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An Alcubierre Drive is a hypothetical device that can move a place someplace else faster than the speed of light without violating known laws of physics. This paper provides some equations as to the energy density in such a field, as well as proposals about testing the theory experimentally and finding modes of applications.

One application is described as follows:

the spacecraft departs earth and establishes an initial sub-luminal velocity $v_i$, then initiates the field. When active, the field’s boost acts on the initial velocity as a scalar multiplier resulting in a much higher apparent speed, $v_{eff}= γ v_i$ as measured by either an Earth bound observer or an observer in the bubble. Within the shell thickness of the warp bubble region, the spacecraft never locally breaks the speed of light and the net effect as seen by Earth/ship observers is analogous to watching a film in fast forward. Consider the following to help illustrate the point – assume the spacecraft heads out towards $\alpha$ Centauri and has a conventional propulsion system capable of reaching $0.1c$. The spacecraft initiates a boost field with a value of 100 which acts on the initial velocity resulting in an apparent speed of $10c$. The spacecraft will make it to $\alpha$ Centauri in 0.43 years as measured by an Earth observer and an observer in the flat space-time volume encapsulated by the warp bubble.

A formula to arrive at $\gamma$ is given in the paper. Now ...

  • I assume that the lower limit to create such a "warp bubble" is independent of the method or device or whatever used, there's a hard lower physical limit - is this even correct?

  • Now, as an example, how much energy would be needed to create a boost factor $\gamma$ of 100 for a golf ball (or any other arbitrary sized volume)?

  • Where does this energy go?

While White mentions an experimental setup to test the theory with only a ring of capacitors, others mention exotic matter to be neccessary. I don't want to go into details of implemementation in this question. Since my question is about the physical limits, it's here on Physics SE and not over at Space exploration.

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"without violating known laws of physics": It basically violates all the laws of physics, as Jerry Schirmer rightly points out. The existence of such a phenomenon would change the causal nature of the universe - any experiment could be altered by things warping in from spacelike-separated regions or even the future, meaning all predictability and indeed all science as we know it just stops working. –  Chris White Aug 6 '13 at 20:25
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It's very, very highly likely that none of these schemes are workable for the following reasons:

  1. 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, the superluminal observer will just be an ordinary superluminal traveller in special relativity. Have two of them travelling in opposite directions, and they can start communicating with each other out of temporal order
  2. They require exotic matter-- all of these schemes require that you have some negative energy density matter in order to create the stretching behind/contracting ahead effect in spacetime that makes the scheme work. This nasa paper claims that you can do this with Casimir potentials, but there is no real indication that Casimir energies are even negative in the GR sense, or that you can get enough of this energy to:
  3. These schemes require a LOT of exotic matter. It's difficult to bend spacetime on macroscopically significant scales because gravity is weak relative to the other forces. Futhermore, the bending of spaceitme has to be large enough to create an effect, but over a large enough region so that our travellers don't get torn apart by tidal effects. The "new" update of the design reduces the amount of exotic mass required from something like several stars to "just" a Jupiter's worth of exotic matter. Even if it was 1 mg, you still have the issue that exotic matter hasn't been observed, and many GR theorems break (cosmic censorship, causality [see 1.], the singularity theorems, etc.), and relativity ceases to be the strong, predictive theorem that we know.
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+1 for concisely criticising the warp hype (though I have to think about a few things), but that does not answer the question yet. Is my assumption about a calcuable lower limit to energy demand wrong? Is it the case that creating a warp bubble would not require significant energy, but "just" configuring exotic matter in a special way (so my question would be meaningless in the form I asked and this exotic matter may not exist, possibly can't exist due to your point 1.)? –  mart Aug 7 '13 at 6:35
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You'd need to create a jupiter's worth of exotic matter, so the required energy would be something like $m_{\rm jupiter}c^{2}$ –  Jerry Schirmer Aug 7 '13 at 14:44
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