NumberTenOx

In a family conversation about time travel last night (we had just watched T2), my son came up with an idea about why it might not be possible that I hadn't heard before. My guess is that this idea has been presented before, and I'm wondering if anyone has heard it.

Assume that time travel backward to an arbitrary point in history is possible, even easy. Assume that the time traveler can interact with the past, and that the interaction will change the future.

In this case, each time traveler will dramatically change the future, with even a small perturbation in the past compounding as time marches forward. This will affect the discovery of time travel itself, so that maybe it's discovered at a different time by a different person.

This situation will continue, with new time travelers going backward and different changes going forward - until you reach a configuration where the future is changed so that no one discovers time travel, at which point the process "sticks".

So even if time travel is possible, it inevitably deletes itself.

(Of course, one way to be sure that no one discovers time travel is to reach a configuration where humanity destroys itself. However, it's possible that leaps in animal complexity in the past were enabled by mass extinctions, so that even if we destroyed ourselves, another even-smarter race would arise and be even more likely to invent time travel.)

I feel compelled to add that I don't believe that time travel is possible, or if it is possible that it would take this form, but it's still fun to think about.

SimplyAtheistic

I personally believe time travel is simply impossible. It is a wish many people try and support, but it is illogical at its core.
Edit: Except into the future, I was stating travel into past time is impossible.
Jake

Jesse

Science fiction author Larry Niven had a similar idea, which is described in this entertaining page on "SF Chronophysics":

Of course, most physicists who theorize about the possibility that time travel might actually be possible (through traversable wormholes, for example) believe we are living in what the page above calls a "Type One" timeline, where all of history is a fixed four-dimensional entity and it is therefore impossible to change the past, and any of your actions in the past were a part of history all along. This would eliminate all potential for paradoxes.

NumberTenOx

Larry Niven, I should have known...

Thanks for the cool pointer!

Jesse

Well, why do you think it's illogical/impossible? Again, the fixed timeline theory would eliminate the possibility of paradoxes.

goat37

I don't see how one could travel into the past. However, the future may be a different story.

Just traveling at near the speed of light would be enough to make a significant difference between your passage of time and someone not traveling near the speed of light. For instance, if you were to conceive a baby on earth, and conceive a baby right on the outer edge of an event horizon (just far enough so you didn't get sucked into the black hole) the conceived baby on earth, would be born, grown up, re-produced, died, and gone through many generations before the conceived baby on the outer edge of the event horizon would be past a fetus stage.

Therefore, if you were somehow able to traverse space at even nearly the speed of light, it would be like a big fast forward button through time. If you saw the movie contact, when Matthew McConoughey talks to Jodie Foster about it, he says that while she will have only aged 4 years, over 50 years will have passed by on earth. That's pretty much what it would be like if you could travel even close to the speed of light.

A good book to read, that goes over physics in general, is The Elegant Universe by Brian Greene. It doesn't specifically go over time travel, but it gives you a good understanding of the forces involved.

Jesse

General relativity allows time travel into the past via wormholes, although no one knows whether this will still be possible when physicists find a complete theory of quantum gravity. Kip Thorne's book Black Holes & Time Warps contains a good discussion of these issues.

The_Unknown_Banana

I also don't think backward time travel is possible, but then I'm no expert. Anyway, here's my thinking on this scenario, and why it doesn't necessarily defeat the possibility.

Time travellers going back in time might change events, for sure. However, who can say that it would lead to the de-invention of time travel? What if it allowed time travel to be developed by someone else even sooner?? Also, if you make room for multiple time lines (Who was it that said something like this? Where every possible action veers off into a different timeline.. besides "back to the future" that is :P) then it would merely change the course time takes, not "delete" any events. I have no idea if I am just talking a bunch of garbage or not though lol.

NumberTenOx

Actually, this is the same scenario. If someone developed time travel even sooner, then some new bloke would travel back in time and change the future again. This keeps happening until some time traveler changes the future into a state where no one discovers time travel. Then the cycle stops.

This is a possibility but a different scenario, where there are multiple timelines. I'm not a fan of these at all, but then what do I know?

Jesse

But the idea of a single changeable timeline would inevitably lead to paradoxes, so there's no way it could be possible in real life, although it's common in science fiction. The only paradox-free options are the multiple-branching-timelines idea and the single-fixed-timeline idea.