All The Myriad Ways

by Larry Niven

  You refuse to believe in my ship? Then think of it as an exercise in speculation. Ridiculous as it may seem, we do get results.

  1) Rate-of-travel of the ship is limited only by mechanical difficulties, that is, by the rate of successive teleportation. The end-teleport drive does not affect the ship's kinetic energy. We change only the position. So there is none of this nonsense about relativity.

  2) We must assume a mechanical limit on rate-of-travel. Otherwise the ship goes off the edge of the universe.

  3) You can take your ffinger off the button. Kinetic energy is teleported along with everything else; and as a perfect image you have free will.

  4) The longer the ship is, the faster it will go, with a given rate-of-teleportation. But: the longer the ship is, the greater is the danger of getting too near a large mass. To land on Earth the ship would have to be less than two feet long.

  In fact, you can't land it anywhere with the end-teleport drive. As with the inertialess drives in Doe Smith's LENSMAN series, you keep an intrinsic velocity which reappears when the drive goes off. To land the ship anywhere you need either inboard auxiliary rockets, or rocket tugs.

  5) What happens if something gets in the way of the ship?

  Good question. Many things definitely will. Light, for example.

  A light beam crosses interstellar space. Suddenly, for an instant, the end-teleport ship is occupying that space. The ship's walls can't stop it, for the light never encountered the walls. A human eye can stop it if the light reaches that eye in time.

  Result: everything on the ship is transparent. If we assume that some light will be picked up by the teleportation field and carried along with the ship, then how transparent everything is will depend on two things: the rate of travel, and the distance of an object from the passenger's eye. His hand is nearly opaque. The further wall is nearly invisible, because so much light is being picked up in the space between wall and eye...and dropped between wall and eye. The cabin in Figure 4 is unnecessary unless the ship carries rocket auxiliaries. With the end-teleport drive going, the stars are visible anywhere you look.

  If the teleportation field will not transport light, the situation becomes more serious. At a useful rate of travel a light beam would have just time to traverse the diameter of a human eye before the eye disappears. So a human eye will still function. But the ship and all its contents, including the passenger, are totally invisible, and each passenger becomes a disembodied viewpoint falling between the stars.

  Travel even faster, and a light beam may have time to touch the retina without first entering the lens of the eye. Now everything becomes a blur. On arrival the passenger becomes a psychiatric patient.

  6) Interstellar dust would also be picked up en route. Most of it could be handled by a tough air conditioning system; but a certain proportion would appear already inside the transitory space occupied by the passenger. Definitely he would need medical attention on arrival.

  7) Interstellar hydrogen would be swept up by the moving ship. Aboard an end-teleport drive there would be absolutely no smoking. Drinking, yes...

  8) As for meteors and larger bodies... we'll use a trick.

  Let's say we're going toward the galactic core, i.e. toward Sagitarius. Okay: Before we leave the system, we take our ship to within a few million miles of the Sun, on the Sagitarius side; and we hover.

  We hover by end-teleporting outward as the Sun's gravity draws us inward. Half an hour of this should give us a respectable intrinsic velocity Sunward. Now we take off toward Sagitarius.

  So we ram something en route. It can happen.

  But... it takes energy to make two solid masses occupy the same space. Chances are we cannot teleport into what we've rammed. A fuse blows and the motor stops. That leaves the ship with its intrinsic velocity, which we have built up hugely in a direction opposite to the direction of travel.

  So the ship backs up at hundreds of miles per second!

  Even if we ram a planet, our intrinsic velocity is higher than escape velocity, and we're safe.

  9) Conservation of energy rears its head once more. The ship becomes fiendishly cold as it leaves the solar system, and body temperature drops simultaneously.

  The reverse occurs as we enter a system. It's a good thing we built a heavy air conditioning system to get rid of all that dust. We'll need it for temperature control.

  VII

  Why do I persist in assuming that the conservation laws hold?

  This question caused a series of soapbox speeches, mostly in my defense (thanks, friends), along the back wall of my Boston audience. The assumptions are important, and I'm going to try to justify them.

  1) The behavior of the universe does not change. In all known cases the laws of conservation of energy and momentum hold rigorously. Now we use them for prediction. The existence and most of the properties of the neutrino were predicted by use of these and other conservation laws. Later the neutrino itself was detected through judicious use of its own proposed properties.

  If today's physicists can use conservation to predict ghost particles, I can use- them to predict the behavior of a teleport system.

  2) In any case, I'm entitled to make any assumptions I like, if they are internally consistent. This is an exercise in speculation, remember? Speculation starts with assumptions. If you don't like mine, try your own; you might get some interesting results.

  3) A passenger teleporting downhill must lose potential energy. Some equivalent gain in energy must appear. But why heat?

  Good question. I myself generally assume that the energy will appear as a jump in electron orbits. Then the electrons drop back, releasing photons. The photons are absorbed before they reach the passenger's skin, giving heat. But almost any reasonable process will ultimately end in heat. Heat is the most general, most randomized form of energy.

  Could the released energy appear as neutrinos? That would not give heat. But it would upset some of the obscure parity laws of nuclear physics (thus upsetting Isaac Asimov, Hal Clement, and thousands of reactionary physicists) and it would make uphill teleportation impossible, for the process would have to destroy neutrinos which weren't there in the first place.

  VIII

  How about a perpetual motion machine?

  See Figure 5 (page 107). The idea is to use open transmitter and receiver booths. The cargo, thirty gallous of water, is teleported to the receiver. It immediately pours out into the open transmitter, which teleports it back to the receiver, et cetera. Put a water wheel in the system and we get power.

  Obviously there's a flaw. If conservation holds, the water freezes pretty quick. Furthermore, thermodynamics says that the energy to run the system will be greater than the maximum energy to be obtained from the continuously falling water.

  But the system is interesting in other ways.

  Let's replace the water with a ton of iron filings. That way we can enclose the whole system in a vacuum chamber and stop worrying about atmospheric friction, water evaporation, and freezing of the water. We let the filings fail under gravity until the mass is a black stream, near absolute zero, moving at seven miles per second. That's nineteen minutes of operation.

  Now we let it go another nineteen minutes. The velocity doubles, and we've let the filings fail the equivalent of twice the distance from infinity to the Earth's surface.

  We could maintain this acceleration forever, provided we do one thing. We will have to build our system at the North (or South) Pole. Otherwise the stream of filings will seem to bend away from the transmitter door as the Earth turns. (BOOM!) So we're at the North Pole...

  In thirty days the mass of the filings has doubled. In sixty days it has quadrupled. Note that while Earth pulls the filings, the filings pull the Earth. Minutely, at first. But the filings aren't really going anywhere, so we have the equivalent of a reactionless drive. Every month the thrust doubles. If we run the system long enough the filings will weigh as much as a star. Obviously we don't want that. Tides! But in its p
resent state, turning off the system would destroy the Earth. So we set up a second receiver at the South Pole.

  The stream of filings goes tearing off through the Earth's atmosphere, a blue flash of iron vapor ramming air. Even the gamma rays are going upward! What a show! Listen to that applause! But all the teevee cameras have melted…

  Well, this is where I quit. But try a few postulates yourself, and see what you get.

  THE THEORY AND PRACTICE OF TIME TRAVEL

  Speculate : (2) To ponder a subject in its different aspects and relations; meditate; esp. to theorise from conjectures without sufficient evidence.

  -Webster's New Collegiate Dictionary, 1959

  Once upon a time a man was given three wishes. He blew the first two, getting himself in such deep trouble that if he let either wish stand, he would suffer terribly.

  Now desperate, he cried, "I wish I'd never had a fairy godmother!" And the past healed to cancel both wishes.

  The first time-travel story was a fairy tale-here drastically condensed.

  Its theme is buried deep in the literature. L. Frank Baum used it in THE WONDERFUL LAND OF OZ. Cabell borrowed it for THE SILVER STALLION. Traditionally the protagonist may change the past without actually moving backward in time.

  H. G. Wells, one of the fathers of modern science fiction, also fathered the time traveling vehicle. This may be the reason Well's spiritual sons tend to treat time travel as science fiction rather than fantasy. But Wells wrote only of travel into the future. He missed the Grandfather Paradox and all the other derivative paradoxes of travel into the past. His time machine was a mere vehicle, no more remarkable than the gravity shielding material, Cavorite.* (*Both were mere philosophical vehicles. Wells liked to preach.)

  Wells also missed the most important aspect of time travel: wish fulfillment. When a child prays, "Please, God, make it didn't happen," he is inventing time travel in its essence. (He will probably give up the idea when he learns good English. More about that later.) The prime purpose of time travel is to change the past; and the prime danger is that the Traveler might change the past. The man who first thought of travel into the past combined the Wells machine with the fairy tale to produce time travel in its present form.

  Time machines come in many forms. Well's man-carrying vehicle was as open as a bicycle seat, with a magnificent view of time flashing past. Poul Anderson's standard issue time Patrol vehicle could do anything Well's could, and fly too.

  More restricted machines may travel only into the future, or may send only subatomic particles into the past, or may be restricted to things even less substantial: thoughts, dreams, emotional states. Others may move only in quantum jumps of a million or sixty million years. A writer who puts severe limits on his time machine, is generally limiting its ability to change the past in order to make his story less incredible.

  THE GRANDFATHER PARADOX is basic to any discussion of time travel. It runs as follows:

  At the age of eighty your grandfather invents a time machine. You hate the old man, so you steal the machine and take it sixty years back into the past and kill him. How can they suspect you?

  But you've killed him before he can meet your grandmother. Thus you were never born. He didn't get a chance to build the time machine either.

  But then you can't have killed him. Thus he may sire your father, who may sire you. Later there will be a time machine...

  You and the machine both do and do not exist Paradox!

  In general we will call any such interference with the past, especially self-cancelling interference, a Grandfather Paradox.

  Travel into the past violates certain of what we regard as laws of nature.

  (1) A vehicle which travels from the thirtieth century AD to the twentieth, may be regarded as appearing from nowhere. Thus it violates the law of conservation of matter. If the vehicle carries a power source of any kind, it also violates conservation of energy...a quibble, as they are both the same law these days.

  To say that an equivalent tonnage of matter disappears a thousand years later is no answer. For ten centuries there was an extra time machine around.

  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 spa
ceflight 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.