[AdmiralRalwood]

You're playing with semantics. He always accepted the greenhouse effect. This word "denialist" is just political slimeballing. What he does is a bit different than the usual denialist, he just doesn't believe there's good evidence for the water vapor positive feedback that the more "alarmist" models predict that the atmosphere has. And then he loses his **** whenever some leftist loses his **** in some newspaper or site or whatever, and it's good fun to watch if you have popcorn by your side.

I think that trash like yourself cannot be debated. It must be destroyed. What is written on the "climate denial" page is just crime, the people who are responsible for it are criminals, and as soon as I get the opportunity to collaborate with someone on their liquidation, I will do it.

(source)

I think Motl does more than enough "political slimeballing" of his own name with his own words.

[jr2]

I would like to point out that if we manage to solve the practical questions (which are not exactly trivial matters) of modifying space-time geometry at will with reasonable energy cost, it would be insanely cool whether Alcubierre Drive as an FTL transportation device works or not.


Things like antigravity, artificial gravity, and reaction massless propulsion systems come to mind*. It would practically be open season for colonizing anything in the Solar System, and it would make it possible to craft large scale, self-sufficient space colonies possibly capable of traveling to nearby star systems. It'd just take a while.

QFT   

[Luis Dias]

I think Motl does more than enough "political slimeballing" of his own name with his own words.

Oh I didn't say it was "undeserved" or anything. I said it was false.

[Bobboau]

reaction massless propulsion systems

this was White's main area of interest that led him to Alcubirre type drives IIRC, his other big thing before this was quantum thrusters, ion drives that use virtual particles as their propellant.

[watsisname]

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 provided the reference frame lacks curvature or other such departures from special relativistic mechanics.

Fixed that for you.  It doesn't take a degree in physics to state the "obvious", but it might take one to state what is correct.   Read the supplementary material provided here for more information.

[Luis Dias]

I'm not debating that this kind of "superluminous travel" using space itself doesn't exist, of course I accept that given the premise that space is stretching and so on then all of this is possible. What I am suggesting is that the process must be viewed in its entirety and what seems like a smart cheat against FTL prohibitions is really not when you see the whole process in its entirety. Inflation was a very special event that was, apparently, very homogeneous in nature and precise, in the sense that all space knew at the start the kind of stretching it would undertake and all that was (apparently) required for it to slow down at some point was a combination of gravity and this weird astro constant. But all of this was already in place. You didn't need to superluminally "tell" space to stretch itself.

Not here. IOW, if you are a light hour away from me, no matter how powerful your Alcubierre Drive may be, "space" has to know by the means of some piece of information wave how precisely it would stretch. This information is necessarily driving in c speeds inside the "previous / current" curvature of space.

After having done this, then you can proclaim all you want in how your ship was (in a common sense view) travelling faster than c and I have no issues with that part of the process claim. Of course that having bent space-time, all bets are off.

[Phantom Hoover]

this is all very 'obvious' luis but until you can convince me that it actually follows from the einstein field equations i am going to continue dismissing you

[Bobboau]

ok, let's put this into slightly different terms. lets say you are near the event horizon of a very large black hole, lets say it is a light hour wide, lets also say this black hole is rotating extremely fast. now for the sake of argument you have really really powerful thrusts that can output arbitrarily powerful thrust of finite amount for as long as needed. you are not inside the event horizon, only near it, and you are thrusting directly away as exactly enough thrust to not get pulled in. the black hole is rotating extremely fast and as a result there is an extremely pronounced frame dragging effect on the space you are occupying. So much so that from a third party several light years (and year years obviously) further/later away that it seems as if you orbit the black hole every hour even though from your perspective you are thrusting directly away from the black hole remaining motionless relative to it. Note the part about how you seem to trace out a path of >1(light hour) * PI every hour.

relativity is very 'obvious' it's why it was so easy for us to figure it out.

[Luis Dias]

That's still a very standard spacetime distortion story. The third person is not even observing the real distortion of spacetime for the light ray paths have also been distorted while trying to get out of that gravity well. In that sense, the spaceship is "travelling" even faster than you imply. But here's the thing: that's not a FTL story. There's no way that spaceship can "use" the black hole's fast rotation in order to travel faster than light, by for instance get inside that particular "orbit" and get out of it in the "other side" as a FTL quick spin to the other side of the BH. This is because when you calculate everything, you've used so much energy and "time" (from the third party point of view!) getting into that position and out of it that if you just closed you eyes in the beggining of that operation and opened in the end of it, the spaceship wouldn't have traveled from A to B in any FTL fashion.

The only way the maths would have made it FTL would require the ship to go through the event horizon but then it can't go back...

Now I know I'm probably oversimplifying here. Lots of **** going on in fast rotating BHs. Still. All that "FTL" shenanigan was just an useless illusion. Alcubierre proposed something remarkably different from this, that this is an illusion that actually works, is actually useful.

But now consider BHs again. They are a useful comparison in this sense: they don't warp space around it in FTL fashion. They always use graviton waves to do the job. I'll ask it again: how does the space itself in the path ahead of the warp bubble "knows" how to bend? It cannot know this unless something told it this information. This information cannot be carried superluminally, or alternatively we are talking about an infinite regression: we need a warp drive to build a warp drive.

[AdmiralRalwood]

But now consider BHs again. They are a useful comparison in this sense: they don't warp space around it in FTL fashion. They always use graviton waves to do the job.

What? You have proof that gravitons exist? Share with the class, please.

[Luis Dias]

Fine. If you want to stick with "observed" entities, substitute "Graviton waves" with "Gravitational waves".

[watsisname]

What I am suggesting is that the process must be viewed in its entirety and what seems like a smart cheat against FTL prohibitions is really not when you see the whole process in its entirety.

It is, otherwise we would have no idea what the proper travel time would be for those on the ship vs. distant observers.

You have some confusion in that you think we are looking at what happens locally because it is difficult or impossible to examine it globally, therefore we are missing something important about what's actually going on.  That is not the case.  If it was, then we would have no way of comparing proper time intervals for those on the ship vs. distant observers.  When we say we are looking at what happens locally, we do that because that is how you examine motions within curved space-time.

This is an important point that cannot be overstated -- general relativity upholds what we already knew from special relativity.  Light still always propagates at c, and allowed time-like trajectories are still less than c.  The difference is that in the flat space-time with unaccelerated observers of special relativity, it doesn't matter how far apart the observers are -- provided an understanding of Lorentz transformations they'll all agree on whether an object has a space-like, time-like, or null trajectory.  But in the curved space-time of general relativity, distance between observers does matter, and if you don't account for it then observers can easily be confused and think a trajectory is space-like when it is not.

Examples:  Objects within black hole event horizons and ergospheres, galaxies at large cosmological distances, wormholes, and Alcubierre drives.

One way of dealing with curvature is to examine it on sufficiently small scales.  Just as the (roughly) spherical surface of the Earth seems flat when you're standing on it, space-time curvature appears flat if examined on sufficiently small scales.  Those who are familiar with calculus might be reminded of tangent lines, and the similarity is not superficial.  By examining motions locally, you see what they actually are through the space-time itself.  The global view in turn will tell you what the curvature is (provided you understand its source) and how motions are affected by it.

So in a nutshell, the central problem that Luis and Aard have here is that they're trying to assert what is or is not allowed according to general relativity on a global perspective, yet general relativity doesn't actually assert such a thing.  If it did, then the observed motions of galaxies at large distances would violate general relativity.  On the contrary, a proper understanding of GR's principles would reveal that the motions of these galaxies is actually a brilliant proof that it works.  The Alcubierre drive is a fine solution to GR as well -- it works as long as it is possible to transform space-time geometry in the stated way, which comes down to the three energy conditions that it does indeed violate.  (Note, though, that the energy conditions are not "laws", like the 2nd law of thermodynamics.  There are known phenomena that violate one or more of them.)

I strongly suggest that Luis et. al refine their argument from "This solution violates basic principles of GR" to "is it possible for the required transformation to occur?"

[Ghostavo]

I'll ask it again: how does the space itself in the path ahead of the warp bubble "knows" how to bend? It cannot know this unless something told it this information. This information cannot be carried superluminally

Apparently, light cones are not all they're cracked up to be...

[Luis Dias]

I strongly suggest that Luis et. al refine their argument from "This solution violates basic principles of GR" to "is it possible for the required transformation to occur?"

Very fine comment, all of it, but it didn't address any of my concerns, since it dealt with basic questions on space-time curvature that I had already agreed with from the start. My concern isn't about the Bubble itself being "anti GR", it's about "how do you tell spacetime itself to do this without informing it so superluminally".

For instance, I do get it that if spacetime bends at the front of the bubble in order to become extremelly small, then you can travel it really "fast" without being fast at all. What I don't get is how you can teach spacetime in very precise coordenates to do this without signalling it. And if spacetime at the front isn't "bent", then all the information that gets to it must do so either at "c" or lower.

Regarding "global concerns", I only ask this question. Are we speaking or not about a drive that can go faster than light or not? I mean, at some point, we are indeed talking about something that is meaningful in some way. My initial point about how looking at this from a more distant point of view is that if we abstract all of this engine into some kind of black box (and in this black box I might include the wave and so on), then what theoretically happens is that there's something at point X in time 0 in a perfectly smooth plane spacetime, and then this thing is at x = 1 light hour at time = 1 second. If I seem to be speaking from a "global reference frame", think about someone watching this in a perpendicular angle from some light hours away. Now this "black box" could be spoken as a kind of a weird (exotic?) particle. And this "particle" seemingly has the ability to travel faster than light (when all things are said and done).

I'm incredibly skeptical at this, and not just for the violations you mention. Although they can be mathematically related.

[Phantom Hoover]

Fine. If you want to stick with "observed" entities, substitute "Graviton waves" with "Gravitational waves".

"ok lemme just juggle physics terminology until i come up with something that makes sense"

Very fine comment, all of it, but it didn't address any of my concerns, since it dealt with basic questions on space-time curvature that I had already agreed with from the start. My concern isn't about the Bubble itself being "anti GR", it's about "how do you tell spacetime itself to do this without informing it so superluminally".

This just further exemplifies your general mistake here: you do not understand GR. You do not know or understand the mathematical framework underlying it. You think you can get by using nice approachable heuristics like this but you just can't. Stop trying to poke holes in the work of people who actually do: you are not going to achieve anything above petty bickering.

[Dragon]

To be fair, that's a very common mistake made by laymen. The only way to truly describe real physics is by mathematics. English language is only capable of approximating them, and working off those approximations leads to many mistakes. That said, this is one of the first things you learn when you start seriously working with physics, so this is a typical layman's mistake no professional would make (or at least, they'd correct themselves quickly after checking the mathematics). Indeed, the whole point of having mathematics is to provide a "language" which can be used to unambiguously (and thus "correctly" in the very essence) describe even the most complex physical phenomena. Whether our description is actually representing reality is, of course, a subject to revision, but a mathematical model (unlike an explanation in English) can be a perfect representation of processes that actually occur. This is an useful thing to know, too. By knowing what kind of process or object a mathematical concept may represent, it's easier (for some) to understand the concept itself.

In that case, it's a classic "it's hard to describe, but it works". That's the best response you can get at an "internet forum with no LaTeX support" level. You can talk about tensors and transformations, but in the end, the best explanation would be purely mathematical, and probably full of nablas, tensor products and other terrifying symbols, to boot.

[Bobboau]

as an aside that I hope doesn't derail the thread too much, I've wondered for a while now, what the **** is the difference between a tensor and a matrix?

[watsisname]

Are we speaking or not about a drive that can go faster than light or not?

Are these observable galaxies moving faster than light or not?

The answer to your above question is both "Yes and no."  Again, and with elaboration having been already provided, faster than light motion does not violate general relativity unless it is faster than light locally.  Such motion does not occur anywhere in the Alcubierre solution as far as anyone has yet been able to demonstrate in a convincing manner.

Which brings me back to what I said a few pages ago:

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.

It appears that the answer to this question is a resounding "No." 

You may attempt to counter all you wish that "physicists don't take these papers seriously", but it's pretty difficult for me to take such a counter seriously, and it seems a number of our fellow forumites hold the same view.  I would like to think that if one of these people you cite had a valid refutation of the Alcubierre solution, they would want to submit it to a relevant journal in addition to posting it on their blog, at the very least to have experts in the field review it and ensure that they are right.  If it is attention that they are after, they'd get that, too.  If not, then why are they posting about it on a blog at all?

[watsisname]

as an aside that I hope doesn't derail the thread too much, I've wondered for a while now, what the **** is the difference between a tensor and a matrix?

A matrix is simply a grid of quantities (which can themselves be almost anything -- scalars, vectors, derivatives, etc), and they may represent something physical (like pixel values in an image, or temperature on a map) or they can be totally random entries.

A tensor on the other hand (which can be expressed with a matrix!), is a geometric object which describes quantities that transform under a particular set of rules.  Another popular way of thinking of tensors is that they are 'machines', which accept certain inputs and spew out outputs, again according to a particular set of rules.  This is pretty vague (e.g. what rules?), but basically what they do is allow us to describe things irrespective of choice or change in coordinate system.  Thus their importance in relativity and other branches of physics.

So to be brief, a matrix is a grid of quantities, while a tensor is a grid of quantities which follow a particular structure.

[Dragon]

A good example of a tensor would be moment of inertia. For example: you input a force (a vector) and you get a rotational acceleration vector. A high-school level approach is to use a coordinate system connected with principal inertia axes (which diagonalizes the tensor) and then apply the force along one of those axes (usually the simplest one), which means you can use a single scalar, which is what they call "moment of inertia". Obviously, such a neat situation is rarely the case in real life. If it isn't, you're gonna work with a MOI tensor.

Tensors are used everywhere, from relativity through classical mechanics and electricity to quantum mechanics. Rank two (representable by a 3x3 matrix) tensors are particularly common.