The Rules of Time Travel

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When preparing to either run or play in a time-travel game, it is essential to learn the rules of time travel. Each game will have its own internal rules, but here are the basics to make a time-travel universe make sense. Note that these rules don’t apply to GURPS Infinite Worlds campaigns as it is dimension-travel rather than time-travel, even if you arrive in what appears to be to a world just like your own but in the past.

The Grandfather Paradox: You can’t change an event because you know that it happened. Edit

The Grandfather Paradox is named after the traditional question: what happens if you go back in time and kill your grandfather before he sired your father? The answer is that you are never born, therefore you can’t go back in time to kill your grandfather, so he does live to produce your father, so you can be born to go back in time and kill him…

The Grandfather Paradox excludes any time agent from deliberately changing an event which they know to have happened. Anything which tries to breach the Grandfather Paradox sets up a self-closing time loop. By doing something to stop the event, they make sure that it never happens, so there is no impetus to go back and do it.

A further example: Adolf Hitler was a bad man, and most people feel that the world would have been better off without him. Time Agents could quite easily go back in time and strangle him in his cradle, or suffocate him with a pillow while he was recuperating after being gassed in the First World War. But if they do that, he would have never entered the history books, so there would be no reason for them to attack him. The fact that the history books say that he did survive to trigger the Second World War is proof that any attempt they make to kill him before that is doomed to failure.

Note that this doesn’t stop you changing known past events unintentionally, unless the change would remove you from existence: two agents could go back into the past, for instance, and one kills a suspect’s ‘local’ henchman in a shootout, only to see the other vanish because the henchman was the second agent’s grandfather.

When time is changed, any agent involved in the mission will remember the original timeline because their consciousness is tied to the resulting history – when they return to their ‘home’ time they will notice the differences. Thereafter, going back to undo the changes they have made is possible. But may be difficult: firstly they will need to convince the agency that sent them back that the change happened (although they will at least believe it to be possible), and that the ‘alternative’ history the agent remembers is a better one than the one they know.

Of course, the Grandfather Paradox still applies. Because they’re going back to undo something they know to have happened, they have to find another way to sort it out: merely putting a bulletproof vest on the grandfather won’t change anything, because it sets up another self-cancelling time loop: they save the grandfather’s life, so the agent doesn’t vanish, so they don’t know they need to give him the vest, so he dies…

The Theory of Conservation of Reality: Most changes don’t make a differenceEdit

Chronoscientists believe in the Theory of Conservation of Reality. There should be an impossibly-large-to-count number of realities. Not only is there one for each of the possible outcomes of every possible option which has ever happened/not-happened, but one for each of the possible combinations of happened/not-happened.

An easy example for gamers is the roll of dice. 1d6 can have six outcomes, so for each roll six possible alternative realities are created. Add another die, and you don’t have twelve possibilities, but thirty-six. Both dice score 1, or the red die rolls 1 and the blue die 2… Roll three dice, and you have 216 almost-parallel universes. Consider the number of options in an entire universe every second, and it’s clear that the number of parallel universes would be as close to infinite as makes no difference.

In fact, there seems to be a limited number of realities. Chrono-technologies can only detect a limited number of parallel universes, which split at crucial moments in history. These ‘pivot points’ are rare: in most circumstances any changes appear to be damped out by new factors which bring history ‘back on course’.

Suppose a Southern States extremist went back in time to ensure that the North would lose the American Civil War. By the Grandfather Paradox, he knows that he can’t assassinate Abraham Lincoln, so he chooses to help out subtly. He plants a minefield in the path of a crucial Yankee advance. He expects that as well as killing more Northern soldiers, this will boost Confederate morale because it looks like their artillery is being unusually effective. But although the Yankees lose more men, they still have enough to overcome the Confederates. The bonus to Rebel morale from their ‘superior artillery’ is counterbalanced by the knowledge that losses from that fire won’t stop the Northern troops from beating them. The regiment which crossed the minefield has such high casualties that it is sent back from the front line to guard duties, and away from risk. It turns out that the only people killed by the minefield were those who were going to be killed anyway in later actions which the depleted regiment now misses, or were going to live out the rest of their lives without doing anything significant to affect the future. Overall, there is no impact on the war.

Only by hitting the ‘pivot points’ can a Time Agent change history and set up a new parallel universe. Even then, the core reality from which that time agent derives still exists. At this point it’s up to the GM to decide which of two possible versions of chronoscience is true. Either the agent finds himself in the new reality he set up, as in the example under the Grandfather Paradox, or he will return to his own timeline and discover how the Law of Conservation of Reality ‘undid’ his changes in his history, but chronoscientists can find the new alternative reality through their instrumentation. At its extreme, the agent will return to find that events he remembers triggering – like the assassination of Hitler – simply didn’t happen, because they dropped into a self-closing time-loop.

The twist, of course, is that no-one can predict where the pivot points are until a parallel universe springs into existence. That’s why Time Agents have to act as if Agents from rival organisations might stand a chance of succeeding in any attempts to ‘change history’. Although they won’t succeed in your universe, they may set up a parallel universe in which billions of people must suffer the effects.

The Infinite Monkeys Rule: You can’t solve a problem by sending the solution back to yourself after it has worked. Edit

This is named after the idea of using random chance to solve a problem. If you put an infinite number of monkeys in front of typewriters, they will eventually produce the complete works of Shakespeare. Afterwards, an agent could tell which monkey did it, and the experiment could actually be run with just that one monkey! But that doesn’t happen, because of the Infinite Monkeys Rule.

A more realistic scenario for gaming. Suppose you need to win a chess match against a Grand Master. Time Agents could set up a chess computer to generate a random series of moves, then make those moves regardless of their opponent’s game. In the vast majority of cases, the Grand Master would win soundly, but in one instance, the agent would win, and could then send the solution back in time to before the match, ensuring that the agent could learn the correct solution, and win the match without using the randomly-generated moves.

In fact, one of the quirks of time-travel is that information cannot be sent back to the past which generated it. Some chronoscientists believe that this is due to a subset of the Grandfather Theory, which decrees that if the Time Agents couldn’t solve a problem without receiving information from the future, the future in which that solution could be passed back in time never arises. Others contend that that wouldn’t stop the information being passed back in the universe where it was going to work anyway, and prefer the theory that it is the working of the Conservation of Reality rules, which says that although there could be millions of possible universes in which messages are passed back in time, they are not the ones which emerge from the theoretical dimensions into actual universes. The times and places to which you might want to pass information back in time are not the pivot points at which reality changes can be made. If the information is sent, the potential resulting universe becomes one of the many which never actually become a reality.

In game-terms this means that the players have to come up with the solutions themselves. They can’t simply say that “After this is all over, we’ll send a message back to ourselves to tell us how we did it” or “…where the Bad Guy’s secret lair is.” If they don’t do it on their own, they can’t send back a message, and if they do it without help from a later point of their own lives, they won’t need to send back the message. They can’t even have a warning from another agent that they are about to walk into an ambush – a person travelling back in time with a verbal message is information just as much as any other form of transmission.

It is possible, but difficult, to provide physical assets to help out a past version of yourself.Edit

Theoretically, any Time Agent should be able to do this. Suppose your character is researching in a library when the rival agents attack you. A Time Agent can leap to a handy section of shelving, pull aside the books, and produce a firearm. Later, they can go back in time and plant the gun there because they know where it will be needed.

To maintain game-balance, this has to be either forbidden by the game mechanics, perhaps by creating a rule which says that you can’t be in the same place twice, no matter how far apart they are in time. Alternatively, it offers a chance for a new Advantage:

New Advantage: Equipment Placer (15 points per level)Edit

(Pre-requisite: Time Travel which the character can control.)
A character can produce any physical equipment they need at any point in the game by going back in time after the adventure and pre-placing it. Only physical equipment can be provided: no information can be sent back which will help the players to sidestep the adventure challenges, like solutions to puzzles or the identity of mystery villains. The player must specify what is in the cache at the time of finding it.
The items must be hidden to ensure that no-one has moved them in the meantime, but they will be wherever the character looks – after all, they will be placed there from the character’s memory of where they found them.
One cache of equipment can be found per level of the advantage per adventure.

The restrictions prevent possible breaches of the Infinite Monkeys Rule: you can’t solve a problem by using information which is based on the fact that you have already solved it. In character terms, it can be stated that there is something about the nature of time-travel which prevents a traveller getting too close to where they have already been, but some people have auras which allow them to stretch the limit and be somewhere they will be again in the relatively near future.

Another reason for the restrictions is gaming practicality: the referee can’t predict what will be necessary in the future. If the players later decide that they need a car to be placed at adventure locale B because their driver crashed their transport on arrival, the referee can’t tell them that the cache found at locale A contains car keys simply because at that point he doesn’t know that they will need the car. They will have to ‘find’ another cache at locale B – if the character has two levels of this advantage.

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