Warp drives

[K^2]

Well, the funding for "Breakthrough Propulsion Program" ended in 2002. So it's outdated by over a decade. There is current experimental research on warp drive technology being done at NASA right now. So it went from "it's purely theoretical," to "we are doing experiments on it," in the meantime.

Inaccuracies are in pretty much everything. Alcubierre Warp Drive is not a speculation. It is a prediction of an extremely well-tested theory. It is yet unclear if all the requirements can be met, but this isn't something that people are just making up. Neither is warping of space-time speculative. Again, it's a prediction of General Relativity. Theory without which the GPS satellites would not work. I mean, at this point, you might as well just say that gravity is just a speculation.

And Special Relativity has nothing to do with the discussion of the FTL propulsion. All of the limitations with infinite energy, etc., only happen if you try to breach the speed of limit in flat space-time. Newsflash, it's not flat. Speed of light is a local limitation, and so long as you obey it locally, absolutely none of the problems the article raises show up. Global causality is violated, but there is absolutely no reason why it shouldn't be.

[Bunsen]

Well, the funding for "Breakthrough Propulsion Program" ended in 2002. So it's outdated by over a decade. There is current experimental research on warp drive technology being done at NASA right now. So it went from "it's purely theoretical," to "we are doing experiments on it," in the meantime.

One should be careful to say exactly what they're doing experiments on. What White is testing is the spacetime curvature induced by ordinary, positive mass, and everyone already knows how this will turn out because it's already been measured with more sensitive instrumentation in many other contexts. There are no plans to measure the unknown part, because the necessary material is hiding somewhere between Jimmy Hoffa's body and a bunch of honest politicians.

Inaccuracies are in pretty much everything. Alcubierre Warp Drive is not a speculation. It is a prediction of an extremely well-tested theory. It is yet unclear if all the requirements can be met, but this isn't something that people are just making up. Neither is warping of space-time speculative. Again, it's a prediction of General Relativity. Theory without which the GPS satellites would not work. I mean, at this point, you might as well just say that gravity is just a speculation.

This embodies the same error as above -- the consequences of the Alcubierre metric are not in dispute, nor is the way that spacetime curves around normal matter. Nobody is arguing with that part. The existence of matter with negative mass would be brand new physics. That's the speculative part, and it's indispensable for the whole idea. The fact that it also depends on well-understood science does not make the scheme any more solid.

... Global causality is violated, but there is absolutely no reason why it shouldn't be.

That's a strong statement, and you'll find quite a number of physicists who disagree¹. It's not a question with a mathematical resolution, true. But you'll also find nothing in the equations that directly prohibits the existence of magnetic monopoles (div(B)=0 is an empirical statement, not one derived from other more fundamental laws), yet those seem to be in short supply for some reason. The absence of a known mathematical prohibition does not guarantee, nor even suggest, existence.

Alcubierre's work can be seen as reducing the question of "Can global causality be violated?" to "Can negative mass be created?" What evidence we have thus far points toward the negative on both counts. The understanding behind it is weak enough, particularly in light of cosmological-scale negative curvature, to sustain doubt, but it's a gross mischaracterization to claim that the existence of suitable negative mass is more than speculation.

¹Myself among them, obviously. My reasons for it could be approximately summarized as "Nature likes single-valued solutions." This line of discussion heads rapidly toward philosophical wankery, though, so I'll freely admit that it's unresolved. But I've got twenty bucks on causality until I go back in time and tell myself not to place that bet.

[K^2]

Except there are known solutions involving only positive definite stress-energy tensor that allow for closed time-like loops. In other words, GR allows for time travel without need for exotic matter. So you can roll your causality in a pipe and smoke it. Not to mention that once you realize that you are dealing with a field theory, all causality "paradoxes" are trivially resolved.

As far as the Alcubierre Metric itself goes, yes, it requires negative energy densities. But first of all, there is nothing that suggests that positive-definite solution does not exist. In contrast, existence of "time machine" solutions suggests the opposite. And it is still not clear how Casimir Effect factors into all of this. There have been a lot of theoretical work done on possibility of using Casimir Effect to stabilize traversable wormholes, which have the same problem as Warp Drive. Known solutions to both require negative energy densities. So it's entirely possible that we already know how to achieve such a state, if only on microscopic levels.

And don't knock the interferometry experiments. What they are doing to test the warp drive goes far beyond any direct, laboratory measurements of space-time curvature we have done to date. Yes, we are all pretty certain what the outcome is going to be. This is more of the test of precision to which we can solve these equations numerically and to which we can conduct measurement. But you have to agree, calling space-time curvature speculative when people are measuring it in the lab is quite misinformed.

nothing in the equations that directly prohibits the existence of magnetic monopoles (div(=0 is an empirical statement, not one derived from other more fundamental laws)

That is actually wrong. Standard model predicts div(=0. It follows from gauge symmetries of the QCD Lagrangian.

Edited July 1, 2013 by K^2

[WestAir]

I have no idea what anyone is saying, but I'm amazingly intrigued.

[Scotius]

But the message sent to FTL travelling ship would have to be faster than light, right? Unless the ship is coming in your direction of course.