But things are even worse if a Grandfather Paradox is involved. One can imagine a centuries-old time machine resting in a museum, inside a glass-and-steel case made from the glass and the steel which would have been used to build the time machine, if anyone had gone ahead and built that time machine, which nobody did, because of interference with the past via that same time machine.

(2) If one cannot send matter through time, perhaps one can send signals-information.

But even this violates conservation of energy. Any signal involves energy in some form.

Furthermore, relativity laws state that information cannot travel faster than c, the velocity of light in a vacuum. A signal traveling back through time travels faster than infinity!

(3) Physical time travel clearly violates any law of motion, as motion always relates to time. This affects conservation of momentum, statements about kinetic energy, and even the law of gravity. Anybody's law of gravity.

(4) What about drawing information from the future?

If precognition and prophecy are only very accurate guesswork by the subconscious mind, then no laws are violated. But if precognition really has something to do with time-

I cite the Heisenberg Principle. One cannot observe something without affecting it. If one observes the future, there must be an energy exchange of some kind. But that implies that the future one is observing is the future; that it already exists; that information is flowing into the past.

I've demonstrated that this violates relativity and conservation of energy. It also involves a Grandfather Paradox, if information drawn from one future is used to create another. And if the information can't be used to change the future, then what good is it?

What was that about the stock market?

(5) Travel into the future is no more difficult than suspended animation and a good, durable time capsule. But you can't go home without traveling into the past.

Does any of this seem like nitpicking? Sure it Is. Are we to regard the laws of relativity and conservation as sacred, never to be broken, nor even bent by exceptions? Heaven forbid.

But time travel violates laws more basic than conservation laws.

Our belief in laws of any kind presupposes a belief in cause and effect. Time travel reverses cause and effect. With a Grandfather Paradox operating, the effect, coming before the cause, may cause the cause never to come into effect, with results which are not even self consistent

Characters in time-travel stories often complain that English isn't really built to handle time travel. The tenses get all fouled up. We in the trade call this problem Excedrin Headache number V -3.14159.

To show it in action, I'd like to quote from one of my own stories, BIRD IN THE HAND. The characters have done catastrophic damage to the past, and are discussing how to repair it.

"Maybe we can go around you." Svetz hesitated, then plunged in. "Zeera, try this. Send me back to an hour before the earlier Zeera arrives. Ford's automobile won't have disappeared yet. I'll duplicate it, duplicate the duplicate, take the reversed duplicate and the original past you in the big extension cage. That leaves you to destroy the duplicate instead of the original. I reappear after you've gone, leave the original automobile for Ford, and come back here with the reversed duplicate. How's that?"

"It sounded great Would you mind going through it again?"

"Let's see. I go back to-"

This was less of a digression than it seemed. The English language can't handle time travel. We conclude that the ancestors who made our language didn't have minds equipped to handle tithe travel. Naturally we don't either; for our thinking is too dependent on our language.

As far as I know, no language has tenses equipped to handle time travel. No language on Earth. Yet.

But then, no language was ever equipped to handle lasers, television, or spaceflight until lasers, television, and spaceflight were developed. Then the words followed.

If time travel were thrust upon us, would we develop a language to handle it?

We'd need a basic past tense, an altered past tense, a potential past tense (might have been), an altered future tense, an excised future tense (for a future that can no longer happen), a home base present tense, a present-of-the-moment tense, an enclosed present tense (for use while the vehicle is moving through time), a future past tense ("I'll meet you at the bombing of Pearl Harbor in half an hour."), a past future tense ("Just a souvenir I picked up ten million years from now"), and many more. We'd need at least two directions of time flow: sequential personal time, and universal time, with a complete set of tenses for each.

We'd need pronouns to distinguish [you of the past] from [you of the future] and [you of the present]. After all, the three of you might all be sitting around the same table someday.

Meanwhile (if, God willing, the word still has meaning), time travel must be considered fantasy. It violates too many of the laws of physics and reason to be thought otherwise.

But it's a form of fantasy superbly suited to games of logic. The temptation to work out a self-consistent set of laws for time travel must be enormous. So many writers have tried it!

Let's look at some of the more popular possibilities:

DEFENSE OF TIME TRAVEL #1: Assume that (1) One can travel only into the future. (2) The universe is cyclic in time, repeating itself over and over.

This works! All you've got to do is go into the future past the Big Collapse when the universe falls in on itself, through the Big Bang when it explodes again, and keep going until you reach the area of the past you're looking for. Then you murder Hitler in 1920, or use the H-bomb on the damyankees at Appomatox, or whatever your daydream is. There is no Grandfather Paradox.

You merely get a new future.

True, the next version of you will not make the trip. You've eliminated his motive. Thus on the next cycle the damyankees will win the Civil War, Hitler will lead Germany into WWII, and so forth. But you've merely introduced a double cycle. There is no paradox.

Further, your time machine need be nothing more than an EXTREMELY durable time capsule.

OBJECTIONS: Three. First, some people don't believe in cyclic time. (I don't.) Second, locating the proper era is a nontrivial problem when you've got the whole lifetime of the universe to search in. You'd be lucky to find any section of human history. Third, removing your time capsule from the reaction of the Big Bang could change the final configuration of matter, giving an entirely different history.

DEFENSE OF TIME TRAVEL #II: Known as the theory of multiple time tracks.

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.)