wormhole like objects

[idobox]

Science fiction is less spectacular without FTL, but FTL leads to time paradoxes.

One way to circumvent this is to use pre-existing shortcurts in the universe, wormholes whose ends are cuasally connected and move slower than light.
Do you know mechanisms other than schwarzshild wormholes iwth exotic matter that could have that kind of behaviour?

[zmatt]

how do wormhole prevent time paradoxes that ftl doesn't? you are still getting to x location before your light does.

[Winter Man]

Does it really matter if you get there before your light? Your light could take a complex route through a bunch of mirrors and arrive at the other end of the hallway after you've walked it and time won't tear itself apart.

[Technical Ben]

Does it really matter if you get there before your light? Your light could take a complex route through a bunch of mirrors and arrive at the other end of the hallway after you've walked it and time won't tear itself apart.

More that "the speed of light" is also the speed of the universe. So when mentioned, it's not the actual photons we are worried about, but the universe around us. So you cannot go faster than, or arrive before, the rest of the universe. Worm holes can provide a way of getting around this, as the universe (the wormhole) has already got to the point you need to get to. At least AFAIK. I'm not sure how wormholes prevent relativity problems, but it may have to do with all observers seeing the wormhole as the common relative point?

PS my explanation on the "universe" being the problem, might be wrong. It's just an example of how to picture it. I'm sure someone will give the correct term. Perhaps "observer" is a better description?

[idobox]

With a wormhole, you don't arrive before your light, because your light also travels through the wormhole.
It is nothing like a teleport, more like a shorter way than what we usually consider to be straight line.

[Robert'); DROP TABLE *;]

Wormholes don't prevent the FTL == time travel problems, except if they're magically exempt from time dilation. (But that causes even more problems than FTL does originally.) As Ben mentioned, c is not so much the speed of light as the speed of the universe, and arriving before the rest of the universe does, no matter how you do it, is going to cause problems.

For instance, imagine you've got two wormholes, A and B and you stick B on very fast spaceship to Alpha Centuari, and it arrives there in what it thinks is one year. (Due to time dilation) If you step through the wormhole and look at Earth with a (impossibly good) telescope, what do you see?

[idobox]

Images form earth delayed by the flight time fromEarth to Alpha Centaury.Now if someone on the ship looks through the wormhole, it's the same as if the ship did a round trip at relativistic speeds.

If you consider the wormhole to be a shortcut, proxima centauri is not 4 ly away, but only a few UA. Light still propagate normally.
There are a few paths for light from the sun to Earth. One goes to the nearest black hole and back, and is considerably longer than the straight line, yet travelling along the straight line does not cause time travel paradoxes. I see a wormhole as yet another path, only much shorter than the straight line.

[Winter Man]

Wormholes don't prevent the FTL == time travel problems, except if they're magically exempt from time dilation. (But that causes even more problems than FTL does originally.) As Ben mentioned, c is not so much the speed of light as the speed of the universe, and arriving before the rest of the universe does, no matter how you do it, is going to cause problems.

You're looking at it like they're two objects that allow teleportation between them. If you look at it like an actual, physical link between to places using more space-time, gravity and warping and such propagate through the wormhole just as they do elsewhere. The time dilation caused by accelerating B would carry over to A, meaning they're in the same reference frame.

edit: Quoted the wrong bit. Made NO sense.

[gmalivuk]

The time dilation caused by accelerating B would carry over to A, meaning they're in the same reference frame.

It doesn't matter. Causality can be violated even if instantaneous travel only happens between points that aren't moving relative to each other.

[grythyttan]

I propose that we break causality then. It's been messing with our bussiness for way too long anyway.

[Winter Man]

The time dilation caused by accelerating B would carry over to A, meaning they're in the same reference frame.

It doesn't matter. Causality can be violated even if instantaneous travel only happens between points that aren't moving relative to each other.

It does matter, because it's not instantaneous travel. We're talking about normal, sub-luminal travel through a region of space-time. They're only shortcuts.

[Aelfyre]

I have thought about this a bit and I think the issue is that a wormhole will not prevent a timetraveling paradox if it in fact is merely a shortcut through the same 3 dimensional spacetime fabric that you would be travelling thru the more circuitous "straight line method" It would be possible for a photon to travel through one end of the worm hole emerging from the other before it left and having enough time to make it back to the entry portal and repeat the journey this time arriving even father in the past and heading back to the entry hole this time with its original self as a traveling companion... this process would repeat as an infinity loop creating a UV Catastrophe of sorts in infinite feedback and the only resolution I can think of is the instantaneously building intensity would destroy the wormhole.

It seems to me what you need to do in order to exceed C without violating causality would be to involve a higher spatial dimension. The only way i can sensibly explain this would be to simplify the picture by collapsing one of the dimension to make it observable from our perspective.

so back to Flatland.. I am sure you all know the tale of a 3 dimensional visitor peering down upon a 2 dimensional plane with 2 dimensional inhabitants. So just hypothetically lets say we remove one spatial dimension from the universe leaving 2 of space and 1 of time. Our observer from the "god's eye view" who previously was 4 dimensional (well 5 including time but were gonna leave that be for now) is now a 3 dimensional entity like you and me..

This being looks down upon the 2 dimensional universe and can see vast expanses of it.. and we are gonna get a step freakier and say that he can actually *see* time itself radiating thru the universe as it expands... yes I know.. but bear with me I am theorizing. In effect he has access to a vast variety of reference frames from each point on the spacetime sheet.. he calibrates a clock at two distinct locations separated by lets say 10 light minutes on two separate worlds that are capable of observing each other thru telescopes or whatever means... we're gonna call this dude X btw

so anyway he picks up subject A from world 1 and places him over on world 2 next to subject B who sees him arrive and then picks up subject B on world 2 and places him next to subject C back on world 1. The trip each way only takes a few seconds from his perspective. They have traversed the distance *much* faster than the light/time radiating from their respective worlds which to the X appears as slowly spreading rings like ripples on a pond.. very slow ripples..

Ok so lets say you are on world 1 and look at a video feed of the clock on world 2. It reads 10 minutes prior to your clock since that is what time it said when that information left and headed your way. The same applies for world 2 observing a video feed of World 1's clock, it appears to be 10 minutes slow. Now a little less than 10 minutes later subject A appears on the video feed for world 2 as viewed by Subject C who has been having a conversation with Subject B. Subject A would have traveled far faster than light but he would not have actually traveled in time. He would appear on the feed at time 0 plus a few seconds according to the the video feed. In fact if X picked subject A back up and returned him to world one after a few minutes then subject A could watch himself appear and then disappear on the monitor.

Now if we reinflate the universe back to the normal amount of dimensions this leaves us with a 4 dimensional X who could pluck us out of our 3 dimensional space and put us somewhere else at apparent superluminal velocities but it would not violate causality.

I know I am missing something here and I have a sneaking suspicion it relates to an assumed absolute time reference frame but I can't put my finger on it... I am just going with an event either has occured yet or it has not regardless if the information regarding that event has had time to reach you. Basically if we were to use this method to travel to a galaxy a billion light years away we would arrive at whatever that galaxy looks like a billion years into the future from our starting reference point because that what it actually is.. it just looks younger due to the time delay in the information leaving us..

basically because X calibrated the clocks to his outside perspective.. if for instance world 1 were to observe the video feed and calibrate their own clock to match the time on world 2 then even an instantaneous trip through the 5 dimension would deposit them on the destination world 10 minutes in the apparent "future"

I hope that makes sense.. it's the best stab I can take at it.

[idobox]

I don't see how a shortcut would violate causality.

Proxima centauri is 4.2 ly away is you travel in a straight line. If we assume a magical shortcut exists, and if you take it, proxima centaury is only 0.2 ly away, how do you propose the photon can come back to the entrance before it left?

It will take 0.2+0.2 - 0.4 years to do a round trip through the shortcut, and 4.4 years to do a round trip, one way through the shortcut, and the other through conventionnal space.
Of course, for an observer, the photon might seem to arrive 4 years earlier, but that because the observer made a bad assumption on the shorter way.

I don't know if a wormhole would behave remotely like this magic shortcut, or if you need some other fictionnal science to make it happen though.

[Tass]

I don't see how a shortcut would violate causality.

Proxima centauri is 4.2 ly away is you travel in a straight line. If we assume a magical shortcut exists, and if you take it, proxima centaury is only 0.2 ly away, how do you propose the photon can come back to the entrance before it left?

It will take 0.2+0.2 - 0.4 years to do a round trip through the shortcut, and 4.4 years to do a round trip, one way through the shortcut, and the other through conventionnal space.
Of course, for an observer, the photon might seem to arrive 4 years earlier, but that because the observer made a bad assumption on the shorter way.

I don't know if a wormhole would behave remotely like this magic shortcut, or if you need some other fictionnal science to make it happen though.

Poke you head through the hole and then send it to Centauri at close to c. When you are .2 years older your head arrives at Centauri, your feet are still on Earth. You now move back again. When your head is back on earth nine years have passed, but it is only .4 years older. No paradox there except it is still attached to your feet, which have never left Earth, and are also only .4 years older on a .4 years older Earth. Pull your feet through and it is 2020, pull your head back and it is 2012. The two mounds are still connected, but they are eight years apart.

[PM 2Ring]

Time travel

The theory of general relativity predicts that if traversable wormholes exist, they could allow time travel.[2] This would be accomplished by accelerating one end of the wormhole to a high velocity relative to the other, and then sometime later bringing it back; relativistic time dilation would result in the accelerated wormhole mouth aging less than the stationary one as seen by an external observer, similar to what is seen in the twin paradox. However, time connects differently through the wormhole than outside it, so that synchronized clocks at each mouth will remain synchronized to someone traveling through the wormhole itself, no matter how the mouths move around.[20] This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a point in time prior to its entry.

For example, consider two clocks at both mouths both showing the date as 2000. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2005 while the stationary mouth's clock read 2010. A traveler who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2005, in the same region but now five years in the past. Such a configuration of wormholes would allow for a particle's world line to form a closed loop in spacetime, known as a closed timelike curve.

It is thought that it may not be possible to convert a wormhole into a time machine in this manner; the predictions are made in the context of general relativity, but general relativity does not include quantum effects. Some analyses[who?] using the semiclassical approach to incorporating quantum effects into general relativity indicate that a feedback loop of virtual particles would circulate through the wormhole with ever-increasing intensity, destroying it before any information could be passed through it, in keeping with the chronology protection conjecture. This has been called into question by the suggestion that radiation would disperse after traveling through the wormhole, therefore preventing infinite accumulation. The debate on this matter is described by Kip S. Thorne in the book Black Holes and Time Warps, and a more technical discussion can be found in The quantum physics of chronology protection by Matt Visser. There is also the Roman ring, which is a configuration of more than one wormhole. This ring seems to allow a closed time loop with stable wormholes when analyzed using semiclassical gravity, although without a full theory of quantum gravity it is uncertain whether the semiclassical approach is reliable in this case.

[Talith]

Can we not consider a wormhole to be a pointwise identification made on the 3d manifold of the universe (at any particular time t) at the two ends of the wormhole and then just apply our normal general relativity on the new manifold? I'm sure that's a pretty naive statement to make but I don't see why we can't consider keeping the physics we have but just changing what kind of shape our universe is when we start building wormholes all over the place.

[PM 2Ring]

Can we not consider a wormhole to be a pointwise identification made on the 3d manifold of the universe (at any particular time t) at the two ends of the wormhole and then just apply our normal general relativity on the new manifold? I'm sure that's a pretty naive statement to make but I don't see why we can't consider keeping the physics we have but just changing what kind of shape our universe is when we start building wormholes all over the place.

Well, we do just apply our normal general relativity on the new manifold. And that allows the time travel stuff to happen. In GR there is no such thing as absolute locations in space, so there's no way to fix the wormhole mouths in place. And thus there's no way to prevent the scenario I quoted above from taking place that turns your wormhole into a time machine.

When you add a wormhole to the universe you change its topology. But topology is a global property of the manifold, it's not something that only has local effects. To give a simple example, if we have a simple 2D spherical surface as our universe, and then we build a wormhole that connects the north pole to the south pole, we've changed its topology to a torus. Now, we might like to think that the wormhole only affects the regions immediately adjacent to the poles, but it doesn't. I'm sure you've played a video game that uses a toroidal universe, ie when you go off the top of the scree you re-appear at the bottom, and when you go off one side you reappear at the other side. In such a toroidal universe, there are no special places corresponding to the hole through the middle of the torus, all locations are topologically equivalent.