Exploring Causality: Can Time Travel Break the Rules?

[JesseM]

why are you quoting my own post back at me? Did you mean to write a reply to that post?

 

[byron178]

why are you quoting my own post back at me? Did you mean to write a reply to that post?

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]

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?

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.

 

[byron178]

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.

does this also fall under the quantum gravity ruling it out?

 

[JesseM]

does this also fall under the quantum gravity ruling it out?

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

 

[byron178]

Yeah, you might have missed it but that's what I meant when I said:

So Time travel back in time might be ruled out all together by quantum gravity?

 

[JesseM]

So Time travel back in time might be ruled out all together by quantum gravity?

Yes.

 

[byron178]

Yes.

what about the time travel without violating causality? could that be ruled out also by quantum gravity?

 

[JesseM]

what about the time travel without violating causality? could that be ruled out also by quantum gravity?

Can you explain what you mean by "time travel without violating causality"?

 

[byron178]

Can you explain what you mean by "time travel without violating causality"?

one example would be tachyons.They travel faster than light but don't violate causality,how does that work?

 

[JesseM]

one example would be tachyons.They travel faster than light but don't violate causality,how does that work?

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]

i also was reading that virtual particles can be tachyonic and travel at speeds faster than light,would this not violate causality?

 

[Ryan_m_b]

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

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?

 

[JesseM]

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?

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.

 

[JesseM]

i also was reading that virtual particles can be tachyonic and travel at speeds faster than light,would this not violate causality?

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]

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.

So virtual particles are not real at all?

 

[JesseM]

So virtual particles are not real at all?

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

 

[byron178]

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

so they are as real as the bogeyman,so he is saying they are not real at all?

 

[JesseM]

so they are as real as the bogeyman,so he is saying they are not real at all?

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.

 

[Ryan_m_b]

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.

Yes but how would a wormhole 'know' if was within the others light cone? If I accelerated wormhole B up to near light speed and back down again to at rest relative to A it is out of sync and out of the light cone. If I accelerate it up again but now heading back the way I came it will become more out of sync but now be in A's light cone.

 

[JesseM]

Yes but how would a wormhole 'know' if was within the others light cone?

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.

 

[byron178]

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.

so virtual particles do in fact travel backwards in time?

 

[JesseM]

so virtual particles do in fact travel backwards in time?

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]

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)

So they exist for a fraction of a second?

 

[JesseM]

So they exist for a fraction of a second?

Are you pulling my leg? I said a few times you don't necessarily have to consider them "real" at all...

 

[byron178]

Are you pulling my leg? I said a few times you don't necessarily have to consider them "real" at all...

No i was being serious,i was reading that virtual particles in fact travel faster than light and backwards in time but don't violate causality,but i can't seem to wrap my head around this one.

 

[JesseM]

No i was being serious,i was reading that virtual particles in fact travel faster than light and backwards in time but don't violate causality,but i can't seem to wrap my head around this one.

If you don't consider them real, but just a mathematical tool for making calculations about observable effects, why is this a problem? No actual measured effects go FTL or backwards in time.