[kneexyFreedly]

Those two statements appear to be contradictory.

Not really. One applies to the ship...the other applies to the space/time bubble surrounding the ship.

[suilusargaino]

Not really. One applies to the ship...the other applies to the space/time bubble surrounding the ship.

I'm looking at the centre of mass of a region, including a space ship, at two points in time, neither of which include a space-time bubble.

[Wgnhqhlg]

Why would the momentum be different. The location of the masses may of moved but wouldn't the relative velocities still be the same before warp movement starts and after warp movement has completed, thus momentum is still the same.
I cant see what moving the centre of mass has to do with it, as long as the centre of mass maintains the same velocity.

Note: I am only considering before warp movement starts and after warp movement ends and not considering any effects during the time warp movement occurs as no one knows how this would occur if it can.

[suilusargaino]

Cros - the point is that if you take any system with no external forces its centre of mass will remain at rest (or constant velocity), no matter what goes on internally. This is straight from Newton's 1st Law of Motion.

So if you have a system where the centre of mass has moved (or is no longer on the same straight line), without the application of external forces, then the conservation of momentum has not applied, even if the total momentum at the start and end is the same.

[voksveta]

What if there is no ether, does that say any more about the situation?

[beepbeet]

I will define a "warp drive" as any mechanism that accelerates a body by warping space/time around it, rather than by applying a force to the body and an equal and opposite force to some other bodies (which will accellerate in the opposite direction).

Okay, but that wouldn't necessarily provide faster than light travel would it? Doesn't there has to be some path shortening involved for that to happen?

[Thomas12400]

How about we get the dark matter rearranged so there's no change in momentum. After all, dark matter has got to be useful for something.

[beepbeet]

Anyway, I think you end up with the alternatives: that space-time itself is a store of momentum (but how can that be meaningfully interpreted?), momentum isn't conserved, or you can't do this.

[suilusargaino]

Anyway, I think you end up with the alternatives: that space-time itself is a store of momentum (but how can that be meaningfully interpreted?), momentum isn't conserved, or you can't do this.

Pretty good summary IMHO.

[Vikonbarius]

"Warp drive" doesn't exist within any realistic physics.

[Alliopeti]

Lousy headline but a sensible article by Adam Frank, a professor of physics and astronomy at the University of Rochester:

Alone in the Void

Short of a scientific miracle of the kind that has never occurred, our future history for millenniums will be played out on Earth and in the “near space” environment of the other seven planets, their moons and the asteroids in between. For all our flights of imagination, we have yet to absorb this reality. Like it or not, we are probably trapped in our solar system for a long, long time. We had better start coming to terms with what that means for the human future.

Of course, we know this, on some level. But in a culture saturated with inbred notions of “progress” and an obsession with worlds seemingly just beyond our grasp, there is an expectation that sooner rather than later, we will be building an interstellar culture. In a kind of cosmic version of Manifest Destiny we assume that, unless something terrible happens, our science will be taking us to the stars sometime in the next few hundred years. Simply say “warp drive” to just about anyone and see if they know what you mean.

From “Star Trek” to “Star Wars,” from warp drive to hyperdrive — the idea of rapid interstellar space travel is such a deep meme for cultural visions of space and our future that Hollywood films don’t even have to waste time introducing them to the audience. You pull a lever and zap — you are in a new star system. How many people would be surprised to know that warp drive isn’t even a coherent concept, let alone a near-future technology?

[suilusargaino]

"Warp drive" doesn't exist within any realistic physics.

So are you saying that it's not even possible in principle within the Standard Model or any realistic alternative?

[Maypeevophy]

I think the physics and engineering required to grasp the necessary concepts of how to create a warp drive in the near future would be akin to Neanderthals envisaging our current technology. To them our technology would appear to like magic, so too the concepts of create warp drives and getting them functional.
It may be IMHO just plausible to create a warp drive but just not anytime soon, I don't think our(human) mindset is ready for such technology yet.

[kneexyFreedly]

It may be IMHO just plausible to create a warp drive but just not anytime soon, I don't think our(human) mindset is ready for such technology yet.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


What's soon? :-]


Wasn't it Sagan who said, "Any sufficiently advanced civilisation would appear as magic to us"...or maybe Clarke?

[blackjackiisre]

Apologies for coming in late on this one. I hope I can be of help.

I think the problem may be the neglect of the energy-momentum that can be propagated by space-time itself eg in gravitational radiation.

I am no expert in GR let alone Alcubierre warp bubbles, but I presume that the warp bubble is carrying a fair amount of momentum. Indeed UIVMM the original solution for the warp bubble was for test (ie massless) particles, so there was no other momentum being carried.

I also note that you can't just expect to just stitch together an extended reference frame in GR and expect it to be useful, but that is a lesser problem.

[blackjackiisre]

As noted above, in the references BC gave and on Wiki, the fact that the warp bubble is a solution to the equations is not thought to mean that it is physically realizable, there are a range of reasons why it might not be: not only does it require negative energy, it requires a shedload of it such that it might take -(the energy of the observable universe) to transport a small object. And that's just one difficulty.

[blackjackiisre]

Personal *bump* for the Rev who doesn't seem to be able to find his way to p2 of the Forum :-)

[suilusargaino]

As noted above, in the references BC gave and on Wiki, the fact that the warp bubble is a solution to the equations is not thought to mean that it is physically realizable, there are a range of reasons why it might not be: not only does it require negative energy, it requires a shedload of it such that it might take -(the energy of the observable universe) to transport a small object. And that's just one difficulty.

Thanks for the personal bump

It seems strange then that NASA should say there is nothing in the laws of physics that would preclude a warp drive.

I'm not suggesting that it is certain that they are impossible in practice, but it seems quite likely that they are.

[blackjackiisre]

It's a 'generous' statement.

It's more accurate to say: we are not sure that these are against the laws of physics, although they might turn out to be. There's good reason for thinking them to be unfeasible in any case, although we might learn new tricks that mean they won't be.

I was actually more interested in your response to the conservation of momentum issue.

[opergolon]

http://www.analogsf.com/0505/altview.shtml

"OUTLAWING"
WORMHOLES AND WARP DRIVES

I have written a number of columns in this magazine about wormholes, warp drives, and other constructs of Einstein's general relativity (GR) that appear to offer a good physics foundation for faster-than-light travel and even for travel back in time. All of these GR constructs come from a particular non-standard way of using Einstein's theory, an approach that might be described as "metric engineering." Instead of considering a particular arrangement of mass and energy and asking how space would be warped and what effects would be produced by such an arrangement, in metric engineering we specify how we want space to be warped in order to produce these effects, and then ask what arrangement of mass and energy would be required to accomplish this. The usual outcome of this kind of GR calculation is that a certain quantity of negative mass-energy would be needed. For example, to stabilize a wormhole, a significant quantity of negative mass-energy is needed near the wormhole's throat.

more at link.