The Andromeda paradox has nothing to do with anything actually traveling into the past, it's just an issue of the relativity of simultaneity. Might be better to google "closed timelike curves" or "traversable wormhole".granpa said:google Andromeda paradox
byron178 said:Can something travel backwards in time i.e back to the future sort of thing and not violate causality? Or is the very presence of something traveling to the past a violation of causality?
JesseM said:The Andromeda paradox has nothing to do with anything actually traveling into the past, it's just an issue of the relativity of simultaneity. Might be better to google "closed timelike curves" or "traversable wormhole".
Yes, they do. But it isn't guaranteed that the laws of physics will preserve causality, CTCs do arise in certain solutions to the equations of general relativity. But physicists expect general relativity will turn out to just be an approximation to a future theory of quantum gravity such as string theory, so it's an open question whether this final theory of quantum gravity would allow CTCs in the situations where GR predicts them.byron178 said:sorry I am answering twice,but,dont ctc's violate causality?
JesseM said:Yes, they do. But it isn't guaranteed that the laws of physics will preserve causality, CTCs do arise in certain solutions to the equations of general relativity. But physicists expect general relativity will turn out to just be an approximation to a future theory of quantum gravity such as string theory, so it's an open question whether this final theory of quantum gravity would allow CTCs in the situations where GR predicts them.
I'd say it probably will but that's just a personal hunch...there are apparently some results in string theory that suggest this though, see the "New Scientist" article posted here.byron178 said:So A future theory of quantum gravity will problaby rule out closed timelike curves?
JesseM said:I'd say it probably will but that's just a personal hunch...there are apparently some results in string theory that suggest this though, see the "New Scientist" article posted here.
No, because GR solutions like traversable wormholes that allow you to create a time machine in a region that didn't have one before (as opposed to cosmological solutions where time travel is possible everywhere in the universe for all eternity, like the Godel metric) don't actually allow you to travel back to a time before the time machine was created, so for example if a traversable wormhole is created in the year 3000 then people in the year 3020 could use it to travel back to 3010 but couldn't use it to travel back to before 3000. Hawking actually makes this point himself in http://www.hawking.org.uk/index.php/lectures/publiclectures/63 , where he says:byron178 said:Can't we say we have evidence that ctc's don't exist since like hawking said we have not seen time travelers?
A possible way to reconcile time travel, with the fact that we don't seem to have had any visitors from the future, would be to say that it can occur only in the future. In this view, one would say space-time in our past was fixed, because we have observed it, and seen that it is not warped enough, to allow travel into the past. On the other hand, the future is open. So we might be able to warp it enough, to allow time travel. But because we can warp space-time only in the future, we wouldn't be able to travel back to the present time, or earlier.
This picture would explain why we haven't been over run by tourists from the future.
JesseM said:No, because GR solutions like traversable wormholes that allow you to create a time machine in a region that didn't have one before (as opposed to cosmological solutions where time travel is possible everywhere in the universe for all eternity, like the Godel metric) don't actually allow you to travel back to a time before the time machine was created, so for example if a traversable wormhole is created in the year 3000 then people in the year 3020 could use it to travel back to 3010 but couldn't use it to travel back to before 3000. Hawking actually makes this point himself in http://www.hawking.org.uk/index.php/lectures/publiclectures/63 , where he says:
Nope, a traversable wormhole is different from an Einstein-Rosen bridge (the first kind of 'wormhole' discovered, which is impossible to actually cross before it snaps shut), although an Einstein-Rosen bridge consists at different times of either two black holes or two white holes, the mouths of a traversable wormhole aren't black holes or white holes since they don't have event horizons.byron178 said:but in order for wormholes to exist wouldn't you need a white hole?
JesseM said:Nope, a traversable wormhole is different from an Einstein-Rosen bridge (the first kind of 'wormhole' discovered, which is impossible to actually cross before it snaps shut), although an Einstein-Rosen bridge consists at different times of either two black holes or two white holes, the mouths of a traversable wormhole aren't black holes or white holes since they don't have event horizons.
Yup, and as it turns out it'd be needed for all GR "time machines" that are created in some finite region of space, Hawking proved a theorem showing that this was the case. Some type of exotic matter/energy seems to be possible via the Casimir effect but it's unknown whether it would be possible to have exotic matter/energy with all the necessary properties, see here.byron178 said:ok so exotic matter is needed for traversable wormholes?
JesseM said:Yup, and as it turns out it'd be needed for all GR "time machines" that are created in some finite region of space, Hawking proved a theorem showing that this was the case. Some type of exotic matter/energy seems to be possible via the Casimir effect but it's unknown whether it would be possible to have exotic matter/energy with all the necessary properties, see here.
No, just that we don't know yet whether they can exist or not. My guess is that if they had to bet, most experts would bet that they will turn out not to be possible in a complete theory of quantum gravity, but that's just my guess, I don't know of any polls or anything like that.byron178 said:What Would most experts say about closed timelike curves? that they exist?
JesseM said:No, just that we don't know yet whether they can exist or not. My guess is that if they had to bet, most experts would bet that they will turn out not to be possible in a complete theory of quantum gravity, but that's just my guess, I don't know of any polls or anything like that.
What do you mean by "trouble"? It would mean causality violations.byron178 said:if ctc's existed would that mean trouble?
JesseM said:What do you mean by "trouble"? It would mean causality violations.
No, unless you believe in miracles, the only way CTCs could exist would be if the laws (like GR) allowed them.byron178 said:wouldn't laws be at threat by ctc's?
JesseM said:No, unless you believe in miracles, the only way CTCs could exist would be if the laws (like GR) allowed them.
Only if it could travel back into its own past light cone. If I could travel "back in time" in the coordinates of some frame, but to a point with a spacelike separation from the point I departed, there wouldn't necessarily be any problem with causality, see the example I give in [post=3268604]this post[/post].byron178 said:if something were to travel backwards in time,would it violate causality?
JesseM said:Only if it could travel back into its own past light cone. If I could travel "back in time" in the coordinates of some frame, but to a point with a spacelike separation from the point I departed, there wouldn't necessarily be any problem with causality, see the example I give in [post=3268604]this post[/post].
Going back in time relative to some inertial frame doesn't necessarily violate causality, but if there's a spacelike separation between departure and arrival, then only in some frame will the arrival be before the departure, in others it'll be after so these frames will say you didn't go backwards at all. On the other hand, if your arrival is in the past light cone of your departure, all inertial frames agree you arrived at an earlier time than you departed.byron178 said:So your saying time travel backwards does not always violate causality?
JesseM said:Going back in time relative to some inertial frame doesn't necessarily violate causality, but if there's a spacelike separation between departure and arrival, then only in some frame will the arrival be before the departure, in others it'll be after so these frames will say you didn't go backwards at all. On the other hand, if your arrival is in the past light cone of your departure, all inertial frames agree you arrived at an earlier time than you departed.
No, look at the bottom section here for some other examples:byron178 said:is the only way to do this is through wormholes?
JesseM said:No, look at the bottom section here for some other examples:
http://en.wikipedia.org/wiki/Template:Time_travel
None have been observed and a lot of physicists think the CTCs which appear in general relativity solutions will probably end up getting ruled out in a theory of quantum gravity.byron178 said:there is an error when i try to click on the page.EDIT Nevermind it works.are all these examples possible in reality?
JesseM said:None have been observed and a lot of physicists think the CTCs which appear in general relativity solutions will probably end up getting ruled out in a theory of quantum gravity.
JesseM said:Only if it could travel back into its own past light cone. If I could travel "back in time" in the coordinates of some frame, but to a point with a spacelike separation from the point I departed, there wouldn't necessarily be any problem with causality, see the example I give in [post=3268604]this post[/post].
I think probably, but it's a matter of opinion since we don't actually have such a theory yet.byron178 said:so all the examples you talk about will probably be ruled out by quantum gravity?
JesseM said:I think probably, but it's a matter of opinion since we don't actually have such a theory yet.
JesseM said:why are you quoting my own post back at me? Did you mean to write a reply to that post?
Yes I have read that one, it's a great book for learning about the subject. On the subject of quantum gravity, check out the discussion on p. 505-507, continued on 516-520 (hopefully the page numbers in your edition are the same as mine), about how as soon as the two mouths of a wormhole are brought into a position where time travel could become possible (when one mouth enters the past light cone of the other), it's possible that ordinary electromagnetic waves or quantum "vacuum fluctuations" could loop through the mouths over and over, building up to arbitrarily high energy density which presumably would destroy the wormhole (or make it impossible to enter the wormhole without being destroyed). The idea that quantum vacuum fluctuations would destroy or seal off the wormhole mouths as soon as they are moved into a configuration that would allow time travel is one big idea about how wormhole time travel would be prevented by quantum gravity, as they say on p. 516 this is part of Steven Hawking's chronology protection conjecture.byron178 said:yes i did sorry about that,what i was trying to say is if this example another one of those that might me ruled out by quantum gravity? also I've been reading black holes and timewarps by kip thorne.In the final chapter it talks about time travel without violating causality,i was wondering if you have read it?
JesseM said:Yes I have read that one, it's a great book for learning about the subject. On the subject of quantum gravity, check out the discussion on p. 505-507, continued on 516-520 (hopefully the page numbers in your edition are the same as mine), about how as soon as the two mouths of a wormhole are brought into a position where time travel could become possible (when one mouth enters the past light cone of the other), it's possible that ordinary electromagnetic waves or quantum "vacuum fluctuations" could loop through the mouths over and over, building up to arbitrarily high energy density which presumably would destroy the wormhole (or make it impossible to enter the wormhole without being destroyed). The idea that quantum vacuum fluctuations would destroy or seal off the wormhole mouths as soon as they are moved into a configuration that would allow time travel is one big idea about how wormhole time travel would be prevented by quantum gravity, as they say on p. 516 this is part of Steven Hawking's chronology protection conjecture.
As for the part about time travel without violating causality, I think you're talking about the "Matricide Paradox" section that starts on p. 508 and goes to 516, you might look at the wikipedia article on the Novikov self-consistency principle for more on this idea. Technically this form of time travel still counts as a violation of "causality" since it allows you to travel into your own past light cone and have a causal influence on events you already remember happening, but this idea would at least prevent any paradoxes from arising.
Yeah, you might have missed it but that's what I meant when I said:byron178 said:does this also fall under the quantum gravity ruling it out?
The idea that quantum vacuum fluctuations would destroy or seal off the wormhole mouths as soon as they are moved into a configuration that would allow time travel is one big idea about how wormhole time travel would be prevented by quantum gravity
JesseM said:Yeah, you might have missed it but that's what I meant when I said:
Yes.byron178 said:So Time travel back in time might be ruled out all together by quantum gravity?
JesseM said:Yes.
Can you explain what you mean by "time travel without violating causality"?byron178 said:what about the time travel without violating causality? could that be ruled out also by quantum gravity?
JesseM said:Can you explain what you mean by "time travel without violating causality"?
They would violate causality if they could be used to transmit information FTL, and if the laws governing their behavior worked the same in all inertial frames as required by relativity, see the tachyonic antitelephone.byron178 said:one example would be tachyons.They travel faster than light but don't violate causality,how does that work?
JesseM said:Yeah, you might have missed it but that's what I meant when I said:
The idea that quantum vacuum fluctuations would destroy or seal off the wormhole mouths as soon as they are moved into a configuration that would allow time travel is one big idea about how wormhole time travel would be prevented by quantum gravity
Being out of sync alone isn't a problem, the problem is when they're out of sync enough so that the region of spacetime where you enter one is in the past or future light cone of the region where you exit the other one (ignoring light that actually goes through the wormhole with you when defining light cones). If you remember, I explained this in [post=3268604]this post[/post] from the "Portal" thread.ryan_m_b said:So would this mean that the moment wormholes came into existence (assuming they could) they would collapse unless kept absolutely at rest relative to each other? Surely even a tiny movement of one of them would put it out of sync with the other one, even though the difference would be phenomenally small?
Virtual particles are just used in calculating probabilities in quantum field theory, they can't be measured directly and it's questionable whether they should even be considered "real", see this FAQ.byron178 said:i also was reading that virtual particles can be tachyonic and travel at speeds faster than light,would this not violate causality?
