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.

I

I was watching the news when the change came, like a flicker of motion at the corner of my eye. I turned toward the balcony window. Whatever it was, I was too late to catch it.

The moon was very bright tonight.

I saw that, and smiled, and turned back. Johnny Carson was just starting his monologue.

When the first commercials came on I got up to reheat some coffee. Commercials came in strings of three and four, going on midnight. I'd have time.

The moonlight caught me coming back. If it had been bright before, it was brighter now. Hypnotic. I opened the sliding glass door and stepped out onto the balcony.

The balcony wasn't much more than a railed ledge, with standing room for a man and a woman and a portable barbecue set. These past months the view had been lovely, especially around sunset. The Power and Light Company had been putting up a glass-slab style office building. So far it was only a steel framework of open girders. Shadow-blackened against a red sunset sky, it tended to look stark and surrealistic and hellishly impressive.

Tonight...

I had never seen the moon so bright, not even in the desert. Bright enough to read by, I thought, and immediately, but that's an illusion. The moon was never bigger (I had read somewhere) than a quarter held nine feet away. It couldn't possibly be bright enough to read by.

It was only three-quarters full!

But, glowing high over the San Diego Freeway to the west, the moon seemed to dim even the streaming automobile headlights. I blinked against its light, and thought of men walking on the moon, leaving corrugated footprints. Once, for the sake of an article I was writing, I had been allowed to pick up a bone-dry moon rock and hold it in my hand....

I heard the show starting again, and I stepped inside. But, glancing once behind me, I caught the moon growing even brighter -- as if it had come from behind a wisp of scudding cloud.

Now its light was brain-searing, lunatic.

The phone rang five times before she answered.

"Hi," I said. "Listen --"

"Hi," Leslie said sleepily, complainingly. Damn. I'd hoped she was watching television, like me.

I said, "Don't scream and shout, because I had a reason for calling. You're in bed, right? Get up and... can you get up?"

"What time is it?"

"Quarter of twelve."

"Oh, Lord."

"Go out on your balcony and look around."

"Okay."

The phone clunked. I waited. Leslie's balcony faced north and west, like mine, but it was ten stories higher, with a correspondingly better view. Through my own window, the moon burned like a textured spotlight.

"Stan? You there?"

"Yah. What do you think of it?"

"It's gorgeous. I've never seen anything like it. What could make the moon light up like that?"