Time Travel Paradox

I am quite confused. Along time ago I was considering time travel. I conducted several thought experiements and came up with several paradoxes.First the pure logic without the thought experiement.A is cause. B is the effect of the cause A.
A causes B.B is the action of going back in time and preventing A from happening.
A never happened, therefore B never happened. But if B never happened, then A did happen, meaning B did happen. But if B did happen then A didn't happen. But A is the cause of B so then B didn't happen. But if B didn't happen then A did happen which means B did happen.How do I resolve this? I don't see how we are going to perform time travel if we can't get around this.Now for the thought experiement.Lets say we have a table with raised edges. There is one ball on the table. There are two holes, one on the right side of the table and one on the top side of the table. The right hole leads to the top hole10 minutes ago.Now, the ball rolls for 10 minutes (this is a really long table and a fast moving ball) from the left of the table for the duration of 10 minutes into the right hole. Half way there, the same ball comes out of the top hole from the future and collides with the other ball. Now the other ball is moved away from the hole so that it cannot enter it.What happens now?
Does the future ball suddenly dissapear? Does that mean the future ball was never there? And if the future ball was never there does that mean that the present ball automatically jumps back to where it was in order to move into the hole. But if it is going into the slot then doesn't the future ball come and stop it?At what point in time does the change in history take affect? Why should the paradox occur only once the collision happens? Does it occur before the present reaches the time of when the collision occurs?Is there a way to mathematically figure out what ultimatley happens to the ball?I would really like some answers.Edited by Guido Arbia, : No reason given.Edited by Guido Arbia, : No reason given.

Thread moved here from the Coffee House forum.

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A is cause. B is the effect of the cause A.
A causes B.

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B is the action of going back in time and preventing A from happening.
A never happened, therefore B never happened. But if B never happened, then A did happen, meaning B did happen. But if B did happen then A didn't happen. But A is the cause of B so then B didn't happen. But if B didn't happen then A did happen which means B did happen.

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When you travel back and change the history you spilt off into an alternative timeline - isn't that the standard answer to that problem?

Yeah, in weak sci-fi But the usual way is "consistency". In General Relativity, any possible space-time is a full 4d entity: past, present, future. You cannot start with some fixed past and generate arbitrary futures. The space-time comes as the whole package. Does wonders for thoughts of pre-destination and free-will.A paradoxical scenario as described by Guido is not a valid space-time... hence it can't happen. Only consistent space-times are allowed.By the way, we actually have a name for Guido's experimental set-up: wormhole billiards, the subject of my very first research. One really intersting part of this is if we fix the initial and final conditions, say ball rolls towards the in-hole, and ball rolls away from out-hole, then classically there should only be one solution.But with the time-machine aspect, we get an infinite number of solutions which we must sum over to get some average: ball goes in in-hole, comes out of out-hole; ball goes in in-hole, comes out out-hole, goes back in in-hole and finally come out of out-hole again; ball isn't going in, but collides with it-self, knocking it in... it comes out, just in time to make the collison with itself and heads out; etc, etc, etc.This is suspiciously like Feynman sum-over-histories quantum mechanics...Edited by cavediver, : No reason given.

Nice thought experiment Check out my reply to CK above. We wait for the "time-lines to adjust" whenever I hear that in Star Trek (all series) I reach for my gun...Edited by cavediver, : No reason given.

hey cave, i had someone sugest that warpdrive is possible, what do you think of that?
i know OT, but i wanted to know if anyone thinks anything in startrek is possible ever

I wonder how do QM probabilities fit in this neat little plot? no kidding!

Well, the obvious answer is that they don't and won't until we have a more concrete idea of quatum gravity.However, I personally do not believe* in quantum probabilities as such. QM is wholly deterministic and the probabilities only come from this bodge known as wave-function collapse/state-vector reduction. My vote is with decoherence, where the classical regime is reached through wave-functions evolving into more and more sharply peaked "classical" distributions through countless yet deterministic interactions with the environment. The graviton background may be a major player in this environment. The QM probabilities are just averages over these unobserved environmental interactions. Not at all! Wormholes as we now know them were discovered by Misner and Thorne by the direct request of Sagan whilst he was writing Contact! He needed a reasonable FTL scenario. This spawned a spate of papers in the mid to late eighties.Any mechanism that allows efective FTL travel has the capacity to form Closed Time-like Curves (CTCs), commonly known as backwards time-travel.*as in, 75% of the time I don't believe, and 25% I do Only a fool would ever be definite about any of this stuff...

There is a theoretical version of it known as Alcubierre’s Warp Drive, bt it's not something you would carry on a ship! It would probably require some roped off corridor of space between to fixed points to work properly. Not that OT. As I mentioned in the post above, any FTL mechanism is automatically a time machine and will give rise to Guido's original concerns.Don't try to follow Star Trek to closely on its physics. Voyager could have been home in a week, but it would have been difficult squeezing in 5 or more series

Yes, indeed. The postulate of the collapse of the wave function is he weakest link of the QM theory. That's interesting. But I don't see how one could acommodate that "simulated collapse" with the quantum entaglement phenomenon short of introducing faster than light communication between the particles. (I suppose that might not be a problem since you're already allowing for time travel in the picture. But it still seems paradoxical)

does it not come down to the classic answer .. ..................if time travel is possible some one in the future will have already done it and so we would be living in the effects ....................... this means either the time polcie are very very good and hidding any evidence ............................ like the dinosaurs died out from over hunting by visitors from the future or new new improved coke will not come out for another 27 years instaed of in 1973 .................. OR time travel is not possible

You can change the four dimensional object if only there is a fifth dimension so that the cause is actually outside the four dimensional object so that changes in this 4 dimensions will not change what is in the fifth dimension.

Ok, time for me to look stupid.In your A-B paradox, it seems that you are leaving out the distance traveled going back to A.Since time and distance are the same thing, there must be some sort of trail going back to A, that wouldn't get erased.Another idea. Since time and space are curved, wouldn't going back in time cause the space curve to curl back on itself, creating a loop? Once you get back to A, you would have to reduce the radius of the curve needed to get back there, and start going forward again.I think time, is not a linear thing, and may have demensions of it's own. When I imagine time, or a timeline, I think of it as traveling in a particular direction, which is probably untrue.Crap my head hurts trying to imagine it.
Sorry, just my ignorant uneducated thoughts.

Not necessarily. Any time-machine cannot send you back earlier than its own first moment of existence. So, you cannot build a time-machine to go back in time (sorry!), but can build one to return to that time.The absence of time-travellers only suggests that we haven't managed to exploit naturally occuring or alien time machines. Or as you say, the time cops are good...The boring answer is that time travel, although theoretically possible with our current understanding, is practically impossible. Every time-machine envisioned is dogged by problems, sufficiently so that Hawking suggested the Chronology Protection Conjecture (CPC) that states the universe contrives to make (backwards) time-travel impossible.

Just imagine for a moment that wormholes exist. You take one wormhole and you accelerate one end while leaving the other alone. Get where I'm going with this?