From ``Time for the Stars'', by Robert A. Heinlein.

Pat was sounding out ticks like a clock; I was told to repeat them ... and every time I did another peak of light flashed on the display scope. Babcock watched it, then turned me around so that I couldn't see and taped a microphone to my voice box. ``Again.''

Pat said, ``Stand by--'' and started ticking again. I did my best to tick right with him but it was the silliest performance possible. I heard Babcock say quietly, ``That cut out the feedback and the speed-of-sound lag. I wish there were some way to measure the synaptic rate more closely.''

Frick said, ``Have you talked to Dev about it?''

I went on ticking.

`` A reverse run now, young lady,'' Babcock said, and slipped headphones on me. I immediately heard a ticking like the ticks Pat had been sending. ``That's a spectral metronome you're listening to, young fellow, times by monochrome light. It was synchronized with the one your brother is using before we left Earth. Now start ticking at him.''

So I did. It had a hypnotic quality; it was easier to get into step and tick with it than it was to get out of step. It was impossible to ignore it. I began to get sleepy but I kept on ticking; I couldn't stop.

``End of run,'' Babcock announced. The ticking stopped and I rubbed my ears.

``Dr. Babcock?''

``Huh?''

``How can you tell one tick from another?''

``Eh? You can't. But O'Toole can, he's got it all down on film. Same at the other end. Don't worry about it; just try to stay in time.''

This silliness continued for more than an hour, sometimes with Pat sending, sometimes myself. At last O'Toole looked up and said, ``Fatigue factor is cooking our goose, Doc. The second differences are running all over the lot.''

``Okay, that's all,'' Babcock announced. He turned to me. ``You can thank your brother for me and sign off.''

Commander Frick and Anna left. I hung around. Presently Dr. Babcock looked up from his desk and said, ``You can go, bub. Thanks.''

``Uh, Dr. Babcock?''

``Huh? Speak up.''

``Would you mind telling me what this is all about?''

He looked surprised, then said, ``Sorry. I'm not used to using people instead of instruments; I forget. Okay, sit down. This is why you m-r people were brought along; for research into the nature of time.''

I stared. ``Sir? I thought we were along to report back on the planets we expect to find.''

``Oh, that-- Well, I suppose so, but this is much more important. There are too many people as it is; why encourage new colonies? A mathematician could solve the population problem in jig time just shoot every other one.''

Mr. O'Toole said, without looking up, ``The thing I like about you, Chief, is your big warm heart.''

``Quiet in the gallery, please. Now, today, son, we have been trying to find out what time is.''

I must have looked puzzled as I felt for he went on, ``Oh, we know what time it is . . . but too many different ways. See that?'' He pointed at the display scope, sill tirelessly making a peak every second. ``That's the Greenwich time tick, pulled in by radio and corrected for relative speed and change of speed. Then there is the time you were hearing over the earphones; that is the time the ship runs by. Then there is the time you were getting from your brother and passing it to us. We're trying to compare them all, but the trouble is that we have to have people in the circuit and, while a tenth of a second is a short time for the human nervous system, a microsecond is a measurably long time in physics. Any radar system splits up a microsecond as easily as you slice a pound of butter. So we use a lot of runs to try to even out our ignorance.''

``Yes, but what do you expect to find out?''

``If I `expected,' I wouldn't be doing it. But you might say that we are trying to find out what the word `simultaneous' means.''

Mr. O.Toole looked up from the console. ``If it means anything,'' he amended.

Dr. Babcock glanced at him. ``You still here? `If it means anything.' Son, ever since the great Doctor Einstein, `simultaneous' and `simultaneity' have been dirty words to physicists. We chucked the very concept, denied that it had meaning, and built up a glorious structure of theoretical physics without it. Then you mind readers came along and kicked it over. Oh, don't look guilty; every house needs a housecleaning now and then. If you folks had done your carnival stunt at just the speed of light, we would have assigned you a place in the files and forgotten you. But you rudely insisted on doing it at something enormously greater than the speed of light, which made you as welcome as a pig at a wedding. You've split us physicists into two schools, those who want to class you as a purely psychological phenomenon and no business of physics -- these are the `close your eyes and it will go away' boys -- and a second school which realized that since measurements can be made of whatever this is you do, it is therefore the business of physics to measure and include it . . . since physics is, above all, the trade of measuring things and assigning definite numerical values to them.''

O' Toole said, ``Don't wax philosophical, Chief.''

``You get back to your numbers, O'Toole; you have no soul. These laddies want to measure how fast you do it. They don't care how fast -- they've already recovered from the blow that you do it faster than light -- but they want to know exactly how fast. They can't accept the idea that you do it `instantaneously,' for that would require them to go to a different church entirely. They want to assign a definite speed of propagation, such-and-such number of times faster than the speed of light. Then they can modify their old equations and go right on happily doing business at the old stand.''

``They will,'' agreed O'Toole.

``Then there is a third school of thought, the right one . . . my own.''

O'Toole, without looking up, made a rude noise.

``Is that your asthma coming back?'' Babcock said anxiously. ``By the way, you got any results?''

``They're still doing it in nothing flat. Measured time negative as often as positive and never greater than inherent observational error.''

``Your see, son? That's the correct school. Measure what happens and let the chips fly where they may.''

``Hear, hear!''

``Quiet, you renegade Irishman. Besides that, you m-r's give us our first real chance to check another matter. Are you familiar with the relativity transformations?''

``You mean the Einstein equations?''

``Surely. You know the one for time?''

I thought hard. Pat and I had taken first-year physics our freshman year; it had been quite a while. I picked up a piece of paper and wrote down what I thought it was:

``That's it,'' agreed Dr. Babcock. ``At a relative velocity of `v' time interval at first frame of reference equals time interval at second frame of reference multiplied by the square root of one minus the square of the relative velocity divided by the square of the speed of light. That's just the special case, of course, for constant speeds; it is more complicated for acceleration. But there has been much disagreement as to what the time equations meant, or if they meant anything.''

I blurted out, ``Huh? But I thought the Einstein theory had been proved?'' It suddenly occurred to me that, if the relativity equations were wrong, we were going to be away a mighty long time -- Tau Ceti, our first stop, was eleven light-years from the Sun . . . and that was just our first one; the others were a lot farther.''

But everybody said that once we got up near the speed of light the months would breeze past light days. The equations said so.

``Attend me. How do you prove that there are eggs in a bird's next? Don't strain your gray matter: go climb the tree and find out. There is no other way. Now we are climbing the tree.''

``Fine!'' said O'Toole. ``Go climb a tree.''

``Noisy in here. One school of thought maintained that the equations simply meant that a clock would read differently if you could read it from a passing star . . . which you can't . . . but that there was no real stretching or shrinking of time -- whatever `real' means. Another school pointed to the companion equations for length and mass, maintaining that the famous Michelson-Morley experiment showed that the length transformation was `real' and pointing out that the increase of mass was regularly computed and used for particle-accelerator ballistics and elsewhere in nuclear physics -- for example, in the torch that pushes this ship. So, they reasoned, the change in time rates must be real, because the corollary equations worked in practice. But nobody knew. You have to climb the tree and look.''

``When will we know?'' I was still worrying. Staying several years, Einstein time, in the ship I had counted on Getting killed in the course of it, the way Uncle Steve said we probably would, I refused to worry about. But dying of old age in the Elsiewas not what I had counted on. It was a grim thought, a life sentence shut up inside these steel walls.

``When? Why, we know right now.''

``You do? What's the answer?''

``Don't hurry me, son. We've been gone a couple of weeks, at a boost of 124% of one gee; we're up to 9,000 miles per second now. We still haven't come far -- call it seven and a half light-hours or about 5,450,000,000 miles. It will be the better part of a year before we are crowding the speed of light. Nevertheless we have reached a sizable percentage of that speed, about five per cent; that's enough to show. Easy to measure, with the aid of you mind readers.''

``Well, sir? Is it a real time difference? Or is it just relative?''

``You're using the wrong words. But it's `real;', so far as the word means anything. The ratio right now is about 99.9%.''

``To put it exactly,'' added Mr. O'Toole, ``Bartlett's slippage -- that's a technical term I just invented -- his `slippage' in time rate from that of his twin has now reached twelve parts in ten thousand.''

``So you would make me a liar for one fiftieth of one per cent?'' Babcock complained. ``O'Toole, why did I let you come along?''

``So you would have some one to work your arithmetic,'' his assistant answered smugly.