Isaac Asimov: Buy Jupiter and Other Stories

“But only if very delicately handled.

“If the endochronic effect lags a little behind the timedilatation effect, the ship will return after two months to find an Earth four months older. This is not much, perhaps; it can be lived with, you might think; but not so. The crew members are out of phase. They feel everything about them to have aged two months with respect to themselves. Worse yet, the general population feels that the crew members are two months younger than they ought to be. It creates hard feelings and discomforts.

“Similarly, if the endochronic effect races a little ahead of the time-dilatation effect, the ship may return after two months to find an Earth that has not experienced any time duration at all. The ship returns, just as it is rising into the sky. The hard feelings and discomforts will still exist.

“No, graduates, no interstellar flight will be considered successful in this star fleet unless the duration to the crew and the duration to Earth match minute for minute. A sixty-second deviation is a sloppy job that will gain you no merit. A hundred-twenty-second deviation will not be tolerated.

“I know, graduates, very well what questions are going through your minds. They went through mine when I graduated. Do we not in the endochronic ship have the equivalent of a time machine? Can we not, by proper adjustment of our endochronic device, deliberately travel a century into the future, make our observations, then travel a century into the past to return to our starting point? Or vice versa, can we not travel a century into the past and then back into the future to the starting point? Or a thousand years, or a billion? Could we not witness the Earth being born, life evolving, the sun dying?

“Graduates, the mathematical-johnnies tell us that this sort of thing creates paradoxes and requires too much energy to be practical. But 1 tell you the hen with paradoxes. We can't do it for a very simple reason. The endochronic properties are unstable. Molecules that are puckered into the time dimension are sensitive indeed. Relatively small effects will cause them to undergo chemical changes that will allow unpuckering. Even if there are no effects at all, random vibrations will produce the changes that will unpucker them.

“In short, an endochronic ship will slowly go isochronic and become ordinary matter without temporal extension. Modem technology has reduced the rate of unpuckering enormously and may reduce it further still, but nothing we do, theory tells us, will ever create a truly stable endochronic molecule.

“This means that your starship has only a limited life as a starship. It must get back to Earth while its endochronicity still holds, and that endochronicity must be restored before the next trip.

“Now, then, what happens if you return out-of-time? If you are not very nearly in your own time, you will have no assurance that the state of the technology will be such as to enable you to re-endochronicize your ship. You may be lucky if you are in the future; you will certainly be unlucky in the past. If, through carelessness on your part, or simply through lack of talent, you come back a substantial distance into the past, you will be certain to be stuck there because there will be no way of treating your ship in such a fashion as to bring it back into what will then be your future.

“And I want you to understand, graduates,” here he slapped one hand against the other, as though to emphasize his words, “there is no time in the past where a civilized astronautic officer would care to spend his life. You might, for instance, be stranded in sixth-century France or, worse still, twentieth-century America.

“Refrain, then, from any temptation to experiment with time.

“Let us now pass on to one more point which may not have been more than hinted at in your formal school days, but which is something you will be experiencing.

“You may wonder how it is that a relatively few endochronic atomic bonds placed here and there among matter which is overwhelmingly isochronic can drag an with it. Why should one endochronic bond, racing toward water, drag with it a quadrillion atoms with isochronic, bonds? We feel this should not happen, because of our lifelong experience with inertia.

“There is, however, no inertia in the movement toward past or future. If one part of an object moves toward the past or future, the rest of the object does so as well, and at precisely the same speed. There is no mass-factor at all. That is why it is as easy for the entire universe to move backward in time as for this single ship to move forward-and at the same rate.

“But there is even more to it than that. The time-dilatation effect is the result of your acceleration with respect to the universe generally. You learned that in grade school, when you took up elementary relativistic physics. It is part of the inertial effect of acceleration.

“But by using the endochronic effect, we wipe out the time-dilatation effect. If we wipe out the time-dilatation effect, then we are, so to speak, wiping out that which produces it. In short, when the endochronic effect exactly balances the time-dilatation effect, the inertial effect of acceleration is canceled out.

“You cannot cancel out one inertial effect without canceling them all. Inertia is therefore wiped out altogether and you can accelerate at any rate without feeling it. Once the endochronic effect is well-adjusted, you can accelerate from rest relative to Earth, to 186,000 miles per second relative to Earth in anywhere from a few hours to a few minutes. The more talented and skillful you are at handling the endochronic effect, the more rapidly you can accelerate.

“You are experiencing that now, gentlemen. It seems to you that you are sitting in an auditorium on the surface of the planet Earth, and I'm sure that none of you has had any reason or occasion to doubt the truth of that impression. But it's wrong just the same.

“You are in an auditorium, I admit, but it is not on the surface of the planet, Earth; not anymore. You-I-all of us-are in a large starship, which took off the moment I began this speech and which accelerated at an enormous rate. We reached the outskirts of the solar system while I've been talking, and we are now returning.

“ At no time have any of you felt any acceleration, either through change in speed, change in direction of travel, or both, and therefore you have all assumed that you have remained at rest with respect to the surface of the Earth.

“Not at all, graduates. You have been out in space all the time I was talking, and have passed, according to calculations, within two million miles of the planet Saturn.”

He seemed grimly pleased at the distinct stir in the audience.

“You needn't worry, graduates. Since we experience no inertial effects, we experience no gravitational effects either (the two are essentially the same), so that our course has not been affected by Saturn. We will be back on Earth's surface any moment now. As a special treat we will be coming down in the United Nations Port in Lincoln, Nebraska, and you will all be free to enjoy the pleasures of the metropolis for the weekend.

“Incidentally, the mere fact that we have experienced no inertial effects at all shows how well the endochronic effect matched the time-dilatation. Had there been any mismatch, even a small one, you would have felt the effects of acceleration-another reason for making no effort to experiment with time.

“Remember, graduates, a sixty-second mismatch is sloppy and a hundred-twenty-second mismatch is intolerable. We are about to land now; Lieutenant Prohorov, will you take over in the conning tower and oversee the actual landing?”

Prohorov said briskly, “Yes, sir,” and went up the ladder in the rear of the assembly hall, where he had been sitting.