[jr2]

Hey, deathfun...

Kudos for getting the point of the thread, have a cookie.



Not referring to anyone who was actually discussing the theory etc, just referring to the 'gaaaaaaa alcubi doesn't work, burn, heretic!' tone that was going on for a bit. I mean, it was mainly about the art, guys.

Sorry I couldn't post screenies, the site wouldn't allow the pictures to be menu-clicked. That would have helped set the tone I imagine. Bah, mobile browsing sucks sometimes...

[Mikes]

Aaaand everyone misses the gorgeous model. Go figure.

re: alcubi drives not working: read up on all the science disproving yhe Wright brothers' hair-brained heavier than air flying machine concept.

Does this prove it will work? No. It simply means, that just because science says something doesnt work, doesn't necessarily mean it won't. You might just be making the attempt in the wrong way. Then again, there might be no feasible way! Haha! But it's always fun to try, you learn more regardless of the results being successful or not.

The case against the Alcubierre drive is much more solid than that, and you should really know better than to equivocate the two. The requirement to have a bunch of exotic matter, which has never been observed to exist and for which no method of manufacturing exists, is a massive dealbreaker.

Are you sure this is not because we are looking back at the Wrights with so much more acrued knowledge now, while Alcubierre is still a big question mark?

Can you really be sure that with another 100 or 200 years of more knowledge we won't be looking at both of them in the same way?

I mean.... not so long ago "breaking the sound barrier" or hah traveling to the moon.... was completely in the realm of science fiction and people speculated whether it will be possible at all ... yes, now we know it's easy... the point is that before we knew that we had all kinds of educated people arguing that it's impossible and utter nonsense


I only know that I can't say with certainity whether it will or will not work at some point in the future - with the limited information we do have at this point, wouldn't you agree?

[Black Wolf]

Are you sure this is not because we are looking back at the Wrights with so much more acrued knowledge now, while Alcubierre is still a big question mark?

Can you really be sure that with another 100 or 200 years of more knowledge we won't be looking at both of them in the same way?

I mean.... not so long ago "breaking the sound barrier" or hah traveling to the moon.... was completely in the realm of science fiction and people speculated whether it will be possible at all ... yes, now we know it's easy... the point is that before we knew that we had all kinds of educated people arguing that it's impossible and utter nonsense


I only know that I can't say with certainity whether it will or will not work at some point in the future - with the limited information we do have at this point, wouldn't you agree?

I guess that the distinction is that, for the Wrights, they could look at birds, bats, insects - lots of examples that what they wanted to do was physically possible. It was purely an engineering challenge that was always going to be solved sooner or later, because nature had already solved it. Similarly, for things like breaking the sound barrier, or landing on the moon - there were challenges, absolutely. But each one was simply a matter of refining and improving existing techniques and technologies. The physics was nothing spectacularly new - even for things like turbulent flow at mach 1, the physics was never exotic - just vastly complex.

An Alcubierre drive is in an entirely different category. We have zero evidence that anything, under any circumstances, under any reference frame can move at a velocity of greater than C, and there are some very, very fundamental physical laws that say that we almost certainly will never find any. The methods people propose to get around it are ingenious, and maybe one day they will work - I really, really hope that they will - but surely you can see the difference between an engineering challenge exploiting known physics and a physics challenge trying to challenge our fundamental understanding of reality?

[The E]

I only know that I can't say with certainity whether it will or will not work at some point in the future - with the limited information we do have at this point, wouldn't you agree?

No. I do agree that we can't know what the future will hold, but I am not holding my breath for the small possibility that the future holds exactly the kind of breakthroughs that would enable us to turn the Alcubierre drive into reality.

I mean, if you're taking the analogy to the early days of powered flight further, remember all those insane machines people came up with? The ones that, to a modern mind, are clearly not going to work, ever? It's far more likely that this drive system is going to end up in the same pile than it is for it to work.

[Dragon]

Note, the "sound barrier" analogy is somewhat better one. It was, at one point, presumed that aerodynamic forces will tend to infinity as one approaches speed of sounds. Turns out Prandtl-Glauert model simply failed at Mach 1, as the air behaves very differently after that. Granted, they did have an example, too (bullets went supersonic long before planes), but it demonstrates a situation where a normally working theory stops to apply.

But before breaking the sound barrier, maybe we should learn to fly? If White's "mini-Alcubierre" works, then it's a good thing for FTL proponents, since it'd demonstrate the concept is sound, even if the speed is limited. However, if it doesn't work, well, that's good too, because it means that a theory we're using isn't good (since it postulates STL Alcubierre drive is definitely possible), and that we don't know a lot more than we think. So either way, it opens up new perspectives in the field. So I'd say, let's wait for White to get some better results.

Also note, you don't need FTL to travel to the stars. If Alcubierre drive is limited by the speed of light, that would probably also mean it's subject to time dilation and Lorenz contraction, much like everything else. Assuming the numbers quoted before hold for an STL system, you could accelerate with the thing to some ridiculous speed like 9.999c and let relativity work for you. You won't travel faster then light, but at this point, distance between you and the target shrinks, so you'll still be at your destination much faster. Of course, the people planetside will think otherwise, but that's their problem, isn't it? And with some imagination, you could write stories just as interesting with this kind of drive (in fact, some did. "Forever War", for instance, deals with using time dilation to cover long distances).

[Ace]

Just thinking... bigger warp engines means more exotic mass and more speed.

So really an optimal design would be deconstructing a small moon, have an equatorial warp ring and a warp field extending just above it's surface.

Probably contain water and fuel storage within the outer sphere. Then put rotational habitat rings inside of it. (probably layer a few at different angles)

So it'd basically be a combination of Rama and the Deathstar.

[Mikes]

Just thinking... bigger warp engines means more exotic mass and more speed.

So really an optimal design would be deconstructing a small moon, have an equatorial warp ring and a warp field extending just above it's surface.

Probably contain water and fuel storage within the outer sphere. Then put rotational habitat rings inside of it. (probably layer a few at different angles)

So it'd basically be a combination of Rama and the Deathstar.

And lucky for us we got one of those small moons in orbit already ....... uh

[watsisname]

No, I didn't "read an article". I consulted with physicists, read quite a lot of material regarding this, and engaged in critical thinking.

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.

Not only you have to generate and efficiently manage this energy, you'll even have to turn it into negative energy, which is something we either believe is impossible to exist, or just mindblowingly difficult if we ever come across with some "fluke" like the Casimir Effect and so on.

But cosmological observations indicate that a negative value for vacuum energy does exist, and it works perfectly fine in the framework of general relativity as well.  The problem is not 'are repulsive effects on space-time possible?' but rather 'how do we achieve them on a local scale?'.  The latter might not be possible or practical, but this is quite a difference from saying the mechanism itself doesn't exist.

This idea of creating "lanes" is good for scifi, but please. Anything that is going faster than light from any other perspective is either creating paradoxes or hidden from us by event horizons, and anything that is beyond event horizons can never "go back" to us.

Admittedly I am not very familiar with the 'lanes' idea, but I don't think this is an attempt of explaining away paradoxes except by people who aren't up to snuff on general relativity and/or don't fully understand how the drive works.  Your argument here sounds very persuasive, but it suffers similar flaws.

The first part of your argument is an assertion that objects moving with v>c (i.e. tachyons, or having a space-like trajectory through space-time) either invokes paradoxes or is hidden by an event horizon.  The first consequence is correct, the second is not.  Tachyons do not produce event horizons (just follow the null geodesics coming off of it), but they do probably invoke paradoxes, because they enable situations where effects can precede their own causes in certain reference frames.

The second part deals with objects that are behind an event horizon, with an assertion that they cannot 'return to us'.  This is correct in a general sense, e.g. particles that have fallen into black holes or which lie beyond the cosmological event horizon.  They cannot return to us because in order for them to do so, they must follow a space-like trajectory which invokes the problem above (among others, like the presumed acceleration past c).

But Alcubierre drive is neither of these cases.  The trajectory of the ship is not space-like, but time-like, and feeling zero accelerative forces.  The motion is instead due to changes in the space-time metric around the ship.  Event horizons may be produced, but these are of very different form than those of black holes or the large-scale universe.  They are non-static and non-permanent.

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.

[Aardwolf]

@Alcubierre Fanboys:

Proposition:
Changes to the curvature of space-time cannot propagate faster than the speed of light.

Discuss.

[The E]

@Alcubierre Fanboys:

Proposition:
Changes to the curvature of space-time cannot propagate faster than the speed of light.

Discuss.

@Aardwolf

That is not how discussion works. Please cite papers that support your position.

[Aardwolf]

I assumed at least someone would be familiar with this.

Speed of Gravity on Wikipedia.

The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c. Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space. This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of "light" is also the speed of gravitational waves and any other massless particle. Such particles include the gluon=http://en.wikipedia.org/wiki/Gluon (carrier of the strong force), the photons that make up light, and the theoretical gravitons which make up the associated field particles of gravity (a theory of the graviton requires a theory of quantum gravity, however).

Emphasis mine.

[watsisname]

@Alcubierre Fanboys:

Proposition:
Changes to the curvature of space-time cannot propagate faster than the speed of light.

Discuss.

And they don't in the Alcubierre solution.  Read the paper linked to in the post above yours.

The most important lesson in general relativity, the lesson they drill into you in the first chapter/lecture in any good course/textbook of the topic, is that space-time physics must be looked at locally.  Locally, nothing moves faster than light in the Alcubierre solution, and space-time perturbations propagate at c.  You're suffering the same misconception people have when they claim that superluminal motions at great cosmological distances in the expanding universe defies relativity.

Ed:  I find it really mind-boggling that people think they can so easily refute a solution of general relativistic field equations with only the most basic notions of general relativity.

[Aardwolf]

Let's try a graphical approach to describe how far the effects of the Alcubierre drive have reached at any time. If "a picture is worth a thousand words", I'm making a sevenfold profit here.

Let's work in one dimension (so we have x and t), and use c=1, so that in undistorted space a particle moving at light-speed is a line parallel to x=t.

Let's use a simplified model of the distortion, since we're only concerned with how far the farthest extent of it has reached. We'll say that space is either distorted or not, and that in distorted space, a particle moving at light-speed is a line parallel to x=2t.

At t=0, we emit a "distortion front". Its trajectory is x=t.
At t=1, we emit another. Its trajectory is initially x=2(t-1). This intersects x=t at (2,2). It has now reached undistorted space, and its trajectory changes to match the original.
At t=2, we emit another. Its trajectory is initially x=2(t-2). This intersects x=t at (3,3). The same thing happens.
     .
     .
     .



You're probably going to tell me this isn't how it works. Well since I'm going to the trouble of describing graphs of how I think the effect would propagate, you can go to the trouble of correcting this model.

[watsisname]

You're right, I'm going to tell you that that is not how it works.

You're yet again trying to examine motions by invoking a global inertial reference frame.  This doesn't work because your reference frame contains non-static curvature which cannot be removed by a simple coordinate transformation, and is therefore not an inertial frame of reference at all.  Therefore your conceptions about what must happen do not hold valid.

This is the whole reason general relativity was derived in the first place -- to deal with motions when global frames are not available.  Space-time physics must be examined locally.

Try reading the supplementary material I have been providing you with before continuing to make arguments out of ignorance of the subject.

[Aardwolf]

Can't read Greek, and learning Greek wouldn't be time-economical.

Ok, tell me this then: the instant the Alcubierre drive is turned on, what is the farthest distance its distortion effect could have reached? How can there not be a "front" beyond which the effect has yet to be observed?

[watsisname]

Can't read Greek, and learning Greek wouldn't be time-economical.

If you do not understand geometrodynamics, then why are you participating in a discussion of it as if you have any expectation of educating those who do?  If you want to understand why your conceptions are wrong, then you need to educate yourself.  The means of doing so have been kindly provided.  Sorry if you don't have the time, but I have no sympathy for one's choice in continuing to deliberately argue out of ignorance.

Your latest question suffers the same conceptual problems that I explained in the prior post.  I won't repeat it again, you know how to read.

[Aardwolf]

Is there or is there not a "front" beyond which the effects of the Alcubierre drive are not felt?

The 5 seconds it takes to respond "there is" or "there isn't" are less valuable than the 10 hours it would take for me to learn even the basics of geometrodynamics.

[watsisname]

Can a quantum system which was prepared to have σ_z=+1 have the state σ_x=+1 and σ_z=+1?  It takes 5 seconds to respond "yes" or "no" and this is more valuable than however long it takes to explain why it is an improper question by explaining the quantum uncertainty principle.

[/equivalent question for purpose of demonstrating why it is ill-posed]

The avenue that you are trying to pursue is that the propagation speed of Alcubierre's distortion field must propagate faster than c in order for it to achieve its claimed effect of superluminal travel, yet it cannot do this because fields cannot propagate faster than c.  Therefore there is a logical inconsistency and therefore the drive is impossible.

Congratulations, you, a person who doesn't understand nor want to put in the effort of understanding general relativity apparently just showed that an entire community of people who are experts in it that they are wrong.  Shall we believe this?  No.  In much the same way as I don't believe a creationist disproved evolutionary theory by any criticisms they've yet made.  They simply have misconceptions just as you currently do.

The logical inconsistency is remedied by investigating motions with an appropriate reference frame in an appropriate coordinate system.  If you do so, you will find that locally, the ship is at rest while distortions to the field propagate at c, and globally, these appear faster than c.  An analysis of the metrics shows that this does not produce paradoxes nor does it violate general relativity.  Quite the contrary, it is a solution to it.

[Black Wolf]

Just for the record, this thread is awesome. It's always great to listen to people who know their **** arguing physics.

[watsisname]

As an aside, as I don't wish for anyone to misunderstand me:  I am not some sort of 'Alcubierre fanboy'.  I'm not convinced that it is feasible.  I'm not convinced that it is even possible.  I actually suspect that there may be a mistake in the formulation of the Alcubierre solution, or that it can't be used to produce effectively superluminal speeds.

I'd love to examine it myself, but I lack the skills.  I study astrophysics and cosmology, I have taken courses on general relativity, own the Gravitation textbook (a landmark text in the field) I linked to in an earlier post, and am quite comfortable with the language and mathematics.  Nevertheless, thoroughly examining the details of Alcubierre's solution is still way beyond my ability.  I don't know that it is right.  I only know that people a hell of a lot more capable than myself think it is, and to my knowledge nobody has shown otherwise yet.  I am completely serious when I ask people who think it's wrong to try to submit a critique to a journal.

To semi-quote Herra from a prior thread, I have yet to see someone examine the equations and say "hey, this minus sign needs to be a plus", or "this space-time interval doesn't properly equate with that one."  The only things I have seen, including in this thread, are people trying to argue that it can't work through the use of simplistic arguments of general relativity that have never failed to be based upon an inadequate understanding of how general relativity actually works.