Re: Tracking

Erik Max Francis wrote:
Mark L. Fergerson wrote:

I keep thinking about the interface between the volume of the Alcubierre bubble and ordinary spacetime. Light travels at c inside the bubble, _and_ outside it, but what happens at the interface?

Remember that locally (in the vicinity of a point), general relativity decays to special relativity. It's only global spacetime curvature that can result in weird situations like this. That's why things like inflation, which don't seem like they'd make logical sense, are possible in general relativity even though they'd obviously result in objects moving faster than light relatively to each other if one could simply measure their separation speeds. Because of this global curvature, it makes measuring speeds at a distance essentially meaningless. This is, for instance, why you get the weird behavior you're probably familiar with near horizons where distant observers "see" an infalling object hover near the horizon, but an actual observer falling in hits the singularity in finite proper time.

The same thing happens here. At the interface, things are still locally special relativistic. It just seems weird when you look at it globally. (And "globally" by itself doesn't make unique sense because they're still subject to relativity, and what they see depends on their speed.)

So the answer is that light still travels at c at the interface. It _always_ travels at c.

Yes, of course, thanks. Sort of a "false acceleration" situation across the interface, which is where I got the "light faster than light" mental imagery by inferring that what's observed across the interface from outside is in a consistent frame. Duh.

Yep, but we live in STL-spacetime, and nobody knows what things would look like from FTL-spacetime (assuming any such thing exists)

But without a coherent theory this devolves into speculation. But you're talking about an Alcubierre drive. That _is_ something modeled by general relativity. Now, of course, it may well be unphysical -- and probably is -- but general relativity will tell you how it behaves.

OK, the interface probably cannot have zero thickness, or it'd simply collapse into an oddly-shaped singularity. There's plenty of room for the "false acceleration".

Sure, you can handwave something entirely new, but when you do that, you're left with pure speculation with no substance. Which is fine if you're handwaving for a story, I suppose, but beyond that it's not going to really go anywhere -- especially when you're talking about things that completely violate our conception of how the Universe works, such as luxons travelling at speeds other than c. At that point the game is up and the rules are so badly broken that there are no rules left.

Yet we keep finding exceptions to rules- OK, actually sub-rules we didn't know about- that MAY allow such things as Alcubierre.

If it is allowed there may be some externally-observable gravitational equivalent to Cerenkov along the lines I described and arrogated my name to; now, is it a loss mechanism? Does an Alcubierre bubble require constant expenditure of energy to maintain it?

I recall one or more pre-Alcubierre tales involving FTL drives that basically turn a ship into a "giant tachyon" that requires no energy input to maintain, but mechanisms for telling when to turn back into a tardyon are unclear; steering may be irrelevant to such a drive but knowing when to stop does seem to require some way to see where you are, and that to me implies at least a potential loss mechanism.


Mark L. Fergerson

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