Exploring the Possibility of Time Travel

[Dr.Calpol3]

does anyone thinks time travel is possible?

 

[smallphi]

SR and GR allow time travel in the future - the twin paradox (experimentally verified). Time travel in the past is thought impossible due to the causality paradoxes it can create like killing your father lol

 

[MeJennifer]

does anyone thinks time travel is possible?

Assuming you mean traveling backwards in time nobody knows.

Time travel in the past is thought impossible due to the causality paradoxes it can create like killing your father

I see no reason whatsoever why closed timelike curves are impossible. Usually those who object resort to "free will" arguments.

 

[Dr.Calpol3]

Time travel in the past is thought impossible due to the causality paradoxes it can create like killing your father lol

you wouldn't want to go and kill you own father anyway, that would result in your own death instantaniously

 

[Dr.Calpol3]

Assuming you mean traveling backwards in time nobody knows. QUOTE]

i meant time travel in general.

 

[cristo]

you wouldn't want to go and kill you own father anyway, that would result in your own death instantaniously

One can't assume that one would simply "disappear" if that happened! This scenario is very popular by science fiction writers, and thus has been discussed quite a lot on the internet. Physicists like to describe such situations using inamimate objects, and so arose a version of this paradox called "Polchinski's paradox." This (like the question you asked on wormholes in your other thread) is discussed in Kip Thorne's popular science book. Give it a go-- there'll probably be a copy in your local university library.

i meant time travel in general.

Well, forwards travel in time is not that interesting-- we are doing it now for example! So, I presume you really do mean traveling to the past. As Jennifer said, it is unknown whether travel backwards in time is allowed. Personally, I think it's possible, but am not sure whether we will ever find out how to. Hawking famously condemns time-travel, and put forward a chronology protection conjecture, which says that the laws of physics prevent time travel. One of his arguements is that, if time travel were possible, then why have we not seen anyone come back from the future to visit?

 

[Dr.Calpol3]

I didn't say you would disappear, i just meant that one wouldn't have been born at all.

 

[cristo]

I didn't say you would disappear, i just meant that one wouldn't have been born at all.

Ok, well then the person's wordline would disappear. Your worldline is a path in spacetime starting at your birth, and ending at your death. I cannot see how one's worldline could just disappear like this! (This has the danger of becoming a rather philospohical discussion if we're not too careful!)

 

[JesseM]

Ok, well then the person's wordline would disappear.

Not according to the resolution suggested in Kip Thorne's book Black Holes and Time Warps which you mention above--his suggestion is just that the timeline must be entirely self-consistent (I think this is justified with QM using the path integral approach), so it would be impossible to "change" any aspect of the past, you could only fulfill things that had been part of your history all along. His analogue of the grandfather paradox is sending a billiard ball through at an angle such that when it comes out the other wormhole mouth in the path, it will be on a trajectory that will knock its younger self out of the way so it never goes back in time--his resolution in this scenario is that the billiard ball from the future will come out on a slightly different trajectory which will deliver only a glancing blow to its younger self, a glancing blow which will slightly alter the angle it goes into the wormhole in just the right way that it will come out in the past with the new trajectory needed to deliver that glancing blow.

 

[cristo]

Not according to the resolution suggested in Kip Thorne's book Black Holes and Time Warps which you mention above--his suggestion is just that the timeline must be entirely self-consistent (I think this is justified with QM using the path integral approach), so it would be impossible to "change" any aspect of the past, you could only fulfill things that had been part of your history all along. His analogue of the grandfather paradox is sending a billiard ball through at an angle such that when it comes out the other wormhole mouth in the path, it will be on a trajectory that will knock its younger self out of the way so it never goes back in time--his resolution in this scenario is that the billiard ball from the future will come out on a slightly different trajectory which will deliver only a glancing blow to its younger self, a glancing blow which will slightly alter the angle it goes into the wormhole in just the right way that it will come out in the past with the new trajectory needed to deliver that glancing blow.

Indeed, however this shows that one cannot go back in time and kill one's own grandfather. I'm not disagreeing with this, but merely responding to that point put forward that one would instantly die one killing ones father, which was brought up above.

 

[JesseM]

Indeed, however this shows that one cannot go back in time and kill one's own grandfather.

Yes, but the full implication of his method of resolving such paradoxes is that one cannot go back and "change" anything whatsoever, the timeline is entirely fixed. (The notion of 'changing' the past is pretty incoherent unless you invoke parallel universes, anyway...usually 'change' means comparing one thing to another and noting a difference, but how can you note a difference between the 'original' version of history and the 'changed' one if the original version not only doesn't exist, but it never did?)

 

[smallphi]

I see no reason whatsoever why closed timelike curves are impossible. Usually those who object resort to "free will" arguments.

Is there actually a physically meaningfull metric (solution of Einstein) with closed timelike curves?

 

[JesseM]

Is there actually a physically meaningfull metric (solution of Einstein) with closed timelike curves?

There are such metrics, but they all involve either unrealistic cosmologies or else violations of the weak energy condition, and I think all the known ones violate the null energy condition too (although in http://prola.aps.org/abstract/PRD/v46/i2/p603_1 Hawking provided a general proof that all time machines in finite regions must violate the weak energy condition, I don't know if there's a similar proof for the null energy condition, I've just seen arguments about it being violated for specific types of solutions like wormholes and the Albucierre drive in papers like this), so whether they are "physically meaningful" is uncertain. I believe it's thought that it is possible to consistently violate the weak energy condition in quantum theory by using the Casimir effect, but based on papers like this I get the impression that it's currently thought that the null energy condition isn't violated by any known effects except in a transient way...someone please correct me if I'm wrong!

Furthermore, it's also thought that even if wormholes held open by the Casimir effect are possible, some sort of feedback loop of vacuum fluctuations in quantum field theory could cause the energy density on their boundary to approach infinity as soon as they were brought into the right position to allow closed timelike curves (one mouth entering the light cone of the other mouth rather than being spacelike separated), destroying them...I believe this is also discussed in the Hawking paper above, and it's definitely discussed in Kip Thorne's popular book Black Holes and Time Warps.