Если мы хотим использовать принцип разветвляющейся Вселенной в нашем сюжете, нам придется определять понятие ключевого события — такого, которое ведет к возникновению развилки. И здесь мы можем с успехом использовать Закон сохранения событий. Предположим, что Вселенная стремится или способна сохранять целостную событийную ткань только до тех пор, пока уровень (например энергетический) воздействия на нее не превосходит определенного порога. После этого уровня возникает развилка. Так резиновая лента способна сокращаться только до определенного уровня растяжения, а после него она рвется. Такое с равным успехом относится и к естественному развитию событий, и к воздействию на прошлое со стороны путешественника. Тогда мы ограничиваем количество вариантов хотя и все равно гигантской, но, тем не менее, поддающейся оценке величиной. И среди них уже можно искать нужный нам. Остается хотя бы с точки зрения видимо-здравого смысла обосновать, что битва при Ватерлоо или на Курской дуге в разряд таких событий попадает.

Кстати, такой подход делает неверным "закон Нивена" и вообще рассуждения о том, почему машина времени не может быть изобретена, если прошлое подвержено изменениям. Стоит только предположить, что Вселенная стремится сохранить не все события, а только "ключевые", оказывающие на нее наибольшее влияние, как его рассуждения идут прахом (а это так — в конце концов, дедушка-то умер, даже если вы заняли его место!) Изобретение машины времени является ключевым событием, которое также должно быть защищено.

В дополнение "закон Нивена" неверен и по другим соображениям. Можно предположить, например, что изменения случаются в одном уголке вселенной, а машина изобретена в совсем другой галактике, на которую данные изменения не влияют и которая по общему соглашению исключена из сферы ведения Войн Времени (вариант: которую создатель машины времени раз и навсегда оградил от перемещений из будущего). Кроме того, изменения в прошлом, пусть и массовые, совсем не обязательно влияют на процесс изобретения машины. Нивен неявно предполагает, что ее создает гениальный одиночка, которому можно просто проломить череп, что ликвидирует и саму машину. Но если это плод работы огромного исследовательского института, базирующийся на долгой истории исследований других коллективов, то исказить его результаты очень и очень непросто. И так далее.

Наконец, следует помнить еще об одной неприятности, подстерегающей путешественника ко времени. Земля НЕ является неподвижным центром мироздания. И если мы хотим прыгнуть в прошлое или будущее, нам придется путешествовать не в одномерном пространстве, а в четырехмерном, иначе после прыжка мы окажемся глубоко в толще планеты или же в глубоком вакууме. Во-первых, уверены ли мы, что у компьютера путешественника хватит математических моделей и вычислительных мощностей для просчета точки назначения?

Во-вторых, в этом случае нам необходимо иметь возможность МГНОВЕННО (или в течение субъективного времени процесса прыжка) перемещаться в пространстве в любую заранее рассчитанную точку, фактически — владеть телепортацией. Единственной альтернативой является встраивание машины времени в звездолет, стартующий из глубокого вакуума и способный догнать точку нашего назначения после прыжка (а если планета успела улететь или еще не долетела до этой точки на световые годы?) Автоматическим следствием из данного наблюдения является тот факт, что если машина времени представляет собой велосипед с моторчиком, как у Уэллса, ее изобретатель заодно нашел и способ телепортироваться. Спрашивается, нафига ему ползать по времени, когда он может просто продать кому-нибудь патент на телепортацию и стать мультимиллиардером?

На самом деле у любого писателя-фантаста есть прекрасная возможность стравить янки из Коннектикута и рыцарей короля Артура, не прибегая к путешествиям во времени. Достаточно лишь предположить, что современный человек (или человек будущего) попадает на планету, населенную когда-то прилетевшими туда и одичавшими колонистами. В качестве дополнительной подпорки сюжета можно ввести инопланетян, которые воруют человека из цивилизации, не научившейся перемещаться между звездами, и забрасывают его на одичавшую планету. Такой подход слегка ограничивает нас в упоминании исторических событий — в конце концов, сложно предполагать, что давным-давно в далекой-далекой галактике жил еще один дедушка Ленин с добрым взглядом и курчавой в детстве головой. Зато он развязывает руки и прочие выступающие части тела нашей фантазии, оставляя ее в рамках научно-фантастического подхода.

Любители читать тексты в оригинале могут найти исходную английскую версию ниже.

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 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.

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