# Faster than light / compress space

by Fed
Tags: compress, faster, light, space
 P: 4 ok, ive heard a theory that it may be possible to compress space in front of a "ship" travel through, or, "over" this compressed space, the result being the ship travelling several light years in minutes. i know bending/stretching of space is proven, is compressing space also proven or just (for now) a theory? if so, how would the space around the compression "react"? thx
 P: 93 yeah that brings up a good point, but i don't have much to say on this subject. Can somebody help me on facts?
 Emeritus Sci Advisor PF Gold P: 9,506 It's called Alcubierre's warp drive. I suggest you guys start with Wikipedia and then search for other posts about it here.
 P: 4 Faster than light / compress space i have but nowhere do i see a theory proven or not about the inevitable stretching of space in front of the space you are compressing if only a small area of space stretches then the compressed space would not ammount to as great a distance as if the stretching was spread over a more vast area like the difference between gum and cloth if you imagine a fixed 1mē piece of chewed gum and keep compressing the centre the immediate area around the compression will just keep thinning out, imagining a ship using ths to travel from a - b they would be limited to traveling as far as the stretching point if like cloth the stretch happens over a wider area meaning much greater travel prospects
 P: 863 Personally I can't see the Alcubierre's warp drive working as described. Given Newton's first law, I don't think the "motion" of a flat region of inside the bubble has any real meaning, or would impart any change of relative motion to the ship, relative to anything else in the Universe. Now once the wave caught up with the ship it would impart an acceleration, like a gravitational field. Like a surfer who must wait on the wave before the wave imparts any motion. But then you would be subject to the time effects of that field. So you could get there really fast, but many more years would still pass back home, like standard high speed relativistic time effects. That's my take on it anyway.
P: 2,193
 Quote by my_wan That's my take on it anyway.
Unfortunately, your take is wrong. Read the paper, it shows quite nicely that there are no time dilation effects and indeed the spaceship (and bubble) can move at an arbitrarily large velocity.

http://arxiv.org/PS_cache/gr-qc/pdf/0009/0009013v1.pdf

It's actually a nice read, even if you're only sort of familiar with GR since he explains a lot of the basic concepts.

With regards to the original question, we have lots of observational evidence for General Relativity at this point, which doesn't really make any distinction between the words "stretching" "compressing" "expanding" etc, it's all just warped spacetime so I prefer to avoid the semantics and just speak of it as such. So yes, the type of spacetime described by Alcubierre isn't hypothetical but exists on solid ground in the theory of GR. However, I wouldn't count on building one of these any time soon, as Alcubierre shows that such a warping of spacetime requires negative energy densities as observed by all observers, which is something of a problem.

Edit: Just for future reference, it seems this thread is better suited for the relativity boards.
 P: 863 By what mechanism is a flat region of spacetime imparting motion to this ship? Let's look at the phenomena in GR it's based on. First is an expansion of spacetime. In the expansion of the Universe, as relatively near points in space obtains space like separations, the discontinuity in simultaneity, as defined by each point relative to each other, increases accordingly. Yet you never actually get anywhere, only farther away from everything else. If you try to emulate this locally, you can't maintain a continual expansion, else it'll have 1 of 2 effects. The first is the location of the expansion doesn't stay local, and must eventually expand to effectively include the entire Universe, pushing you away from everything. The second is that the gravitational field strength must continually increase around the flat region. Ok, let's not have a continual expansion, but merely create this flat region inside a constant field. It's not even generally true that 2 inertial observers with no relative motion between them will share clock rates. Consider a large uniform massive hollow sphere. As you approach this sphere you clocks will slow all the way to the surface. Go inside this sphere and spacetime is flat. Yet the clock rate inside this flat region of spactime will remain slowed to the same rate as the surface with gravity. So flat spacetime itself doesn't get you out of the time dilation effects. So this drive creates an expansion and a depression to balance clock rates inside and outside. But there is no constant expansion 'rate', beyond what Hubble law imposes. Only a stable expanded region. You can move around inside this flat region, like in the hollow sphere, but the motion of the expanded regions are not going to impart any motion to you without an expansion 'rate'. You must supply the motion yourself. If your clock rate matches that outside the region, you only get normal relativistic time dilation effects keeping up with the bubble. If you lower the gravitational depth of this flat region, like in the sphere, and get there much faster locally, but that means your clock is far slower than clocks outside that region. I read the paper, and it moves from expansion to balancing constant fields, as if it's going to accomplish the same thing. Then supposes somehow moving this field isn't going to bring you into contact with the fields gradients, the only thing that can impart local acceleration to you. It seems to understand the need to balance the fields to maintain clock rates, but accelerates coordinate systems as if a coordinate system is a physical thing, it's not. It's called background independence. Changing coordinate transforms of the surrounding space means nothing to your relative velocity to anything in the Universe, assuming your clock rate remained constant relative to the outside, except the fields created by the transform, which then runs you over.
P: 1,555
 Quote by Nabeshin we have overwhelming observational evidence for General Relativity at this point
I would not quite put it like that. There are only a few phenomena that we can actually test by using GR and mostly using weak field approximation methods. Apart from measurements even complete formulaic solutions are very few and numerical approximations are often problematic. On the other hand we can point to observations that puts the validity of GR at odds. Right now the mainstream scientific community approach to this is to postulate things like the existence of dark matter and dark energy to get the data to fit the theory again.

So yes there are some experiments that show GR is a more accurate theory than Newton's theory but I would not call it overwhelming. Perhaps you are simply easier overwhelmed than I.

 Quote by Nabeshin So yes, the type of spacetime described by Alcubierre isn't (purely) theoretical, but exists on solid ground in the theory of GR.
So if something is valid in a theory it is not not (purely) theoretical?
And "on solid ground"? There are an infinite number of solutions that satisfy Einstein's field equation but that does not imply that all of those are automatically physical.

GR is a theory it is not a religion!
P: 2,193
 Quote by Passionflower I would not quite put it like that. There are only a few phenomena that we can actually test by using GR and mostly using weak field approximation methods. Apart from measurements even complete formulaic solutions are very few and numerical approximations are often problematic. On the other hand we can point to observations that puts the validity of GR at odds. Right now the mainstream scientific community approach to this is to postulate things like the existence of dark matter and dark energy to get the data to fit the theory again. So yes there are some experiments that show GR is a more accurate theory than Newton's theory but I would not call it overwhelming. Perhaps you are simply easier overwhelmed than I.
Fine, not overwhelming. Choose your adjective (wisely, someone else might disagree!), but it's on solid ground, no?

 So if something is valid in a theory it is not not (purely) theoretical? GR is a theory it is not a religion!
My distinction obviously isn't clear. I'm trying to distinguish the alcubierre spacetime from something like tachyons, which I think we all agree are purely theoretical constructs. I realize this is somewhat obtuse and confusing, perhaps not even important, so let me think of an example to attempt to elucidate my point...

Imagine you come up with a solution to maxwell's equations, some distribution of E and B fields that has some interesting property (what this property is isn't important). Name this distribution the Passionflower field. Now, the Passionflower field is very difficult to produce, perhaps it even requires infinite charge distributions, so it is a theoretical idea. But, we believe Maxwell's equations to be very sturdy, so the Passionflower field isn't some whimsical dream not grounded in experiment, but rather a non-realized solution to the equations.

Perhaps that helps, probably not though.
P: 15,319
 Quote by Nabeshin My distinction obviously isn't clear. I'm trying to distinguish the alcubierre spacetime from something like tachyons, which I think we all agree are purely theoretical constructs.
They're not even theoretical. The word you're looking for is hypothetical.

There is no evidence, theory or hypothesis that suggests tachyons do or should exist. They are no more than a "suppose". Significantly weaker than theoretical.

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