The discussion centers on the concept of time travel and its implications for causality, exploring whether backward time travel can occur without violating causal relationships. Participants examine theoretical frameworks, including Quantum Field Theory and general relativity, and consider various models such as closed timelike curves and traversable wormholes.
Participants express multiple competing views regarding the implications of time travel on causality, with no consensus reached on whether backward time travel can occur without violating causal principles.
Discussions involve complex theoretical frameworks and assumptions about time, causality, and the nature of particles, with unresolved questions about the implications of various models.
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