Can Time Travel Really Avoid Violating Causality?

[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.