[Aardwolf]

I don't get what the problem is with a "global reference frame". Why not pick a patch of space with no big gravity gradients, scatter a bunch of observers throughout it with zero relative velocity between them, and tell them to keep transmitting "ok" until they detect their velocity relative to the rest of the observer network is (significantly) nonzero. Alcubierre ship starts docked with an observer, gets clear of it before doing its space-distorting business, deliberately runs over some observers with its Alcubierre bubble, then comes to a stop and docks with another observer at the far end of the test area.

Oh. That relies on the bubble having a definite size outside of which its effect is negligible. But it probably falls off at 1 / r², so there can't be an "outside".

But that's ignoring curvature change propagation stuffs. I still think there's something weird with how an Alcubierre drive would "start up".

[watsisname]

Such a network of observers, occupying a region which is flat, experiences proper time equal to coordinate time.  Nothing mysterious there, we're just back to special relativity.  This is a perfectly fine reference frame in which to do physics, until they become interested in the motion of the Alcubierre ship. 

If they think special-relativistically, the network will think the path of the ship is space-like.  They will think this blatantly violates physics if they don't recognize the curvatures involved.  If they are smart, they will suppose curvature must be involved because as far as anyone knows, relativity prevents space-like trajectories.  If they are really smart, they'll test this by examining the gravitational lensing of stars around the ship to map the curvature.

When they do so, they'll find that the ship is not following a space-like trajectory, it is following a time-like one.  In fact, it is following a geodesic.  It is exactly equivalent to a ship in free fall.  Locally, meaning examining sufficiently small reference frames such that the curvature appears flat, there is nothing going faster than light.  The ship is at rest, and the distortions to the space-time around it propagate at c.

Again, this is the whole reason general relativity was derived in the first place.  Many situations in space-time physics involve accelerated observers or deviations from flatness.  When this happens, our intuitions from special relativity go right out the window.  For a fine example, just consider that there are galaxies at very large cosmological distances which are receding much faster than c, and we can see them.  It is only if we think special relativistically that this should freak us out.

Added:

That relies on the bubble having a definite size outside of which its effect is negligible. But it probably falls off at 1 / r2, so there can't be an "outside".

No, the field is extremely localized and does not follow 1/r².  (Graphics of the field are pretty widespread, including in the original paper itself if you had actually bothered to try looking at it.)  This also produces extreme tidal forces as a consequence, but only significantly so outside of the ship if field parameters are carefully chosen.

[Aardwolf]

Mmm. Yes, the pictures I've seen didn't look like 1 / r², but virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.



I don't get how you resolve things when point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon. Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?

[Bobboau]

it's because the space in front of it is getting smaller and the space behind it is getting bigger. the ship is sitting motionless, the space is 'moving' for lack of a better word, the field propagates at C, it moves across space at 299792458m/s meters in front are getting smaller, meters behind are getting bigger.

[watsisname]

Sort of what Bobboau said.  The measure of distances, (and not just distances in space, but distances in space-time) change in front of and behind the ship in anti-symmetrical ways (one expands, the other contracts).  Any and every time you try examining such a system with a global, flat coordinate system, you're doomed to fail miserably at understanding what's happening.  This is why you keep having trouble, Aardwolf.  It would be a lot easier if you had some familiarity with what a reference frame means and how they work within curved space-time.

Trust me, people who haven't taken special relativity usually struggle for a long time before finally grasping it.  I was no exception.  General relativity is even harder.  If you have not studied general and are trying to understand Alcubierre drives, you are going to have a very bad time.

Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?

Nope, this has nothing to do with time differentials.  Or magic, for that matter.

virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.

"Sharp cutoff" = discontinuity = non-differentiable = failure of standard space-time physics.  There is no discontinuity in the Alcubierre field.

The geometry of the field is described by the curvature tensor, which depends on the metric (which is defined explicitly in the paper) and the stress-energy tensor (also defined explicitly).

[watsisname]

I feel strongly compelled to add that your latest method of argument is truly astounding.

"I think this field seems wrong because I've not seen many with that kind of geometry before.  Nevermind that it's a solution of GR and I haven't done any sort of critical analysis, let alone have familiarity with GR."  Brilliant!  You should submit that one immediately. 

[jr2]

Assuming one could be built that functioned as advertised, could an Alcubi drive work on - planet for near instantaneous travel to other parts of the globe (and possibly cheap transit to orbit)?

Or would something interfere?

[watsisname]

Possibly, but I would certainly not recommend trying it.  The tidal effects generated at the edge of the field would pretty much tear apart anything it encounters.

Come to think of it, I imagine this is what the 'safe lanes' thing that Luis was talking about (should be) all about, rather than paradox avoidance.

[jr2]

I thought the latest calculations indicated no tidal forces would be generated? Maybe I'm mistaken.

[Aardwolf]

* Aardwolf looks at his own post

You know why that seems like an odd method of argument? Because it's not an argument. Arguments don't start with "I don't get how you resolve". The next time you feel "strongly compelled" to say something snarky like that, don't.

I don't get how you resolve things when point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon.

If something is in the path of the ship, what does it see? Question mark.

It seems to me that something in the path of the ship could see that all of the space within 1 light-year of it is "flat" (by observing markers placed throughout it), and then less than 1 year later, it's roadkill. It can be roadkill instantly, because the observations of things exactly one light year away are already one year old by the time they are observed. I think I figured it out. You were the opposite of helpful.

[watsisname]

I thought the latest calculations indicated no tidal forces would be generated? Maybe I'm mistaken.

I'm not familiar with latest calculations (please feel free to link to them though!), but any kind of Alcubierre solution is necessarily going to produce tidal forces somewhere.  Tidal forces are consequences of space-time curvature, and you don't have Alcubierre drive without curvature somewhere.

Tidal forces can be described as this:  Two objects separated by some distance, but following initially parallel trajectories, will deviate from parallel if the space-time they occupy is curved.  In language of general relativity, each is following a perfectly straight line (geodesic) through space-time and feels no forces at all, but because the space-time is curved, these straight lines don't remain parallel.  So it seems that the objects are being squeezed together or torn apart by some force, which we call tidal force.

Alcubierre drives produce powerful tidal forces outside of the ship, near edge of the field, because that is where curvature is most extreme.

[watsisname]

* Aardwolf looks at his own post

You know why that seems like an odd method of argument? Because it's not an argument. Arguments don't start with "I don't get how you resolve". The next time you feel "strongly compelled" to say something snarky like that, don't.

>Enters thread with an argument about A.D.
>Calls upon same argument immediately after someone explained why someone else's notions about A.D. are wrong and why, with ****ty attempt at a conversational opener: "Alcubierre Fanboys: …discuss:"
>Is told that the argument is wrong and why in great detail, complete with links to supplementary material and a recommendation of reviewing it before making further arguments.
>Expresses disinterest/inability of self-education and proceeds to make further arguments
>Becomes angry when he is made a fool of.

I've been following a form of tit-for-tat response method, so consider your own behavior if the replies make you angry.  I am happy to explain this stuff to anyone who expresses that they want to learn.  I am less-than-impressed by those who act that they are going to be able to grasp something after being told what they need to do to achieve proper understanding (and why), and refusing to do it.

You'll also note that I've been answering all of your questions directly.

If something is in the path of the ship, what does it see? Question mark.

It first sees changes in curvature of the space time it occupies, then possibly is destroyed by tidal forces, then impacts the Alcubierre ship itself.  The proper time it takes to do so may be extremely short, and this may very well confuse you if you continue to think in perspective of global reference frames.

[Aardwolf]

No. You responded to this post

Mmm. Yes, the pictures I've seen didn't look like 1 / r², but virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.



I don't get how you resolve things when point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon. Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?

with this

I feel strongly compelled to add that your latest method of argument is truly astounding.

WHAT ARGUMENT? There is no argument being advanced in that post. Do you want me to go through it sentence by sentence and demonstrate that for you? You probably don't. But that's even more reason why you need to be shown how you're wrong.

Mmm.

Pensive.

Yes, the pictures I've seen didn't look like 1 / r²,

Concession that the hypothesis from the prior post is, in retrospect, stupid.

but virtually nothing in physics has a sharp cutoff outside of which the field is exactly 0.

Tangent. Illustrations appear to approach zero as distance from the bubble increases. Nonetheless it is possible for a function to be continuously differentiable, be nonzero for part of its domain, and zero for some other continuous subset of its domain; e.g. f(x) = 1 - sqrt(1 - x²) for x < 0, 0 for x >= 0 ...   But since there's not much like that in physics, it probably isn't that.

I don't get how you resolve things when

Expression of bewilderment under a certain circumstance, to be specified forthwith.

point A says it can propagate changes to point B in 1 nanosecond and point B doesn't think changes from point A can propagate to it that soon.

Specification of the aforementioned circumstance, which is seemingly paradoxical.

Does the problem just magically go away because you're taking the limit as dt approaches zero, and so both A and B's perception of dx also approaches zero?

Hypothesis regarding how to resolve the aforementioned paradox.

[jr2]

Well, here's one bit but it's not the one I saw : I'll keep looking for that.


http://www.space.com/17628-warp-drive-possible-interstellar-spaceflight.html


Link to thread discussing this, and in the top to a video by Dr White, maybe they misunderstood him?


http://forum.kerbalspaceprogram.com/threads/82877-Hypothetical-effects-of-the-hypothetical-Alcubierre-drive/page2

[watsisname]

While Aardwolf vents I thought it might be fun to pose a question for anyone who is interested in the extremely non-intuive results that can occur in strongly curved space-time:

Suppose sub-space drives as seen in Freespace are real (this isn't important to the question, but just for fun and to allow the following situation).  Suppose that you are in a ship that has just exited from a sub-space jump gone horribly wrong.  The drive suffered irreparable damage and forced you to exit subspace right next to a supermassive black hole.  Before you can react, you fall inside the event horizon.  You are now unable to escape -- doomed to hit the singularity in some finite amount of time in the future.  But your engines are still operable, and they allow arbitrarily large acceleration in any direction.  You decide that although escape is impossible (and it is), you can at least try to maximize the amount of time you have left before you meet the singularity.  What action would you pursue?  (You may treat yourself and your ship as being impervious to the accelerative forces of your engines, as well as to the tidal forces within the black hole).

@Jr2:  Thanks for the links.  As far as I can tell, the first link isn't discussing tidal forces, but rather the amount of energies required to produce the field, with more recent calculations suggesting these would be not as large as previously supposed.  As for tidal forces, I don't see anyone saying that it produces no tidal forces, but rather that they can be made to be reasonably small within the ship.  Outside the ship, however, they may be enormous.  Basically, whenever you look at a figure showing the geometry of the Alcubierre field, the tidal force at a given point is going to be proportional to how "steep" the field is there.

If there are no tidal forces anywhere, then there is no Alcubierre field, and the drive is not achieving anything.

[Scourge of Ages]

Suppose sub-space drives as seen in Freespace are real (this isn't important to the question, but just for fun and to allow the following situation).  Suppose that you are in a ship that has just exited from a sub-space jump gone horribly wrong.  The drive suffered irreparable damage and forced you to exit subspace right next to a supermassive black hole.  Before you can react, you fall inside the event horizon.  You are now unable to escape -- doomed to hit the singularity in some finite amount of time in the future.  But your engines are still operable, and they allow arbitrarily large acceleration in any direction.  You decide that although escape is impossible (and it is), you can at least try to maximize the amount of time you have left before you meet the singularity.  What action would you pursue?  (You may treat yourself and your ship as being impervious to the accelerative forces of your engines, as well as to the tidal forces within the black hole).

Logic and Kerbal Space program would suggest that the best method would be to accelerate into an orbit around the black hole and hope that you don't hit any objects.

[Mongoose]

Given the non-intuitive disclaimer, I'm going to say that (for whatever reason) accelerating towards the black hole would actually do the trick.  Possibly due to a time dilation effect, but I only have a middling grasp on special relativity, and essentially nothing on the general side.

[watsisname]

Logic and Kerbal Space program would suggest that the best method would be to accelerate into an orbit around the black hole and hope that you don't hit any objects.

This makes me wish there was a fully relativistic version of KSP with ability to fly around/into black holes, but holy crap how hard would that be to do... 

Anyway, a great thought, but unfortunately, there is no such thing as a stable orbit inside the event horizon of a black hole unless the particle or ship can move faster than the speed of light.  Within the horizon, any allowed trajectory is destined to hit the singularity in finite time, so all that can be done is maximize the amount of time it takes to do so.

There's a very weird way of describing this, which is that inside the event horizon, the roles of space and time interchange.  Outside the horizon, you have complete freedom to move in any spatial direction, but you are compelled to move forward in time and the only thing you can do is slow it down (by moving really fast).  Inside the horizon, you have complete freedom to move in time, but you are compelled to move in the direction of decreasing distance from the singularity.  In other words, avoiding the singularity inside a black hole is no more possible than avoiding the progression of time in normal space.

Given the non-intuitive disclaimer, I'm going to say that (for whatever reason) accelerating towards the black hole would actually do the trick.  Possibly due to a time dilation effect, but I only have a middling grasp on special relativity, and essentially nothing on the general side.

Time dilation effects are definitely important here.  It will turn out though that accelerating towards the singularity is a bad choice -- you'll end up hitting it sooner. 

It turns out that accelerating away from the singularity is also a bad choice!  You'll end up hitting it sooner.

It can be shown that the longest proper time (proper meaning it's how much time you experience and count out if you look at your clock) occurs if your path is a geodesic -- freefall.  If you're in the event horizon, the best you can do is do nothing at all.  Any other action you try to take will simply make the journey to the singularity shorter.

This problem can be seen worked out (warning: math) here.  It's number 12-14.
http://dafix.uark.edu/~danielk/Relativity/HW8Soln.pdf

"The more you struggle, the shorter you live."  Black holes are like cosmic quicksand.

[Luis Dias]

Were these physicists you consulted specialists in general relativistic field equations, and did they explain to you why the Alcubierre solution is invalid?  If so, then I urge you to urge them to submit their critique to a relevant journal, perhaps ApJ, so that others can examine it.  If not, then you may want to reconsider your views.

You are assuming these papers are being sufficiently taken seriously by the physics community for them to enact the necessary work...

Look, for me this is very simple and you do not require a degree of physics to state the obvious: you cannot contradict General Relativity and move any piece of information faster than light in any given reference frame. Now these people think they are smart and have gathered a "smart-ass" way to deal with this "prohibition", but when you even everything out, it turns out that you didn't get what you wanted in the first place. Irrespectively of how technically and "locally" you can think you can outsmart Nature, in everyone else's point of reference frame, impossible things are happening right at front of them: an object is at place X=0 at time t=0 and then at place X=1 light minute at time t=1.1 minutes (it took 0.1 minutes to travel 1 light minute). Impossible events (just like the Picard Maneuver) and paradoxes abound here.

I'll grant you all the local wizardry and shenanigans, but what makes the whole point moot is just by looking at the larger vicinity of events. The space around the phenomena is just the typical minkowksy spacetime and in such a thing you cannot have any phenomena going back in time, period. This is strickly forbidden by general relativity. If you allow yourself to say "Well but perhaps it is indeed possible to travel backwards in time!" then the onus of writing a paper saying such a thing is on these revolutionaires, not me! These are the ones going for the Nobel prize, not me!

To truly examine the validity of the Alcubierre solution in context of producing paradoxes or not, you have to examine space-time intervals within the metric.  People have done so and to the best of my knowledge have thus far not found any glaring problems.

They have narrowed their investigations to the very local things that were happening inside the "warp bubble" or immediately outside of it, etc. Sometimes, it's enough to just look at the outside of the box and see how amazingly simple it actually is to disprove these notions. Kinda reminds me of stuff like the Zeno paradox and so on. You just needed to look at the problem from a different angle, you don't need any amazing analytical gifts here.

[AdmiralRalwood]

an object is at place X=0 at time t=0 and then at place X=1 light minute at time t=1.1 minutes

I don't think there's anything unusual about an object moving 1 light minute in 1.1 minutes.