The Alcubierre warp drive metric has been criticized on the points of requiring a large amount of exotic matter with negative energy, and conditions deadly for human travellers inside the bubble. What if the Alcubierre metric is used to just span a tiny bubble around a single spin-1/2 particle to transport either classical or quantum information faster than light? Wouldn't known small-scale effects, such as the Casimir force, suffice to provide the exotic conditions required for an Alcubierre bubble large enough to fit at least one particle? My question explicitly addresses the feasibility of a microscopic bubble using known microscopic effects.
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1$\begingroup$ Exotic matter hasn't been shown to exist. So, a design requiring any amount of it has insurmountable design flaws, at least from a practical point of view. $\endgroup$– Zo the RelativistCommented Apr 1, 2012 at 13:26
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$\begingroup$ @JerrySchirmer: The Casimir effect is sometimes quoted to provide the supposed conditions at a microscopic level. I've updated my question in response. $\endgroup$– blkCommented Apr 1, 2012 at 13:49
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$\begingroup$ @JerrySchirmer, nonsense, if exotic matter could not exist, then we would not be speaking right now, since inflation would not have ocurred. $\endgroup$– lurscherCommented Aug 27, 2012 at 22:46
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5$\begingroup$ @lurscher: inflation is not exotic matter, it doesn't violate the null energy condition. To make a warp drive you need the type of exotic matter which doesn't exist. $\endgroup$– Ron MaimonCommented Aug 28, 2012 at 2:21
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$\begingroup$ Related: physics.stackexchange.com/q/18835/2451 $\endgroup$– Qmechanic ♦Commented Aug 28, 2012 at 11:59
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3 Answers
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The reason why the Alcubierre drive is not feasible as an FTL drive has nothing to do with absence of exotic matter; in fact, we likely had plenty of the required matter during cosmic inflation era, or something with identical geometrical effects. Physicists that dismiss it as impossible because "there is no exotic matter" play ignorance to the fact that inflation era happened, and it cannot be accounted by some magic variation of the cosmological constant, because the cosmological constant is a constant, otherwise you are referring to some scalar field, which is not GR, so dismissing exotic matter (or something equivalent) amounts to dismissing inflation and General Relativity
Now, obviously we don't have any idea about how to create "inflation" fields, but even if we were able to have some way to create some "inflation" field, it would still not help us to create a FTL drive. The reason is that the Alcubierre metric assumes that the field is already distributed over the spacetime at previous times before the tube is formed. Since all this field is arranged over space-like distances, the only way to distribute those fields so that you are able to "surf" over them is by travelling faster-than-light to set them up. So, you need a FTL drive in order to be able to build an Alcubierre drive.
If you want, you can lay out that field over the space beforehand so ships can then use it to travel, but since you are already changing the geometry, you are not travelling FTL anymore, since the metric has been distorted significantly. Also, there are be better ways to construct FTL drives if exotic matter was to be accessible, like wormholes
So in short; no amount of exotic matter will make the Alcubierre metric work as it was intended
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1$\begingroup$ you should define clearly what type of exoticness is required for different things. There is no violation of null energy condition from a positive cosmological constant, since it cancels out on null rays. $\endgroup$ Commented Aug 28, 2012 at 2:22
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$\begingroup$ @RonMaimon, right, the cosmological constant is not exotic (does not violate the null energy condition). The inflation field was exotic though $\endgroup$– lurscherCommented Aug 29, 2012 at 2:32
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5$\begingroup$ The inflaton is not exotic--- the negative stress is just the same as a cosmological constant, and doesn't violate null energy since it cancels on null rays. This is why people use scalars to drive inflation. $\endgroup$ Commented Aug 29, 2012 at 3:14
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$\begingroup$ lurscher, what makes you think that? :"The reason is that the Alcubierre metric assumes that the field is already distributed over the spacetime at previous times before the tube is formed." - As far as I can see, neither Alcubierre's articles nor any other fundamental studies suggest that the distribution of matter along the entire route should be prepared in advance. $\endgroup$ Commented Sep 9, 2022 at 2:17
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The problem is not the availability of exotic matter. It is the manipulation of it in a practical manner that is not known. The stuff of exotic matter (quantum fluctuation) is everywhere. The Casimir Effect is only a demonstration of its existence in a practical manner. To do more with that so called negative energy that the quantum fluctuation can present is the challenge. The basics of the challenge consists of making the negative energy assymetrical. It is akin making the energy of the quantum fluctuations act on only one side of a piece of the more familiar matter we are used to manipulating.
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This can indeed be done, even though instead of encoding information inside the bubble it is easier to exploit the properties of warps interaction with external matter, to capture particles along the warp trajectory and use the emitted particles as probes, encoded as 0-1 bits to perform communication.
See Government-funded Study Explores Warp Drives As Means Of Faster-than-light Communication Through “hyperwaves” for a light reading or the paper:
Hyperwave: Hyper-Fast Communication within General Relativity
Warp-drives are solutions of general relativity widely considered unphysical due to high negative energy requirements. While the majority of the literature has focused on macroscopic solutions towards the goal of interstellar travel, in this work we explore what happens in the small radius limit. In this regime the magnitude of the total negative energy requirements gets smaller than the energy contained in a lightning bolt, more than 70 orders of magnitude less than the original Alcubierre warp drive. Such an amount could conceivably be generated with current technology by scaling up Casimir-like apparatuses. We then describe a tubular distribution of externally-generated negative energy which addresses the major issues plaguing macroscopic warp-drives and propose a concrete mechanism to accelerate and decelerate a warp. A byproduct of warp deceleration is the emission of a ray of high-energy particles. The detection of such particles could be used as the backbone of a faster-than-light communication device, reminiscent of the hyperwave of science fiction, even though significant engineering challenges remain to achieve practical communication.
(disclaimer: arxiv paper author here)