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In attempting to come up with a semi-reasonable method of intergalactic travel, I ended up conceiving a general method of FTL travel relying on brane cosmology and the "bulk" being transversable in some way. More specifically, traveling from the brane through the bulk to another point on the brane acts like a highly non-linear map (in terms of distance traveled in the bulk's relationship to distance on the brane). The complexity involved in figuring out what path has to be transversed through the bulk to get to a specific point on the brane would make all but specifically mapped out routes too dangerous to attempt (sort of approximating a warp-gate network without actually having warp-gates).

My first question is whether or not this is at all compatible with actual theories of brane cosmology (this is really far from my area of expertise)? My second question is, assuming such travel is even theoretically possible, how much energy (order of magnitude) would be necessary to "breach" the bulk?

EDIT:

Having done additional research on the subject, it appears that the standard explanation for why additional dimensions are not apparent in brane cosmology is that material objects are the result of open strings and are "bound" to the brane on which our universe sits. As such, they cannot move orthogonally to the brane with which they are bound, which prevents them from interacting with the bulk (this also provides an explanation for the "weakness" of gravity; gravity is the result of closed string vibrations which are not bound to the brane and can leak out into the bulk). So, it seems that shifting from an open to closed string is necessary to interact with the bulk, which shifts the question to whether or not it is possible to have a string shift between open and closed states and if so how much energy does that require?

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    $\begingroup$ String theory and extra dimensions is super heady stuff. It would be great if someone answered this at a level appropriate for the WB stack. The physics stack has a whole tag for branes. physics.stackexchange.com/questions/tagged/branes?mixed=1. When I look at these questions and their answers I realize how little I know. $\endgroup$ – Willk Nov 20 '17 at 21:30
  • $\begingroup$ @Will it's a fine line we walk here, between the lofty groans of "Oh you're handwaving", and the fearful gasp of "But -- but that's real physics!" ;D $\endgroup$ – akaioi Nov 20 '17 at 22:25
  • $\begingroup$ Welcome to Worldbuilding, nickschachter, and a good question too. Since you are trying to construct a faster-than-light travel scenario with plausible concepts from cosmology, I'd suggest adding the [tag-science-fiction] tag and allow for sufficient hand-waving to bridge the knowledge gaps that might otherwise open up beneath our conceptual feet. $\endgroup$ – a4android Nov 21 '17 at 1:33
  • $\begingroup$ If the physics text book is thick enough, nothing is handwaving. $\endgroup$ – Justin Thyme Nov 21 '17 at 3:05
  • $\begingroup$ The 'cosmological constant' is just another phrase for 'Then a miracle happens". $\endgroup$ – Justin Thyme Nov 21 '17 at 3:07
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Yes your model for intergalactic FTL travel is sufficiently compatible with brane cosmology. The bulk is effectively a higher dimensional space with large, non-compacted dimensions. Effectively, this is something like the conventional science-fictional concept of a "hyperspace". A longstanding trope for attaining FTL travel in science-fiction.

For ease in traversing the bulk it may be best to assume its dimensions are simply a set of dimensions rotated away from the dimensions in our brane. Once a vehicle has accessed the bulk it would travel as if it was in "normal" space. You have assumed correctly that the distances traversed via the bulk will be complex and non-linear.

As for accessing the bulk, it may be easiest to assume this can be done by means similar to the creation of a wormhole. The problem of accessing the bulk is now no more than that of making a wormhole.

Please note: Not all theoretical models of wormholes indicate they will collapse without using exotic matter to keep them open. This make intergalactic FTL travel so much easier.

Also, spacecraft need only arrive at a suitable point in our universe where they can create their wormhole to follow a "safe" path through the bulk to the galaxy that is their destination.

This answer has attempted to follow a similar set of worldbuilding rules implied in the question. Namely, selecting suitable plausible and genuine scientific concepts and using only sufficient hand-waving to make them fit for purpose in a science-fictional context.

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  • $\begingroup$ 'selecting suitable plausible and genuine scientific concepts and using only sufficient hand-waving to make them fit for purpose in a science-fictional context'. Wow. A statement worthy of a politician. Sort of like "our budget includes assumptions that, although completely fictitious, have enough semblance to real economics as to make our projections plausible for the purposes of our political platform." The budget will increase the deficit, but it isn't a REAL deficit. $\endgroup$ – Justin Thyme Nov 21 '17 at 2:57
  • $\begingroup$ Baffledegab beats bullsh*t. $\endgroup$ – Justin Thyme Nov 21 '17 at 2:59
  • $\begingroup$ The problem of accessing the bulk is now no more than that of making a wormhole. So impossible then as passing through a wormhole amounts to passing through a singularity. $\endgroup$ – StephenG Nov 21 '17 at 5:08
  • $\begingroup$ @StephenG I suggest you do more research into wormholes. You're probably thinking of black holes. Even with black holes, wormholes do not need to coincide with a singularity. See Kerr black holes, for example. $\endgroup$ – a4android Nov 21 '17 at 12:34
  • $\begingroup$ My understanding is that, regardless of which theory, you're traversing from one universe to a different universe. That pretty much rules them out as a useful FTL drive. $\endgroup$ – StephenG Nov 21 '17 at 13:19

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