Let there be a myriad of realities, of universes. For every decision made by any form of life, let it be made both ways; or in all possible wars if there are more then two choices. Let universes be created with every choice.

Then conservation of matter and energy holds only for the universe of universes. One can move time machines from one universe to another.

You've got to admit it's flamboyant!

You still can't visit the past. But you can find a universe where things happened more slowly; where Napoleon is about to fight Waterloo, or Nero is about to ascend the throne. Or, instead of changing the past, you need only seek out the universe where the past you want is the one that happened. The universe you want unquestionably exists. (Though you may search a long, weary time before you find it.)

Ersatz time travel becomes a special case of sidewise-in-time travel, travel between multiple time tracks.

The what-if story has fascinated many writers. Even 0. Henry wrote at least one. From our viewpoint, sidewise-in-time travel solves conservation laws, Grandfather Paradox, everything.

I hate sidewise-in-time travel stories.

Let me show you why.

First, they're too easy to write. You don't need a brain to write alternate-world stories. You need a good history text.

In the second place...did you ever sweat over a decision? Think about one that really gave you trouble, because you knew that what you did would affect you for the rest of your life. Now imagine that for every way you could have jumped, one of you in one universe did jump that way.

Now don't you feel silly? Sweating over something so trivial, when you were going to take all the choices anyway. And if you think that's silly, consider that one of you still can't decide...

In the third place, probability doesn't support the theory of alternate time tracks.

There are six ways a die can fall, right? Which makes thirty-six ways that two dice can fall, including six ways to get a seven. Each way the dice can fall determines one universe. Then the chance of your ending in each of the thirty-six universes is one in thirty-six, right?

Then it doesn't matter if the dice are loaded. One chance in thirty-six, exactly, is the odds for each way the dice can fall. One chance in six, exactly, of getting a seven.

Experience, however, shows that it does matter if the dice are loaded.

DEFENSE OF TIME TRAVEL #III: The idea of reversing the flow of time isn't nearly as silly as it sounds. I quote from an article in the October 1969 issue of Scientific American, "EXPERIMENTS IN TIME REVERSAL," by Oliver E. Overseth.

"All of us vividly recognise the way time flows; we take considerable comfort, for example, in our confidence that the carefully arranged marriage of gin and vermouth is not going to be suddenly annulled in our glass, leaving us with two layers of warm liquid and a lump of ice. It is a curious fact, however, that the laws that provide the basis for our understanding of fundamental physical processes (and presumably biological processes as well) do not favor one direction of time's arrow over another. They would represent the world just as well if time were flowing backward instead of forward and martinis were coming apart rather than being created."

Is the universe really invarient under time reversal? Many physicists think not. Overseth and his partner Roth spent almost two years looking for a case in subatomic physics in which invarience under time reversal is not preserved.

They knew exactly what they were looking for. They were watching (via some very indirect instruments) the decay of a lambda particle into a proton plus a pi meson. The anomaly would have been a nonzero value for the beta component of the spin of the proton.

The point is that they failed to find what they were looking for. There have been many such experiments in recent years, and none have been successful. At the subatomic level, one cannot tell whether time is running backward or forward.

Could a determined man reach the past by reversing himself in time and waiting for last year to happen again?

Present theory says that he would reverse both the spin and the charge of every subatomic particle in his body. The charge reversal converts the whole mass to antimatter. BOOM!

Less dramatically, there is conservation of mass/energy. Reverse the direction of travel in time of a human body, and to any physicist it would look like two people have vanished.

Clearly this is illegal. We can't do it that way.

We might more successfully reverse a man's viewpoint: send his mind backward in time. If there is really no difference between past and future, except in attitude, then it should be possible.

But the traveler risks his memory healing to a tabula rasa, a blank slate. When he reaches his target date he might not remember what to do about it.

For there is still entropy: the tendency to disorder in the universe, and the most obvious effect of moving "forward" in time. Entropy is not obvious where few reactions are involved, as in the motion of the planets, or as when a lambda parlicle breaks down. But the mushroom cloud left by a hydrogen bomb is difficult to return to its metal case. That's entropy.

Any specialist in geriatric medicine knows about entropy.

Let's try something less ambitious.

Suppose we found a clump of particles already moving backward in time. (Exactly what Roth and Overseth and their brethren might find in their experiments, if time-reversal turns out to be valid. Though most expect to find just the opposite.) Now we write messages on that clump. Simple messages. "Blue Ben in the sixth, 4/4/72."

But from our viewpoint, we start by finding a message and end by erasing it! And if it went wrong...We find a message: "Blue Ben in the sixth, 4/4/72." We bet on him, and he loses. Now what? Can we unwrite a different message? Or just refuse to erase it at all?

But if it did work, we could make a fortune. And it violates no known physical laws! Practically.

Meanwhile, Roth and Overseth and a number of others are all convinced that there must be exceptions to the symmetry of time. If they find just one, it's all over.

DEFENSE OF TIME TRAVEL #IV: The oldest of all, going back to Greek times. Philosophers call it fatalism or determinism. A fatalist believes that everything that happens is predetermined to the end of time; that any attempt to change the predetermined future is fated, is a part of the predetermined future itself.

To a fatalist, the future looks exactly like the traditional picture of the past. Both are rigid, inflexible. The introduction of time travel would not alter the picture at all, for any attempt on the part of a time traveler to change the past has already been made, and is a part of the past.

Fatalism has been the basis for many a tale of a frantic time traveler caught in a web of circumstance such that every move he makes acts to bring about just the calamity he is trying to avert. The standard plot sketch is reminiscent of Oedipus Rex; when well done it has the same flavor of man heroically battling Fate and losing.

Notice how fatalism solves the Grandfather Paradox.

You can't kill your grandfather, because you didn't. You'll kill the wrong man if you try it; or your gun won't fire.

Fatalism ruins the wish-fulfillment aspect of time travel. Anything that averts the Grandfather Paradox will do that. The Grandfather Paradox is the wish-fulfillment aspect. Make it didn't happen.

The way to get the most fun out of time travel is to accept it for what it is. Give up relativity and the conservation laws. Allow changes in the past and present and future, reversals in the order of cause and effect, effects whose cause never happens...

Fatalistic time travel also allows these causative loops, but they are always simple, closed 1oops with no missing parts. The appearance of a time machine somewhere always implies its disappearance somewhere-and somewhen else. But with this new, free will kind of time travel...

We assume that there is only one reality, one past and one future; but that it can be changed at will via the time machine. Cause and effect may loop toward the past; and sometimes a loop is pinched off, to vanish from the time stream. The traveler who kills his six-year-old grandfather eliminates the cause of himself, but he and his time machine remain-until someone else changes the past even further back.

Between the deterministic and free will modes of time travel lies a kind of compromise position:

We assume a kind of inertia, or hysteresis effect, or special conservation law for time travel. The past resists change. Breaks in time tend to heal. Kill Charlemagne and someone will take his place, conquer his empire, mate with his wives, breed sons very like his. Changes will be minor and local.

Fritz Leiber used Conservation of Events to good effect in the Change War stories. In TRY AND CHANGE THE PAST, his protagonist went to enormous lengths to prevent a bullet from smashing through a man's head.

He was sincere. It was his own head. In the end he succeeded-and watched a bullet-sized meteorite smash into his alter-self's forehead.

Probabilities change to protect history. This is the safest form of time travel in that respect. But one does have to remember that the odds have changed.

Try to save Jesus with a submachine gun, and the gun will positively jam.

But if you did succeed in killing your own six-year-old grandfather, you would stand a good chance of taking his place. Conservation of Events requires someone to take his place; and everyone else is busy filling his own role. Except you, an extraneous figure from another time. Now Conservation of Events acts to protect you in your new role!

Besides, you're already carrying the old man's genes.

Certain kinds of time travel may be possible; but changing the past is not. I can prove it.

GIVEN: That the universe of discourse permits both time travel and the changing of the past.

THEN: A time machine will not be invented in that universe.

For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past.

If we assume that there is no historical inertia, no Conservation of Events, then each change makes a whole new universe. Every trip into the past means that all the dice have to be thrown over again. Every least change changes all the history books, until by chance and endless change we reach a universe where there is no time machine invented, ever, by any species.

Then that universe would not change.

Now assume that there is an inertia to history; that the past tends to remain unchanged; that probabilities change to protect the fabric of events. What is the simplest change in history that will protect the past from interference?

Right. No time machines!

NIVEN'S LAW: IF THE UNIVERSE OF DISCOURSE PERMITS THE POSSIBILITY OF TIME TRAVEL AND OF CHANGING THE PAST, THEN NO TIME MACHINE WILL BE INVENTED IN THAT UNIVERSE.

If time travel is so manifestly impossible, why does every good and bad science fiction writer want to write a new, fresh time travel story?

It's a form of competition. No writer believes that a field is completely mined out. And no field ever is. There is always something new to say, if you can find it.

Time travel can be a vehicle, like a faster-than-light drive. Our best evidence says that nothing can travel faster than light. Yet hard-headed science fiction writers constantly use faster-than-light spacecraft. If a character must reach the Veil Nebula, and if the plot demands that his girl friend be still a girl when he returns, then he must needs travel faster than light. Similarly, it takes time travel to pit a man against, a dinosaur, or to match a modern man against King Arthur's knights.

There are things a writer can't say without using time travel.

Then, time travel is so delightfully open to tortuous reasoning. You should be convinced of that by now.

The brain gets needed exercise plotting a story in a universe where effects happen before their causes; where the hero and his enemy may be working each to prevent the other's birth; where a brick wall may be no more solid than a dream, if one can eliminate the architect from history.

If one could travel in time, what wish could not be answered? All the treasures of the past would fall to one man with a submachine gun. Cleopatra and Helen of Troy might share his bed, if bribed with a trunkful of modern cosmetics. The dead return to life, or cease to have been at all.

Bothered by smog? Henry Ford could be stopped in time, in time...

No. We face insecurity enough. Read your newspaper, and be glad that at least your past is safe.

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