Exploring Causality: Can Time Travel Break the Rules?

[byron178]

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?

 

[PhilDSP]

(I'm new to this argument so by all means please correct me where I'm wrong)

Peskin and Schroeder show that no violation of causality occurs within Quantum Field Theory. Positrons and other particles that mathematically appear to travel backwards with respect to the forward progression of time represent only one part of the symmetry inherent in any particle interactions. The necessary addition of both the field of the forward-in-time propagating particle indicating the probability of the particle existing in a region, and the field indicating the probability of the backwards-in-time propagating particle appearing results in a cancellation of mutual probabilities.

 

[granpa]

google Andromeda paradox

 

[JesseM]

google Andromeda paradox

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".

 

[totentanz]

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?

What do we mean when we say travel to the past or to the future? does the past still EXIST somewhere else?and is the future just a station waiting for us-it is already there-to reach it?
I do not think so,time is just a concept of the mind,or as Kant says"Synthetic a priori"
The only thing that you can speak about is relativity,get a ship with the speed of light...3days after,come back and you see the change,and you will know what happen,BUT imagine that no thing changes...how can you konw the difference? I mean how can you understand the concept of time?
Try Martin Heidegger:Being in Time...this will make it more clear...thanks for giving me your Time

 

[ExecNight]

Well i am guessing it is just mathematically modeled, but a particle going back in time should simply mean dissappearing from existence, from our reference point?

That would mean a particle traveling back in time would look like, popping out of nowhere for us, then dissappearing.

Now this kind of backward causality is confusing. Meaning observations at t, t+1 and t+2 are somewhat related to each other.

At t+1 i see a particle, at t+2 its just dissappeared. Then what about my t observation? So was this particle also going back in time at t+1?

Where was it coming from t+?, where is it going t-?..

Is t+1 simply our crossing point? How does this really flow? t+2 i see no particle but i should be seeing the affectes of that particles passing backwards in time?

So many questions.

 

[byron178]

Don't closed timelike curves violate causality?

 

[byron178]

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".

sorry I am answering twice,but,dont ctc's violate causality?

 

[JesseM]

sorry I am answering twice,but,dont ctc's violate causality?

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]

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.

So A future theory of quantum gravity will problaby rule out closed timelike curves?

 

[JesseM]

So A future theory of quantum gravity will problaby rule out closed timelike curves?

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]

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.

Can't we say we have evidence that ctc's don't exist since like hawking said we have not seen time travelers?

 

[JesseM]

Can't we say we have evidence that ctc's don't exist since like hawking said we have not seen time travelers?

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:

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.

 

[byron178]

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:

but in order for wormholes to exist wouldn't you need a white hole?so what your also saying is string theory might rule out ctc's?

 

[JesseM]

but in order for wormholes to exist wouldn't you need a white hole?

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]

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.

ok so exotic matter is needed for traversable wormholes?

 

[JesseM]

ok so exotic matter is needed for traversable wormholes?

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]

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.

What Would most experts say about closed timelike curves? that they exist?

 

[JesseM]

What Would most experts say about closed timelike curves? that they exist?

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]

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.

if ctc's existed would that mean trouble?

 

[JesseM]

if ctc's existed would that mean trouble?

What do you mean by "trouble"? It would mean causality violations.

 

[byron178]

What do you mean by "trouble"? It would mean causality violations.

wouldn't laws be at threat by ctc's?

 

[JesseM]

wouldn't laws be at threat by ctc's?

No, unless you believe in miracles, the only way CTCs could exist would be if the laws (like GR) allowed them.

 

[byron178]

No, unless you believe in miracles, the only way CTCs could exist would be if the laws (like GR) allowed them.

if something were to travel backwards in time,would it violate causality?

 

[JesseM]

if something were to travel backwards in time,would it violate causality?

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]

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].

So your saying time travel backwards does not always violate causality?

 

[JesseM]

So your saying time travel backwards does not always violate causality?

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]

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.

is the only way to do this is through wormholes?

 

[JesseM]

is the only way to do this is through wormholes?

No, look at the bottom section here for some other examples:

http://en.wikipedia.org/wiki/Template:Time_travel

 

[byron178]

No, look at the bottom section here for some other examples:

http://en.wikipedia.org/wiki/Template:Time_travel

there is an error when i try to click on the page.EDIT Nevermind it works.are all these examples possible in reality?

 

[JesseM]

there is an error when i try to click on the page.EDIT Nevermind it works.are all these examples possible in reality?

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]

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.

so all the examples you talk about will probably be ruled out by quantum gravity?

 

[byron178]

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].

even this example might be ruled out by quantum gravity?

 

[JesseM]

so all the examples you talk about will probably be ruled out by quantum gravity?

I think probably, but it's a matter of opinion since we don't actually have such a theory yet.

 

[byron178]

I think probably, but it's a matter of opinion since we don't actually have such a theory yet.

Actually it's better if the portal allows you to get somewhere FTL (FTL from the point of view of light traveling the "long way", of course you still move slower than any light which travels through the wormhole with you). If the regions of spacetime the portal connects have what's called a "space-like separation", meaning that no signal traveling the regular way could get from one to the other without traveling FTL, then there's actually no danger of time travel here. For example, say there's a star 100 light years away and traveling through the portal on Earth in 2000 would cause you to step out at the location of that star in 1950 (with time being defined relative to the rest frame of the Earth and star). Sure you've gone back in time, but if you send a light signal back towards Earth it won't actually reach them until 2050, and if you step back through the portal in the other direction it takes you 50 years into the future, so you're back at Earth in 2000 (or a little later if you hung out at the star for a while).

On the other hand, suppose there's another star 20 light years away, but stepping through the portal on Earth in 2000 takes you to the star in 2040. In this case the separation is "time-like", meaning that stepping through the portal won't get you to the star faster than a light beam would. It might seem like there's no problem here, but the portal is two-way, meaning if you step back through the portal at the star in 2040, you'll end up at Earth in 2000, and in general if you step through the portal at the star in year Y you'll end up at Earth in Y-40. So now say in 2000 you get in a rocket which flies to the star at 0.8c, covering the 20 light years in 20/0.8 = 25 years. This means you'll arrive at the star in 2025, so if you step through the portal you'll now be on Earth in 2025-40=1985, in your own past! Could this example also be ruled out by quantum gravity?