Can Wormholes and Dark Matter Lead to Time Travel?

[FantasyQueen]

Do many scientists believe in the existence of wormholes? Can further research lead to the invention of the time machine?

 

[Nabeshin]

Pretty much no serious scientist believes that there exist naturally occurring wormholes out there somewhere. However, if you do have a wormhole, it is really quite trivial to construct a time machine from it from the following procedure:

Imagine you have the two mouths of a wormhole, A and B. These two mouths are synchronized in time, that is, passing through the wormhole does not change the time between them. Now, by some mechanism, take the mouth of wormhole B, affix it to a spaceship, and travel at an appreciable fraction of the speed of light (or, equally as good, bring it close to a large gravitational potential, such as a black hole or neutron star). The time dilation effects will induce a time lag between the mouths of the wormhole. All that is to be done now is to move the mouth of B back near to the mouth of A and voila, you have established a time disparity between the two mouths. Now, if the time difference is larger than the time it takes for light to travel the distance between the two mouths, it is possible to travel arbitrarily far into the past or future by simply repeatedly entering the wormhole.

The process I have just described, while seemingly simple, we have numerous reasons to believe that it is unreasonable. For one, some calculations show that as you approach the situation I mentioned where a time machine is about to be attained, the flux of light through the mouth of the wormhole becomes infinite and the entire structure must collapse. Also, it is certainly not clear how quantum fluctuations play their part in something like the scheme above.

 

[Chronos]

There is no credible evidence of anything resembling a wormhole in this universe. The time machine aspect of a wormhole is even less credible. Leakage of energy into the past [or future] would have paradoxical consequences.

 

[Phrak]

Pretty much no serious scientist believes that there exist naturally occurring wormholes out there somewhere. However, if you do have a wormhole, it is really quite trivial to construct a time machine from it from the following procedure:...

Actually you can't construct a time machine with a wormhole in a universe existing for finite time. As measured in the relatively flat background metric of the universe, it takes an infinite amount of time to transit the event horizon. Therefore no problem with causality, just a problem with arriving before you departed.

 

[Nabeshin]

As measured in the relatively flat background metric of the universe, it takes an infinite amount of time to transit the event horizon.

Source?

 

[Phrak]

From a Google search:

Event Horizon (DVD, 2008, Lenticular O-Sleeve) - eBay (item ...
eBay: Find Event Horizon (DVD, 2008, Lenticular O-Sleeve) in the DVDs Movies ... Free option usually ships within 3-5 days, and transit time takes 5-9 days ...
cgi.ebay.com/Event-Horizon-DVD-2008-Lenticular-O...-/360333341868​

5 to 9 days... Wait, that's not it.

Seriously, though, the event horizon is a temporal coordinate singularity by definition. This means that as an asymptotic time interval (an interval where R-->infinity) remains finite, an interval at the event horizon goes to infinity.

http://en.wikipedia.org/wiki/Schwarzschild_metric"

 

[Nabeshin]

Hah.

This is certainly true in the schwarzchild metric, but I'm not interested in this! The much more interesting case are the traversable wormholes with no tidal forces (or horizons, I believe!) that can conceivably be used to construct the time machine I mentioned. Obviously the Einstein-Rosen bridge will not do since one cannot pass through it, and yes with respect to the coordinate observers it takes an infinite amount of time.

 

[dimensional]

I don't know many technical/mathematical details but I believe string theory somewhat supports the notion through the idea of cosmic strings, which are highly dense pockets of warped gravity (or something along the lines of that). Someone please correct me if I'm wrong, and add more to this line of thought.

 

[Nabeshin]

Ok well cosmic strings have nothing to do with wormholes...

Also string theory doesn't really support the notion of a cosmic string so much as it provides a method by which one may be created. The idea of cosmic strings is much older than string theory, and when string theory came along there was a possible explanation for a cosmic string (which have, by the way, never been observed). But string theory by no means predicts cosmic strings, so I wouldn't say it supports them at all.

 

[dimensional]

Well, we might be just disagreeing on semantics here. Perhaps I'm not weighing as much importance to the word "support" as you might be, I was just bringing up the fact that string theory does at least talk about the idea in a sense. I don't know a lot about it though, so I would like to learn. I know the proposal for the idea itself has been around longer than string theory proper, I assumed that it would allow for a more modern interpretation though. What more can be read about the subject?

And regarding the relevancy of cosmic strings and wormholes, I was just drawing a parallel I suppose. Wouldn't they serve a similar function if used for practical purposes, in theory only I mean?

 

[piareround]

I know the last post in this thread was to day ago, but I was hoping I could get a question answered:

From what I have read on Wikipedia http://en.wikipedia.org/wiki/Wormholes" , their is the concept that something with negative energy could bend space time. In fact Wikipedia specifically says this:

[PLAIN]http://upload.wikimedia.org/wikipedia/commons/a/af/Worm3.jpg

Wormholes which could actually be crossed, known as traversable wormholes, would only be possible if exotic matter with negative energy density could be used to stabilize them. (Many physicists such as Stephen Hawking, Kip Thorne, and others believe that the Casimir effect is evidence that negative energy densities are possible in nature). Physicists have also not found any natural process which would be predicted to form a wormhole naturally in the context of general relativity, although the quantum foam hypothesis is sometimes used to suggest that tiny wormholes might appear and disappear spontaneously at the Planck scale.

What exactly is this quote saying here?

What I don't understand is why mathematically would something with negative energy(*) bend spacetime in such a way as to create a wormhole?

Does exotic matter really have to have negative mass? Reason I asked this was, I thought that the concept of negative energy in QFT took care off the need for negative mass.

__________________________
**Note in this specific question I am not trying to asking whether the "something" has to be either quantum foam or exotic. What I am trying to figure out is the exact role negative energy plays in wormhole theory I think I would best understand it if someone could give a mathematical reason why negative energy could create a wormhole.

 

[Nabeshin]

I'm not sure of an intuitive way to explain why, mathematically, negative energy densities are necessary to stabilize a wormhole. What I can say is this:

Normally, when we approach Einstein's theory of General Relativity, which shows us how mass curves spacetime, we ask what the spacetime around a known mass distribution is. This mass distribution can be a star, a black hole, an infinite plane of mass, a flower shape, or anything you can think of. We then solve the equations to see what the spacetime looks like and what kinds of interesting properties it has.

With regards to wormholes, we approach the problem from the other direction. We say we want a spacetime which contains a traversable wormhole, and then ask what mass distribution is necessary for this requirement. It simply turns out that there must be a local negative energy density near the throat of the wormhole for the solution to exist and be stable.

Perhaps one way to think about it, although I wouldn't push this far at all, is that a wormhole throat has a natural tendency to collapse, and negative energy density provides an outward pressure to halt this collapse. Perhaps you can liken this to the effect Einstein's cosmological constant had on the universe as a whole.

 

[piareround]

Hmm... that is a interesting way of thinking about it. Thanks for the help

 

[Chronos]

I would say it takes imaginary energy to hold a wormhole open - emphasis on imaginary.

 

[Phrak]

I would say it takes imaginary energy to hold a wormhole open - emphasis on imaginary.

Yes, but we have plenty of that sort of stuff lying around lately, called dark matter and dark energy.

 

[George Jones]

Yes, but we have plenty of that sort of stuff lying around lately, called dark matter and dark energy.

It is not thought that dark matter violates energy conditions. Dark matter needs to be gravitationally attractive.