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.
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?
JesseM said: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.
Probably depends how you define "real", I'm not really that well-versed in the subject of virtual particles myself, but for another opinion look at chapter A7 of A. Neumaier's physics FAQbyron178 said:So virtual particles are not real at all?
JesseM said:Probably depends how you define "real", I'm not really that well-versed in the subject of virtual particles myself, but for another opinion look at chapter A7 of A. Neumaier's physics FAQ
That's what he is saying, I don't know if his opinions reflect the typical perspective of physicists on this or not though, like I said it's not something I'm very knowledgeable about.byron178 said:so they are as real as the bogeyman,so he is saying they are not real at all?
JesseM said: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.
I believe the idea is that the wormhole is destroyed, either by real particles or vacuum fluctuations, which go from point A back to an earlier point B through the wormhole, then get back to point A by traveling at the speed of light or slower through the regular space outside the wormhole, then the process repeats ad infinitum so the energy density goes to infinity. This would only be possible if you can actually get from B to A through normal space, which only happens when B is in the past light cone of A.ryan_m_b said:Yes but how would a wormhole 'know' if was within the others light cone?
JesseM said:I believe the idea is that the wormhole is destroyed, either by real particles or vacuum fluctuations, which go from point A back to an earlier point B through the wormhole, then get back to point A by traveling at the speed of light or slower through the regular space outside the wormhole, then the process repeats ad infinitum so the energy density goes to infinity. This would only be possible if you can actually get from B to A through normal space, which only happens when B is in the past light cone of A.
They are represented as doing so, but as I said you don't necessarily have to consider them "real", they could just be seen as a method of calculating the value of some observable things like the energy density in some region (aside from wormholes, another example would be the space between plates in the Casimir effect)byron178 said:so virtual particles do in fact travel backwards in time?
JesseM said:They are represented as doing so, but as I said you don't necessarily have to consider them "real", they could just be seen as a method of calculating the value of some observable things like the energy density in some region (aside from wormholes, another example would be the space between plates in the Casimir effect)
Are you pulling my leg? I said a few times you don't necessarily have to consider them "real" at all...byron178 said:So they exist for a fraction of a second?