B Can Hypothetical FTL Technologies Enable Travel to Other Bubble Universes?

[windy miller]

In the eternal inflation scenario described by Guth here: https://arxiv.org/abs/hep-th/0702178
it is usually thought it is impossible to ever travel to one of the other bubble universes as they are separated by inflating space expanding faster than any spacecraft could every travel. However there have been some speculative idea about FTL travel such as warp drives and worm holes Suppose hypothetically such devices were possible. Would the statement that travel to another bubble region is impossible still be valid?

 

[Michael Price]

Wormholes would certainly not enable you reach other pocket universes embedded within the eternally inflationary backgroumd. You have to drag the one of the wormhole ends slower-than-light to your intended destination before you start using them for transit.

 

[windy miller]

thanks for that, and what about a warp drive? Btw I know this is very speculative stuff just wondered what is logically possible.

 

[Michael Price]

Certainly not logically impossible - it would depend on how the warp drive 'worked'.

 

[PeterDonis]

what about a warp drive?

If you mean an Alcubierre warp drive, I don't think the possibility can be ruled out in principle, if it is assumed that such a warp drive can be constructed (which is not considered practically possible because it requires exotic matter).

 

[kimbyd]

In the eternal inflation scenario described by Guth here: https://arxiv.org/abs/hep-th/0702178
it is usually thought it is impossible to ever travel to one of the other bubble universes as they are separated by inflating space expanding faster than any spacecraft could every travel. However there have been some speculative idea about FTL travel such as warp drives and worm holes Suppose hypothetically such devices were possible. Would the statement that travel to another bubble region is impossible still be valid?

Wormholes are probably impossible to traverse (apparently they tend to collapse if you try send so much as a photon through, and can only be held open by matter with very exotic properties that probably doesn't exist).

As others have noted, the wormhole ends would have to be moved. Basically, it's only really possible if the wormhole existed from the time before the inflation causally-disconnected the ends of the wormhole. But as the horizon scale during inflation was microscopic (much smaller than a proton), the wormholes would also be microscopic, and the ends would therefore have evaporated long ago (the ends of a wormhole basically act like black holes).

 

[Michael Price]

Wormholes are probably impossible to traverse (apparently they tend to collapse if you try send so much as a photon through, and can only be held open by matter with very exotic properties that probably doesn't exist).

As others have noted, the wormhole ends would have to be moved. Basically, it's only really possible if the wormhole existed from the time before the inflation causally-disconnected the ends of the wormhole. But as the horizon scale during inflation was microscopic (much smaller than a proton), the wormholes would also be microscopic, and the ends would therefore have evaporated long ago (the ends of a wormhole basically act like black holes).

There are suggestions that when D>4 (e.g. string theory) wormholes become traversable without exotic matter.
https://en.m.wikipedia.org/wiki/Gauss–Bonnet_gravity

 

[kimbyd]

There are suggestions that when D>4 (e.g. string theory) wormholes become traversable without exotic matter.
https://en.m.wikipedia.org/wiki/Gauss–Bonnet_gravity

Usually this kind of thing only works if the extra dimensions are comparable in size to the physical system (in this case, the openings of the wormhole), but current limits on the sizes of extra dimensions are microscopic (nanometers or less).

 

[Michael Price]

Usually this kind of thing only works if the extra dimensions are comparable in size to the physical system (in this case, the openings of the wormhole), but current limits on the sizes of extra dimensions are microscopic (nanometers or less).

Perhaps wormholes will be primarily microscopic - but if they can pass data then that is sufficient to act like transporters.

 

[PeterDonis]

the ends of a wormhole basically act like black holes

Do you have a reference for this? There are some similarities between wormhole and black hole spacetimes, but AFAIK nobody has shown that wormholes emit Hawking radiation. Wormholes, unlike black holes, are not vacuum spacetimes--they have to contain exotic matter to hold the wormhole open. I would expect that to have a significant effect on applying QFT in a wormhole spacetime.

 

[PeterDonis]

Wormholes, unlike black holes, are not vacuum spacetimes--they have to contain exotic matter to hold the wormhole open.

Also, wormholes do not have horizons--no event horizons and no trapping horizons. So at any rate the intuitive logic that "horizons -> Hawking radiation" does not work for wormholes.

 

[kimbyd]

Perhaps wormholes will be primarily microscopic - but if they can pass data then that is sufficient to act like transporters.

But they probably can't even permit the passage of photons without collapsing. Which means they would all have collapsed long ago when the universe was a dense plasma, aside from the possibility of collapsing when you attempt to actually send a message.

Also, wormholes do not have horizons--no event horizons and no trapping horizons. So at any rate the intuitive logic that "horizons -> Hawking radiation" does not work for wormholes.

Fair point.

 

[PeterDonis]

they probably can't even permit the passage of photons without collapsing

There are known wormhole solutions that have null and timelike paths through them; the simplest is the Morris-Thorne-Yurtsever wormhole:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.61.1446

 

[Michael Price]

But they probably can't even permit the passage of photons without collapsing. Which means they would all have collapsed long ago when the universe was a dense plasma, aside from the possibility of collapsing when you attempt to actually send a message.

The may not be true of higher dimensional wormholes. Calabi-Yau space looks like it has wormholes in it.
https://en.m.wikipedia.org/wiki/Calabi–Yau_manifoldYes, I am dreaming of time when we are a K3 civilisation.

 

[kimbyd]

There are known wormhole solutions that have null and timelike paths through them; the simplest is the Morris-Thorne-Yurtsever wormhole:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.61.1446

Having a null or timelike path through the wormhole is not sufficient to show that it's traversable. You'd also have to show that those paths remain even in the presence of the stress-energy of the traversing matter or signal. My understanding is that they don't when using the simplest assumptions.

 

[kimbyd]

The may not be true of higher dimensional wormholes. Calabi-Yau space looks like it has wormholes in it.
https://en.m.wikipedia.org/wiki/Calabi–Yau_manifoldYes, I am dreaming of time when we are a K3 civilisation.

Sure, but as I said above, I'm skeptical that such wormholes can be of anything like a reasonable size if the extra dimensions are small, which they experimentally are. And making the wormhole small will also make it harder to keep open, because it will require less added stress-energy to disturb the space-time paths and prevent traversal.

 

[PeterDonis]

Having a null or timelike path through the wormhole is not sufficient to show that it's traversable. You'd also have to show that those paths remain even in the presence of the stress-energy of the traversing matter or signal. My understanding is that they don't when using the simplest assumptions.

Do you have a reference?

A test object should have negligible stress-energy and effect on the spacetime geometry. At or near a Cauchy horizon, such as the inner horizon of a Reissner-Nordstrom or Kerr black hole, it's problematic whether anything can be a test object, but there are no Cauchy horizons in a wormhole spacetime.

 

[PeterDonis]

making the wormhole small will also make it harder to keep open, because it will require less added stress-energy to disturb the space-time paths and prevent traversal.

I see what you're saying here if it's restricted to a small wormhole, but again, do you have a reference?

 

[kimbyd]

I see what you're saying here if it's restricted to a small wormhole, but again, do you have a reference?

I don't know of a good reference that talks about the issue in general, but here's one paper that examines the stability of a wormhole supported by exotic matter:
https://arxiv.org/abs/gr-qc/0506001

 

[PeterDonis]

here's one paper that examines the stability of a wormhole supported by exotic matter

More specifically, supported by phantom energy, i.e., a perfect fluid with equation of state $p = w \rho$ and $w < -1$. IIRC many wormhole solutions, such as the original Morris-Thorne-Yurtsever one, do not have $w < -1$ and so this paper would not appear to apply to them.

 

[kimbyd]

More specifically, supported by phantom energy, i.e., a perfect fluid with equation of state $p = w \rho$ and $w < -1$. IIRC many wormhole solutions, such as the original Morris-Thorne-Yurtsever one, do not have $w < -1$ and so this paper would not appear to apply to them.

Sure, but if you're interested in why classical wormholes are not considered to be traversable, you can look at the references in that article. Pretty sure it's there.

 

[PeterDonis]

many wormhole solutions, such as the original Morris-Thorne-Yurtsever one, do not have $w<−1$

In fact, strictly speaking, the equation of state $p = w \rho$ isn't quite valid for wormhole solutions (including the "phantom energy" one in the paper), because the pressure is not isotropic; there is radial tension and tangential compression. Instead, it should be $p_r = w \rho$ and $p_t = - p_r$, where $p_r$ is the radial pressure (typically negative indicating tension) and $p_t$ is the tangential pressure (typically positive indicating compression).

For the Morris-Thorne-Yurtsever wormhole, with the above caveat, we have $w = - 1$. The usual expression for the metric of this wormhole is

$$
ds^2 = - dt^2 + dl^2 + \left( l^2 + b^2 \right) \left( d\theta^2 + \sin^2 \theta d\phi^2 \right)
$$

where $l$ is a radial coordinate, but ranges from $- \infty < l < \infty$, and $l = 0$ is the wormhole throat. The constant $b$ fixes the area of the wormhole throat.

The Einstein tensor for this wormhole corresponds to a stress-energy tensor that looks like $\text{diag} \left( \rho, - \rho, \rho, \rho \right)$, where $\rho = b^2 / \left( b^2 + l^2 \right)^2$.