"...something always 'happens' to keep you from changing the past. You stumble and break your neck before assassinating Hitler, or whatever."
- the point being that this is implausibly improbable. Indeed. I will try to copy a relevant passage from a post on the "Logic of Time Travel" blog:
Possible Solutions to the Causality Violation Paradox
1. The Logical Impossibility of Causality Violation in a Single Timeline
Hitler was not killed or kidnapped in childhood. Therefore, either no-one tried to do this or someone tried and failed, even if we do not know the precise reason for failure: accident, apprehension, arrest etc and even if the person who tried had foreknowledge of Hitler’s career and had wanted to prevent it. This remains true even if the source of foreknowledge was time travel from Hitler’s future. In fact, introducing time travel increases the number of possible reasons for failure. It would not be easy for a time traveller from a later period to find the best time and place at which to intervene in order to interrupt Hitler’s early life, which might even be protected by pro-Nazi time travellers. Therefore, causality violation is logically impossible because it cannot be true that something happened when it is already known that it did not happen. More specifically, it cannot be true that someone performed a task when it is already known that it was not performed, thus that, if anyone tried, they failed.
Unfortunately, if time travel became common, then the number of adverse circumstances and accidents necessary to prevent causality violation would become statistically impossible. Therefore, there has to be some reason either why time travel does not occur (e.g. because it is impossible or because it is so difficult that no-one ever finds out how to do it in the whole history of the universe) or why its use is extremely limited (e.g. because it requires too much energy or because the theory is valid which stipulates that any time journey would also have to be a long space journey, thus precluding effortless disappearance and reappearance on Earth’s surface in recent historical periods).
(Added on 21 April 2006: Thanks to Colin G. Mackay for pointing out that there is no such thing as a "statistical impossibility". My argument should be re-phrased: Something must happen to prevent a logical contradiction but, by definition, it is extremely improbable that an extremely improbable event will always occur just in time to prevent any logical contradiction.)
-copied from here.
3 comments:
I believe Larry Niven wrote a story about this. I can't recall the title. It has been sufficiently long ago that I wonder whether it might have been an essay rather than a story.
Essentially, he said that if someone invents time travel, sooner or later it is inevitable--from the perspective of someone looking into the universe and observing causality from the outside, so to speak--that someone, motivated, perhaps, by madness, will travel back in time from the future to kill the inventor of time travel before he can invent it. And the universe will quiver a moment and history will change and, suddenly, a promising young scientist with odd theories died under mysterious circumstances, and time travel was never created in the first place.
And perhaps later on--in the future of the new timeline, that is--another brilliant individual, perhaps even finding the first one's notes, will invent time travel. And a time-traveling assassin will do him in, too, before he can create it. It's inevitable, given a nigh-infinity of future time and a nigh-infinity of future individuals with access to time travel technology. It's like the Heisenberg theory of lion hunting--construct a cage and watch it, as a lion must surely appear in it eventually.
And again and again and again it will happen. And the only possible stable final outcome of all this is: in a universe where time travel is possible, no one will ever create or discover it. From "outside," the timeline will appear to twitch and twist and writhe for perhaps a very long subjective time indeed before it freezes into its final form, but from the perspective of those living within it, time travel was never invented and things have always been like this.
This was one of the themes of one of my very favorite Niven short stories, "Rotating Cylinders and the Possibility of Global Causality Violation," in which the universe itself seems to have a perverse and malevolent will of its own, aggressively destroying civilizations that get too close to creating time travel. For if it were not so, if the nature of the universe were anything else, paradox would destroy the universe altogether. If the universe exists, everyone everywhere who gets too close to creating time travel must die, even if it requires the extinction of entire species by plague or unexpected supernova.
Thank you, Anonymous. Niven wrote an article, "The Theory and Practice of Time Travel," and an even better one on teleportation.
It doesn't require a sentient universe for this to work.
The logic chain goes:
Time travel is possible.
Time is mutable.
If time travel occurs, the past will be repeatedly changed.
The only way for this to stop is for a change in history that prevents the -invention- of time travel.
So changes will continue until that happens.
That's the "rest state". Time travel is possible, time is mutable, but the line of historical development is changed by time travel until nobody ever finds out how to do it.
Post a Comment