The Time Traveller by Isaac Asimov

When It Comes to Time Travel, There's No Time Like the Present

By ISAAC ASIMOV

Modern writers who have attempted science fiction or fantasy have, on numerous occasions, ventured on plots in which the hero or heroine has gone back into the past. Mark Twain's ''Connecticut Yankee in King Arthur's Court,'' published in 1889, was the first. Works by such writers as Jack Finney, Ray Bradbury and L. Sprague de Camp followed. Lately, there have been motion pictures on the theme, including ''Back to the Future,'' released in 1985, and ''Peggy Sue Got Married,'' which closes the New York Film Festival tonight and begins its commercial run Wednesday.

Why this interest in going back in time? For one thing, it enables one to have fun with anachronisms, to say nothing of the possibly hilarious confusion of the time traveler who finds himself out of time and of the equal confusion of those who must deal with such a person. More seriously, it enables the writer to lend a sharper point to satire, since the protagonist (and the reader or viewer) knows the future, while all the other characters in the story don't.

Nevertheless, I suspect that the chief point in concocting such a story rests with the fact that human beings have a hankering to return to the past, so that tales dealing with such an event are quite likely to be popular. I'm sure that every one of us has at one time or another felt the urge to step back into the past, even if only briefly. But, before considering why that should be, let me disappoint you.

In many science-fiction stories, the trip into the past is by way of some futuristic machine that can take you through time at will, as an automobile or airplane takes you through space. Such a machine was first used in H. G. Wells's ''Time Machine,'' published in 1895, and one was used in ''Back to the Future.'' That, however, is totally impossible on theoretical grounds. It can't and won't be done.

Nevertheless, even if time travel is not in the cards, does it not remain a beautiful dream?

I must disappoint you again. It's not a beautiful dream, it's a nightmare. Let me explain.

One reason to want to go into the past is to experience youth again. Why not? Youth is better than old age. How wonderful to have young limbs pumping tirelessly again in place of the creaky old body you now have. You may think longingly of the simple joys of youth; the security of being cared for by your parents; the fun of play, and so on, and so on, and so on.

But in order for this to be meaningful, you must go back into the past with your adult memories intact (as was true for both the Con-necticut Yankee and Peggy Sue). If you're a youngster again, but with only the youngster's memory, you simply live your life once again, without any sense of glory in youth and health and fun. If, however, you do have your adult memories, you can appreciate and delight in the change - except that, knowing the future, you know what lies ahead. You know that at such and such a time you will experience a serious accident, or a terrible disease, or have someone you love die. That would make life unbearable, I promise you. It is only because we can't foresee the future that our lives are bearable right now.

Besides that, youth was not the pleasurable time you may think it was. Our memories are treacherous, eliminating the unpleasant and painting the pleasant in unrealistically bright colors. I took the precaution of keeping a diary from the age of 18, so I know this. If you kept a diary, you would, too. Take my advice and be as you are, old and decrepit though you may be. Why subject yourself to disillusion and find that your dearest memories are cobwebs of illusion?

You may decide, of course, that you don't want to go back and relive your youth. You merely want to see your parents again, your little sister, your old cronies, the old swimming hole -whatever.

Thornton Wilder did this magnificently in his drama ''Our Town,'' produced in 1938. The heroine, who had a chance to go back after death, simply broke her heart. She couldn't endure watching everyone live as though life were eternal, and not cherishing each other while they had the chance.

The truth is even less romantic than this. Undoubtedly, you will find that your parents are not quite as you remember them, nor your family, nor your friends, nor your surroundings. Everything will be smaller, dumber, less interesting. Again for your pains, you will have disillusionment. Far from regaining youth for a second time, you will lose what you had (in memory) the first time.

But then, you may not want to go back into the past to relive your own life or to renew your old memories. You might just want to go back to a simpler time - a time before the problems of today, a time before the nuclear threat, terrorism, drugs, traffic tie-ups, pollution and the myriad of ills society seems heir to now. Thus, Jack Finney, in his ''Third Level,'' written in the aftermath of World War II, had his hero go back to the last decades of the 19th century, and leaves him sitting on a porch, sipping cider through a straw in the quiet twilight.

The hero is living a quiet middle-class life. Let him visit the city slums of the 1880's; however, remember that before the Great Depression, the Government of the United States felt no responsibility whatever for the poor. Or let him continue to sip cider until he feels the need for some entertainment, in which case he'd better enjoy a quilting party, for there were neither movies nor television to amuse him. And he'd better not get sick. No antibiotics, no modern surgical techniques. Then, too, he has to live with the sure knowledge that the 20th century is coming with its world wars, Fascism, Communism and everything else. Restful? I think not.

But hold on. Suppose you're not going into the past merely to relive a simpler or more youthful time in a passive manner. Suppose you're going back for some active point. You are going to change your life. You're going to find the fork where you took the wrong turning (haven't we all gone wrong at some time or other?) and change it. (In the drama, ''Morning's at Seven,'' there is one character who constantly leaned against a tree, saying, ''I've got to find the fork.'') Peggy Sue, for instance, finding herself back in the past, is determined not to marry the very unpleasant young man she had married, because the marriage had proved to be an unhappy one.

However, taking the other road when you come to the fork is not merely going to change one consequence and leave everything else unaltered. The other road is going to lead to innumerable unforeseen consequences, while the first road, left untrodden, is going to remove equally innumerable consequences you might not want removed. For instance, to take a very simple example, if you decide to wipe an unsatisfactory husband out of your life, it may be that he provided you with a child you adore. That child gets wiped out with your husband, of course, and you have to weigh the benefits against the harm. In fact, for all you know, taking the other road may lead to an agonizing death for you the next day. How can you dare touch anything?

Ray Bradbury made that point marvelously in his ''Sound of Thunder.'' Hunting safaris go back into the Mesozoic era to track and possibly kill dinosaurs. This is done in strictly restricted areas and under restricted conditions calculated to prevent any change in the future. One person carelessly steps outside the bounds and unintentionally kills a butterfly. The present world is enormously changed, in consequence.

Or, perhaps, the time traveler doesn't want to change his life but merely use his foreknowledge to become wealthy. Thus, Peggy Sue discovers that way back in the dark ages of 1960, there were no pantyhose. There is a scene, then, where Peggy Sue is shown sewing pantyhose perhaps with the intention of making herself rich, rich, rich. This is not followed up but it is not likely that the mere existence of one pantyhose is going to lead to wealth. It may seem, after the fact, that there is an overwhelming demand for pantyhose, but that demand had to be created by publicity and advertising campaigns that would require a great deal more capital than Peggy Sue would be likely to have.

The same probably goes for any other get-rich-quick scheme the time traveler has. He may know the next Derby winner or what the stock market will do, but his first few winnings will undoubtedly change reality in such ways that his foreknowledge is wiped out.

But, then, the time traveler may be a true idealist. He (or she) may not care what happens to himself as a person. He may be intent on changing the world for the better, overall, and damn the particular consequences.

Thus, it was the Connecticut Yankee's dream to introduce 19th-century American initiative and technology into King Arthur's court, thus wiping out slavery and the backward mummery of Merlin (the villain of the story). Again, in L. Sprague de Camps's ''Lest Darkness Fall,'' the hero, Martin Padway, goes back to Ostrogothic Italy just before Belisarius's campaigns to wipe out what is left of Roman civilization in the West and introduce the ''dark age.'' Padway strives manfully to introduce 20th-century technology and prevent the dark age.

Would such grandiose schemes succeed? Personally, I think not. It is very unlikely that one can graft the technology of one century onto the social system of another. In other words, ''you can't have steam engines until it is steam-engine time.''

Besides, what may seem a desirable change to one person may not necessarily seem so to another. There have been a number of stories written of people who go back into the past and seize an opportunity (or plan one from the beginning) to prevent the assassination of Abraham Lincoln. (Naturally, such a scheme of prevention always fails.) On the other hand, some other person finding himself a few years farther in the past may be an ardent Confederate patriot and may exert himself to bring about the assassination of Abraham Lincoln before he took office.

Would the former change have really prevented the ills of the Reconstruction period? Would the latter change have really led to the Union's loss of the Civil War and the establishment of an independent Confederacy? Do we really know? - And what other consequences would follow in either case? Do we really know?

In short, going back into the past is not going to help anyone. Whatever a time traveler's purpose is likely to be, he will surely be disappointed.

We may continue to dream on (there is no charge for dreams) but let us be glad, then, that time travel is impossible.