Hot answers tagged

17

Edit regarding 3+1 spacetimes and causality I'll keep adding to the answer as I get more information, and hopefully everything will just evolve along. At the very least, I'll have a set of notes to work from in the future :) This is also the first, broadest, cut at an actual answer regarding causality. Alcubierre sets out to find his warp drive metric ...


11

Warp drives are not allowed by the basic laws of physics, in particular the theory of relativity prohibits any superluminal motion or superluminal propagation of usable information. So whatever "exotic matter" or other wordings are proposed to justify the superluminal warp drives is banned, too. The typical "exotic matter" needed for warp drives would need ...


10

Irrepspective of the amount of negative mass matter required, it's still a fact that these schemes require negative mass matter to work, and negative mass matter is not known to exist. And there are still problems with faster than light travel that you would have to resolve--in particular, that a solution like this would enable you to construct a time ...


10

Spacetime can dynamically evolve in a way which apparently violates special relativity. A good example is how galaxies move out with a velocity v = Hd, the Hubble rule, where v = c = Hr_h at the de Sitter horizon (approximately) and the red shift is z = 1. For z > 1 galaxies are frame dragged outwards at a speed greater than light. Similarly an observer ...


8

I don't understand why we are able to see and measure curvature / warping of space at all. The Earth's surface is curved and this can be observed via the vast number of pictures of the Earth from space that now exist. However, the surface curvature can also be "seen" via measurements on the surface itself. For example, if one were start at the North ...


8

It will violate causality globally. There is no way around that. This is the best argument about why building these things is impossible. To see this, all you have to do is zoom out to some scale where the warp drive field becomes a point. Then, the warp ship is just a point moving superluminally against a SR background, and all of the causlity problems ...


7

This is more of a meta answer, since it isn't really Physics, but it got a bit long to put in a comment. You say: I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. You need to think about precisely what this statement means. If we take the Alcubierre drive as an example it is a ...


7

If Alcubierre warp bubbles are physically possible, which is exceedingly unlikely, and if the equivalence principle is correct, you could definitely escape from a black hole in one, because there's nothing locally special about the event horizon. In a large black hole the whole escape process can happen arbitrarily far from the singularity, in a spacetime ...


6

The interior of the bubble is causally disconnected. It's not possible for the bubble to be turned off or steered from the inside. But there is no reason it cannot be affected from an outside agency at a pre-planned points, or even simply have a finite lifetime, naturally deteriorating to stop at the intended destination. that upon traveling to some ...


5

Theoretically, yes. Assuming that our physics for black holes is correct enough to tell us that the escape velocity of a black hole is just greater than the speed of light. If you think about going faster than the speed of light as going back in time, this also serves as a way of "escaping" the black hole, for you would go back in time to a point to where ...


5

Curvature affects how objects in the universe move and interact with one another, and these effects can be measured. Take, for example, the phenomenon of gravitational lensing. Because spacetime curvature can deflect the path of light, we can potentially observe light coming from objects that are directly behind other objects. Here's a nice picture. As ...


5

Relativistic effects happen when you travel near light speed through space-time. With the Alcubierre drive, you don't travel through space-time, but remain stationary and the space-time around you is warped in a way that brings you closer to your destination. So if it only took a week for you, it would only take a week for your observers. It'd be like if ...


5

It expresses infinitesimal distance in space-time coordinates. What's interesting is the long term, representing some kind of wave moving on the $x$ axis. I assume you know the pythagorean theorem $ds^2 = dx^2 + dy^2$. Well, this just adds the $z$ and $t$ coordinates. The long term defines a coordinate that replaces the $x$. It is a combination of $x$ and $...


4

It doesn't require negative mass but negative vacuum energy density, which presumably he is generating using the Casimir effect.


3

It's very, very highly likely that none of these schemes are workable for the following reasons: Superluminal travel violates causality. You can talk about "general relativity loopholes" all you want, but you can always envision that the "warp region" is going to be confined to a finite-sized subspace of the whole spacetime, and outside of this region, ...


3

False. These devices require a type of matter that has not been shown to exist, and would become a time machine if they did exist -- I could use it to send signals into the past.


3

Level 1 multiverses almost certainly exist, and any FTL drive would be able to move between them. As far as i know this includes an Alcubierre drive, assuming you could build one. So yes, an Alcuierre drive could move between level 1 universes. As far as I know the only even remotely plausible realisation of a level 2 multiverse comes from eternal inflation....


3

Pete, please elaborate your question, the original paper by Miguel describes how one can travel back and to a destination using the warp drive. The space in front is contracted to get closer to the destination, and the space behind is expanded. For getting out of the bubble, let's use the picture below: The bubble is created, so to speak, through the ...


3

Exotic matter is matter with negative energy density, or to be more precise, where the energy density tensor trace invariant is negative. There are no known instances of standalone exotic matter in the current universe, but on the other hand, there is no much that we know about the full matter content of the universe, there is a big amount of dark energy ...


3

By "heat" I'm going to assume you mean electromagnetic radiation, because outside of the interiors of stars and other similarly dense regions, heat cannot be conducted or convected. The answer is simply : No. Dark matter cannot be tiny bits of matter that we cannot see. To help you understand it better, let me explain a bit of history first - Dark matter ...


3

Heat can't just live in the vacuum. Because heat is a form of energy and energy is equivalent to mass, heat has to have a material carrier. The most "neutral" type of heat that you probably meant is the electromagnetic radiation. Dark matter can't be made out of electromagnetic radiation because it would escape, in 100,000 years, away from the galaxy. ...


3

If somehow, it is only possible to create one alcubierre drive, and it can never turn around, then you won't get closed timelike curves. Otherwise, any construction is going to have them. The reason is that you can "zoom out" far enough that the distortions to spacetime caused by the drive are no longer present, and the person flying the drive then just ...


3

We can't say much about closed timelike curves with any certainty; they are an artefact of the existence of solutions to the general relativity equations which allow them. It is possible (and quite a few physicists believe this) that a theory of quantum gravity may preclude CTC from occurring, or that CTC may occur but the information might be censored by ...


3

All the treatments of the Alcubierre drive I've seen have not dealt with the acceleration and deceleration. The nearest I've seen is the paper The Alcubierre Warp Drive: On the Matter of Matter, but this is mainly interested in the interactions of matter with the drive and it doesn't deal with the mechanism of acceleration. You'd have to specify how the ...


3

It isn't obvious to me why there should be any relationship between the Alcubierre Drive and gravitoelectromagnetism. The Alcubierre drive is a valid solution provided you can lay your hands on some exotic matter, but then if you have a stock of exotic matter to hand all sorts of problematic solutions to GR are possible. The conclusion that most of us draw ...


3

The problem with general relativity is it is too general and allows solutions that it would be very difficult to believe are physical. The first problem is that the metric is defined upon a topology. For example the physically extremely prohibitive condition that a spacetime is empty and with zero cosmological constant is not enough to uniquely define a ...


3

You never have to make energy come from nowhere, the fact that the Einstein tensor has zero covariant divergence means that you can write any spacetime and the corresponding stress-energy tensor will have zero covariant divergence. So in short if you write down an arbitrary spacetime, and look at conservation of energy and momentum for the required stress-...


3

The Alcubierre drive is like an escalator. Space expands behind the ship, and this is like the moving stairs coming out of the floor behind you, while space contracts in front of the ship, like the moving stairs disappearing into the floor in front of you. This is not, however, a description of the actual Alcubierre process. We live in de Sitter space, ...


3

Alcubierre [1] and others have shown how a suitable shaping of space-time curvature can be used to generate a warp bubble, where the contents of the bubble can be transported to arbitrarily large speeds, even exceeding the local speed of light, without suffering inertial effects. This can be called an inertialess warp drive. “Abstract: It is shown how, ...


2

If you start out in a flat Minkowski spacetime and begin to distort it to create an Alcubierre bubble, you won't actually be able to travel to anywhere that wasn't in the future light cone of your position immediately before you started work. For example if you have a network of clocks synchronized in the rest frame of the galaxy, and in 2000 you start work ...



Only top voted, non community-wiki answers of a minimum length are eligible