, really - That is your job, I think.'

It was, and I left.

I interviewed each one of Tywood's four research students in a volume of chaos called a research laboratory. These student research laboratories usually have two hopefuls working therein, said two constituting a floating population, since every year or so they are alternately replaced.

Consequently, the laboratory has its equipment stack in tiers. On the laboratory benches is the equipment immediately being used, and in three or four of the handiest drawers are replacements or supplements which are likely to be used. In the farther drawers, on the shelves reaching up to the ceiling, in odd corners, are fading remnants of the past student generations - oddments never used and never discarded. It is claimed, in fact, that no research student ever knew all the contents of his laboratory.

All four of Tywood's students were worried. But three were worried mainly by their own status. That is, by the possible effect the absence of Tywood might have on the status of their 'problem.' I dismissed those three - who all have their degrees now, I hope - and called back the fourth.

He had the most haggard look of all, and had been least communicative - which I considered a hopeful sign.

He now sat stiffly in the straight-backed chair at the right of the desk, while I leaned back in a creaky old swivel-chair and pushed my hat off my forehead. His name was Edwin Howe and he did get his degree later on; I know that for sure, because he's a big wheel in the Department of Science now.

I said: 'You do the same work the other boys do, I suppose?'

'It's all nuclear work, in a way.'

'But it's not all exactly the same?'

He shook his head slowly. 'We take different angles. You have to have something clear-cut, you know, or you won't be able to publish. We've got to get our degrees.'

He said it exactly the way you or I might say, 'We've got to make a living." At that, maybe it's the same thing for them.

I said: 'All right. What's your angle?'

He said: 'I do the math. I mean, with Professor Tywood.'

'What kind of math?'

And he smiled a little, getting the same sort of atmosphere about him that I had noticed in Professor Keyset's case that morning. A sort of, 'Do-you-really-think-I-can-explain-all-my-profound-thoughts-to-stupid-little-you?' sort of atmosphere.

All he said aloud, however, was: 'That would be rather complicated to explain.'

'I'll help you,' I said. 'Is that anything like it?' And I tossed the foolscap sheet at him.

He didn't give it any once-over. He just snatched it up and let out a thin wail: 'Where'd you get this?'

'From Tywood's safe.'

'Do you have the rest of it, too?'

'It's safe,' I hedged.

He relaxed a little - just a little: 'You didn't show it to anybody, did you?'

I showed it to Professor Keyser.'

Howe made an impolite sound with his lower lip and front teeth, 'That jackass. What did he say?'

I turned the palms of my hands upward and Howe laughed. Then he said, in an offhand manner: 'Well, that's the sort of stuff I do.'

'And what's it all about? Put it so I can understand it.'

There was distinct hesitation. He said: 'Now, look. This is confidential stuff. Even Pop's other students don't know anything about it. I don't even think I know all about it. This isn't just a degree I'm after, you know. It's Pop Tywood's Nobel Prize, and it's going to be an Assistant Professorship for me at Cal Tech. This has got to be published before it's talked about.'

And I shook my head slowly and made my words very soft: 'No, son. You have it twisted. You'll have to talk about it before it's published, because Tywood's gone and maybe he's dead and maybe he isn't. And if he's dead, maybe he's murdered. And when the department has a suspicion of murder, everybody talks. Now, it will look bad for you, kid, if you try to keep some secrets.'

It worked. I knew it would, because everyone reads murder mysteries and knows all the cliches. He jumped out of his chair and rattled the words off as if he had a script in front of him.

'Surely,' he said, 'you can't suspect me of… of anything like that. Why… why, my career -'

I shoved him back into his chair with the beginnings of a sweat on his forehead. I went into the next line: 'I don't suspect anybody of anything yet. And you won't be in any trouble, if you talk, chum.'

He was ready to talk. 'Now this is all in strict confidence.'

Poor guy. He didn't know the meaning of the word 'strict' He was never out of eyeshot of an operator from that moment till the government decided to bury the whole case with the one final comment of '?' Quote. Unquote. (I'm not kidding. To this day, the case is neither opened nor closed. It's just '?')

He said, dubiously; 'You know what time travel is, I suppose?'

Sure I knew what time travel was. My oldest kid is twelve and he listens to the afternoon video programs till he swells up visibly with the junk he absorbs at the ears and eyes.

'What about time travel?' I said.

'In a sense, we can do it. Actually, it's only what you might call micro-temporal-translation -'

I almost lost my temper. In fact, I think I did. It seemed obvious that the squirt was trying to diddle me; and without subtlety. I'm used to having people think I look dumb; but not that dumb.

I said through the back of my throat: 'Are you going to tell me that Tywood is out somewhere in time - like Ace Rogers, the Lone Time Ranger?' (That was Junior's favorite program Ace Rogers was stopping Genghis Khan single-handed that week.)

But he looked as disgusted as I mast have. 'No,' he yelled. 'I don't know where Pop is. If you'd listen to me - I said micro-temporal-translation. Now, this isn't a video show and it isn't magic; this happens to be science. For instance, you know about matter-energy equivalence, I suppose.'

I nodded sourly. Everyone knows about that since Hiroshima in the last war but one.

'All right, then,' he went on, 'that's good for a start. Now, if you take a known mass of matter and apply temporal translation to it - you know, send it back in time - you are, in effect, creating matter at the point in time to which you are sending it. To do that, you must use an amount of energy equivalent to the amount of matter you have created. In other words, to send a gram - or, say, an ounce - of anything back in time, you have to disintegrate an ounce of matter completely, to furnish the energy required.'

'Hm-m-m,' I said, 'that's to create the ounce of matter in the past. But aren't you destroying an ounce of matter by removing it from the present? Doesn't that create the equivalent amount of energy?'

And he looked just about as annoyed as a fellow sitting on a bumblebee that wasn't quite dead. Apparently laymen are never supposed to question scientists.

He said: 'I was trying to simplify it so you would understand it. Actually, it's more complicated. It would be very nice if we could use the energy of disappearance to cause it to appear, but that would be working in a circle, believe me. The requirements of entropy would forbid it. To put it more rigorously, the energy is required to overcome temporal inertia and it just works out so that the energy in ergs required to send back a mass, in grams, is equal to the mass times the square of the speed of light in centimeters per second. Which just happens to be the Einstein Mass-Energy Equivalence Equation. I can give you the mathematics, you know.'

'I know,' I waxed some of that misplaced eagerness back. 'But was all this worked out experimentally? Or is it just on paper?'

Obviously, the thing was to keep him talking.

He had that queer light in his eye that every research student gets, I am told, when he is asked to discuss his problem. He'll discuss it with anyone, even with a 'dumb flatfoot' -which was convenient at the moment.

'You see,' he said like a man slipping you the inside dope on a shady business deal, 'what started the whole thing was this neutrino business. They've been trying to find that neutrino since the late thirties and they haven't succeeded. It's a subatomic particle which has no charge and has a mass much less than even an electron. Naturally, it's next to impossible to spot, and hasn't been spotted yet. But they keep looking because, without assuming that a neutrino exists, the energetics of some nuclear reactions can't be balanced. So Pop Tywood got the idea about twenty years ago that some energy was disappearing, in the form of matter, back into time. We got working on that - or he did - and I'm the first student he's ever had tackle it along with him.

'Obviously, we had to work with tiny amounts of material and… well, it was just a stroke of genius on Pop's part to think of using traces of artificial radioactive isotopes. You could work with just a few micrograms of it, you know, by following its activity with counters. The variation of activity with time should follow a very definite and simple law which, has never been altered by any laboratory condition known.

'Well, we'd send a speck back fifteen minutes, say, and fifteen minutes before we did that - everything was arranged automatically, you see - the count jumped to nearly double what it should be, fell off normally, and then dropped sharply at the moment it was sent back below where it would have been normally. The material overlapped itself in time, you see, and for fifteen minutes we counted the double material -'

I interrupted: 'You mean you had the same atoms existing in two places at the same time.'

'Yes,' he said, with mild surprise, 'why not? That's why we use so much energy - the equivalent of creating those atoms.' And then he rushed on, 'Now I'll tell you what my particular job is. If you send back the material fifteen minutes, it is apparently sent back to the same spot relative to the Earth despite the fact that in fifteen minutes, the Earth moved sixteen thousand miles around the Sun, and the Sun itself moves more thousand miles and so on. But there are certain tiny discrepancies which I've analyzed and which turn out to be due, possibly, to two causes.

'First, there is a factional effect - if you can use such a term - so the matter does drift a little with respect to the Earth, depending on how far back in time it is sent, and on the nature of material. Then, too, some of the discrepancy can only be explained by the assumption that passage through time itself takes time.'