Like on a scale of one to ten, how plausible are Alcubierre drives in terms of our current understanding of physics? Plausible enough to be considered "hard sci-fi", or are they just another form of handwave like hyperspace?
This isn't asking about how the science behind the Alcubierre drive works (except as needed for other parts of an answer). Rather, it is about whether and how an Alcubierre drive can work in a hard-science level fiction work.
The only real issue with Alcubierre drives is that they require negative matter to work. Negative matter (NOT antimatter, which is very real) is a hypothetical substance that generates negative gravity; i.e. that pushes matter away from it. In the "rubber sheet" analogy, where gravity is depicted as depressions in the fabric of space-time, negative matter creates "bumps" instead.
While mathematically possible under the geometry of special relativity, negative matter has never been observed, nor is there any place in the Standard Model of particle physics where it is expected to occur. So the big issue is, is negative matter possible? If so, an Alcubierre drive is simply a question of energy and engineering. If not, the drive is not possible.
Alcubierre drives are maybe around a 7.5 in sci-fi hardness - not a completely magical hand-wave but still requires an imaginary particle to work. They are more plausible than wormholes (which also require negative matter, as well as the need to 'tear' space-time) and less plausible than Dyson spheres (which require no imaginary physics, only scale and energy).
They are one of the least hand-wavey methods of FTL travel, so if your story needs FTL travel while remaining as plausible as possible an Alcubierre drive is probably the way to go.
The Alcubierre drive is theoretically plausible. It is a mathematically correct solution of the Einstein field equations. Its main problem is that the warped path through spacetime for its trajectory has to be set up before it travels. This makes it more like a FTL railway train than an independent vehicle like a spacecraft.
Segei Krashnikov, another theoretician, was dubious about the effectiveness of the Alcubierre metric and he proposed his own solution the Krashnikov. A spaceship travelling to Alpha Centauri creates the warped spacetime path as it travels to Alpha Centauri at sublight speed. On its return journey the spaceship travels along the Krashnikov tube arriving home faster than if it had travelled at less than lightspeed. Effectively that's faster than lightspeed, but only for the return leg of its round trip.
This example is to illustrate the fact that while the Alcubierre drive is theoretically plausible and mathematically correct, not every scientist working in general relativity accepts the concept.
There is also the exotic matter issue, because it requires negative energy densities to keep open the warp bubble. There are problems about the possible of causality violation, because if Alcubierre drive vessels travel first one way and come back again this allows trip into its own past. OK, this can be regarded as a bonus. With FTL travel, you get time-travel as a free accessory. Still it's a worry for those who hold causality dear.
If a writer was using an Alcubierre drive for their FTL spaceships it is scientifically plausible and can be considered as almost a hard-science concept.
There is a weak sense in which the Alcubierre geometry is a "solution" of general relativity. But anything is a "solution" of general relativity in this same weak sense. General relativity says that the matter distribution in a spacetime is related to the geometry of the spacetime in a certain way. You can take any spacetime geometry (as long as it's twice differentiable), plug it into that equation, get a matter distribution, and then say "if only we could make this matter distribution happen, we could make this spacetime geometry happen", with exactly the same plausibility as the Alcubierre geometry.
For example, suppose you want the Sun to suddenly disappear. I don't mean that it accelerates away, or blows up, or anything like that. It just disappears. Its gravitational field goes to zero, planets fly away in a straight line, etc.
It's easy to do this. You just take a spacetime metric with the Sun in it (the Schwarzschild interior and exterior solutions stitched together), and a spacetime metric without the Sun (Minkowski space), interpolate between them with a smooth (or at least twice differentiable) function of time, and plug that into the Einstein equation. The result will not make much sense: you'll find that the Sun's mass flows out to infinity through a region with zero mass density. Exotic matter to the rescue! By introducing negative-mass matter, you can counter the mass of the outflowing matter and make the total mass density zero in the region where general relativity says it has to be zero.
It's important to understand that this matter isn't "exotic" just in having negative mass. It's exotic in that it doesn't follow any physical laws. It just shows up out of nowhere (literally, out of vacuum) during the disappearance of the sun, then disappears into nowhere. It isn't even subject to cause and effect, much less any more specific physical theory.
The same is true of the exotic matter in Alcubierre's spacetime. It is not a "warp drive", because that would imply that the exotic matter could come from the spaceship that's going to ride the bubble. It can't, because the exotic matter on the outside of the bubble is spacelike. That means that either it travels locally outside the light cone, or it arises independently everywhere along the path. The first case would make the solution uninteresting, since if you can travel outside the light cone then you don't need a general-relativistic warp drive to circumvent light speed. So the exotic matter can't come from the ship. In Alcubierre's geometry, it appears miraculously out of vacuum just before the ship arrives, then disappears into vacuum after it leaves. General relativity is perfectly fine with this. As soon as you add any additional physical laws, it's ruled out.
People have speculated about exotic matter guns that could pre-distribute exotic matter with the right properties. You could likewise speculate about a Sun-vanishing gun. The argument in both cases is "Well, based on everything that we think we know about the world, this is impossible. But if you ignore some of that, the remaining premises aren't strong enough to prove it's impossible any more. So maybe it's possible!" Yes, maybe. All science is subject to revision. But based on everything we know right now, the Alcubierre drive is as scientific as time travel. (In fact, it could be used for time travel.)
The Alcubierre Drive is indeed possible from a scientific point of view. Exotic matter is not hypothetical, it has being observed, but is just very difficult to produce and it is almost impossible for us, right now at least, to make it in amounts large enough. Also, Alcubierre proposed that Casimir vacuum might work instead of exotic matter, for that matter (no pun intended). Whether it violates causality or not that was debated somewhere else.
To me, the real issue is more about practicality. The Alcubierre Drive generates a lot of practical problems, apart from costs. For example:
But any talented sci-fi writer can find clever ways to overcome these issues.
Another thing that most writers overlook is that most of the problems I mentioned do not happen in subluminal speeds, and that makes the Alcubierre Drive interesting for a non-FTL setting becoming the by far fastest way to travel, for example, inside the Solar System.
Alcubierre drive is a sound way, under the current laws of physics, to explain a valid way of travel faster-than-light and the only way (unless wormholes are ever observed) to “break” the speed of light. However, and this is a big however, what makes AD improbable or implausible for our current civilization is how to make it work. For it we would need whether exotic matter with negative mass (which still hypothetical) or harnessing somehow the Casimir effect which causes negative mass. Both things are not impossible, but we still don’t know how to do either yet. So, maybe tomorrow someone would find the way or maybe never.
So, if for example you’re working on science fiction book and wanted it to be hard sci-fi, the “hardish” way to make FTL is with the AD, but for a lot of people even in that case is then no longer “hard” sci-fi (but this is kind of subjective).
It is impossible, by any means, to send information from point A to point B faster than the speed of light in a vacuum. I will give you a simple proof of this:
Einstein constructed Special Relativity using thought experiments that involved clocks positioned at various points in space and information flowing between these clocks. The information flowing between these clocks has to be the fastest possible speed at which information can be communicated, which happens to be the speed of light in a vacuum. If there is a faster max speed by which to communicate information between clocks in space then Einstein would have had to use that faster speed. So for example, if the max speed is 2c instead of c then Einstein would have had to use 2c in his relativistic equation instead of c, otherwise the relativistic equations would produce incorrect results. So let's take the equation E = MC^2. This equation has been experimentally verified to many decimal places https://news.mit.edu/2005/emc2. It uses the speed of light. Therefore the speed of light must be the fastest possible speed to transmit information. If the max speed is 2c then the equation would have to be rewritten as E = 4MC^2.
Now you might say that an Alcubierre drive does not operate in normal space and therefore is exempt from the light speed limitation. But this is not so. It doesn't matter how information is transferred between points A and B in normal space. You could put an Alcubierre drive between points A and B in normal space, A spaceship will start from point A, use an Alcubierre drive after it has left point A and then shutoff the Alcubierre drive and reenter normal space before it reaches point B. In doing so it will outrace a beam of light that started at point A and headed towards point B. Einstein could have used this Alcubierre drive in his thought experiments.
So the fact that the equation E = MC^2, and all the other relativistic equations that use the speed of light, produces correct results proves that the speed of light IS the fastest possible speed at which to transmit information (and astronauts) in space.
There is something that is moving/expanding faster than the speed of light. Space. The expansion of space using the measure of redshift in order to calculate relative speed in reference to us, the observer shows that indeed, some of the farthest astronomical objects billions of light years away are moving away from us faster than light.
The reason this does not violate Einstein's special relativity is because, in between the two objects(us and a distant astronomical object) new space is appearing.
– Sync Jul 02 '22 at 01:14Sorry, this is going to be more of a rant against the concept of "hard sci-fi" than a strict answer to your question, but asking people now in the 21st century to guess at the viability of the Alcubierre drive would be like asking someone from Da Vinci's time how plausible it would be for a helicopter to ever really be built. They, and we, simply do not have sufficient information to answer the question. In fact, the only thing we can be certain of is that we can be certain that we can't be certain.
It depends on your definition of "plausible". I would take it to mean anything not disproven by established fact or available evidence. There is no need for a working demonstration test article to exist just to make a claim for plausibility.
Consider a few established realities of our modern world:
and how plausible any of that would have been to a Jules Verne era audience.
Consider also that NASA has taken the Alcubierre drive seriously enough to fund related projects at their Advanced Propulsion Physics Laboratory.
On the other hand, Star Trek style artificial gravity, superluminal ("subspace") communication, force fields, phasers and Vulcan telepathy are all far less plausible. Nothing in our current understanding of physics permits the existence of any of them. Although teleportation (a "transporter" in Trek-speak) is theoretically possible and even demonstrated to a limited degree at a quantum mechanical level, it is nevertheless implausible on a human scale because of all the difficulties which arise. Without a whole new understanding of physics, none of this belongs in hard sci-fi.
What may have been seen as totally implausible a century and a half ago is now part of our everyday world. Who could have predicted the wireless revolution or the internet at a time when there was no such thing as radio, telephone, or anything more computationally sophisticated than a mechanical adding machine?
Even if it turns out that an Alcubierre drive is impossible to build/operate, it has passed the threshold of plausibility not just in hard sci-fi but even respectable science because:
antitelephoneon Physics SE. – user Aug 01 '16 at 20:33