Let's start with first the form of the Alcubierre drive described in the gr-qc/9906050v4
paper:
C.V.D. Broeck wrote:By making use of the QI, Ford and Pfenning [3] were able to show that a warp drive with a macroscopically large bubble must contain an unphysically large amount of negative energy. This is because the QI restricts the bubble wall to be very thin, and for a macroscopic bubble the energy is roughly proportional to R^2/Δ, where R is a measure for the bubble radius and Δ for its wall thickness. It was shown that a bubble with a radius of 100 meters would require a total negative energy of at least
E =~ −6.2 × 10^62 v_s kg, (2)
which, for v_s =~ 1, is ten orders of magnitude bigger than the total positive mass of the entire visible Universe.
In [6], it was shown that this number is very much dependent on the details of the geometry. The total energy can be reduced dramatically by keeping the surface area of the warp bubble itself microscopically small, while at the same time expanding the spatial volume inside the bubble.
From here.
For the sci-fi application of this, get a manned, macroscopic ship of meters in length into a region of spacetime separate from the rest of the universe, somewhat analogous to a "pocket universe."
Do that without shredding the ship's structure and crew in the process, despite the neck connecting the bubble to regular spacetime being of subatomic diameter. Such would be a cool magic-like ability to practically manipulate reality like that, creating the bubble and putting a spaceship into such unharmed ... somehow doing so with real-world equipment and real-world materials.
Papers studying far-out ideas like the Alcubierre drive are beneficial just in case it leads to advancement of science. But the common sci-fi idea of the Alcubierre drive idea being workable with macroscopic, manned spaceships doesn't have good engineering plausibility.
C.V.D. Broeck wrote:Using this scheme, the required total energy can be reduced to stellar magnitude, in such a way that the QI is satisfied.
From here.
A stellar magnitude of mass-energy is better than billions of times all the energy in the universe, but it is still a large quantity. Let's use the sun as an example.
The sun's power output is equivalent to a few million tons of matter being converted to energy per second, a very small fraction of its total mass cumulatively over the millions of billions of seconds in its multi-billion-year lifespan. That's nothing compared to what one is talking about here for the mass-energy equivalent of the sun's mass. The sun's total mass
that is 1.99E30 kilograms has an energy equivalence of 1.8E47 joules (E = MC^2). For perspective, that's millions of times more than the energy released by the entire Milky Way galaxy in a hour,
since its power output is on the order of 3E36 watts.
And the preceding is not to be positive energy but negative energy.
C.V.D. Broeck wrote:On the other hand, the energy densities are still unreasonably large, and the spacetime has structure with sizes only a few orders of magnitude above the Planck scale.
From here.
It is important to observe the significance of that much energy being involved in creating a structure, a bubble diameter, almost at the Planck scale.
The Planck scale corresponds to the Planck length of ~1.6E-35 meters, e.g. billions of quadrillions of times smaller than a single atom. One is talking about the astronomical amount of energy previously described while expecting that much "negative energy" to be somehow packed into a volume vastly tinier than a quark.
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Now, let's look at another form of the Alcubierre drive idea, attempting for less mass-energy:
The new metric of the Van Den Broeck/Alcubierre warp bubble is like a bulls-eye target with a center (Region 1) surrounded by three concentric rings (Regions 2-4).
[...]
Van Den Broeck makes the radius of Region 1 about 100 meters, and sets to 10^34, so that Region 4 is only about 3 x 10^-32 meters in radius. With such a small radius, if the warp bubble travels at 10 times the velocity of light the amount of negative mass-energy it would require is only about –0.06 grams. Even if it travels at 100 times the velocity of light, it would require is only about –56 kilograms of negative mass-energy.
[...]
First, although the interior of the warp bubble may be quite spacious, its exterior is only 3 x 10^-32 meters in radius, mush smaller than a proton and approaching the Planck length (1.62 x 10^-35 meters) in size. This is close enough to the minimum length-scale of the universe that such a size reduction is doubtful due to quantum effects.
[...]
Van Den Broeck’s warp drive is a large volume of flat space that is connected to normal space by a tiny "neck". It therefore resembles the more familiar general relativity topologies of wormholes or "baby universes" and perhaps has a similar behavior. This raises the issue of how the neck is prevented from pinching off altogether, isolating our space travelers in a new universe of their own rather than transporting them to a new part of the old one.
From here.
Observe the similarity mentioned at the end between the ability to do this and the hypothetical ability to create new universes.
Anyway, in this example, a quantity of 0.06 grams of negative mass-energy (somehow created and concentrated) is to be involved with a volume a mere 3E-32 meters in radius, e.g. on the order of 1.1E-94 m^3 volume. Since 0.06 grams mass-energy equates to 5.4E12 joules, that makes the ratio of the negative mass-energy to the volume be on the order of 4.8E106 J/m^3.
The magnitude of 5 * 10^106 J/m^3 is a quantity that barely fits description. The energy density of the energy released by an exploding nuclear bomb can be on the order of 1E16 J/m^3 (plus or minus an order of magnitude or two), but that's nothing in comparison. This is so many quadrillions of quintillions of septdecillions of times greater concentration of energy, except it is to be negative energy...
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Incidentally, it wouldn't help plausibility much to try to have the negative mass-energy in the form of negative mass as opposed to pure negative energy.
Negative mass is a more far-out hypothetical substance than some readers may be aware. It is what happens if one looks at mathematical equations for the interactions of real-world matter and arbitrarily changes the sign of some quantities, while wondering if such could exist in the real-world. Here's one description:
Negative matter is a hypothetical form of matter whose active-gravitational, passive gravitational, inertial, and rest masses are oppisite in sign to normal, positive matter. Negative matter is not antimatter (which has a positive mass). If an object made of negative matter could be obtained and coupled by elastic, gravitational, or electromagnetic forces to an object containing an equal amount of positive matter, the interaction between the two objects would result in an unlimited amount of unidirectional acceleration of the combination without the requirement for an energy source or reaction mass. This would not violate the Newtonian laws or General relativity.
From here.
Almost anything can be imagined, but "negative matter" as discussed previously is not exactly experimentally supported...
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For likely plausibility, even papers filled with formulas are only as accurate as the assumptions before the math. The papers fundamentally assume the ability to create ("set") what negative matter/energy densities they desire on the scale they desire. There's not an explanation how to do that, not remotely on the level of engineering analysis.
Research is always good, just in case such someday leads to progress.
However, the difference between these papers and a relatively plausible engineering concept is vast. It is a little like the difference between mathematically calculating that a hypothetical material with arbitrary 10^XY V/m dielectric strength would nominally make a capacitor storing energy far more energy per volume than a nuclear bomb, if one chooses the X and Y one wants, versus determining whether one can approach within orders of magnitude of that with real-world materials and complications. So far, the demonstrated plausibility of Alcubierre drives ranks up with hand-held planet-killer devices, as what orders of magnitude are involved matters a lot for plausibility.
The physical limits of equipment built from real-world molecular materials may apply even after eons of technological advancement.
Possibly, possibly the Alcubierre drive's energy densities like the negative energy to bubble exterior volume ratio of ~1E106 J/m^3 may be somehow obtainable in a non-apparent manner (e.g. something weird with mini black holes or the like) ... but the papers do not suggest that. They just show what is possible according to some physics assumptions, if one assumes that one can create more or less whatever negative energy density is desired, on a subatomic scale. They provide no answer to how one performs the space-time modification with equipment made of atoms so many orders of magnitude larger, nor to the other practical engineering questions.
In some ways, this reminds one of the unlikely
femototech idea of the Orion's Arm website, but here one is talking about somehow creating, manipulating, and sustaining far-out space-time manipulation on a scale even more orders of magnitude smaller than a single proton.
But let's ignore that for the moment, and consider the negative energy.
In a way, "negative energy" can be created today. To utilize the Casimir effect, put two plates of matter close together with a narrow vacuum in between
them, and some wavelengths of electromagnetic waves otherwise in regular vacuum are excluded. The region obtains what is called negative energy density because it has less energy density than regular vacuum, and regular vacuum is described as zero energy density by definition.
However, such isn't very applicable to the preceding, not for such astronomical energy density, not for working on a scale many orders of magnitude smaller than a proton. Real-world materials have atoms with spacing on the nanometer scale, able to create only a very weak effect with the smoothest, most-closely-spaced plates.
There is a different paper that does make an attempt to come up with methods to generate substantial negative energy, like some hypothetical laser techniques. It is actually about hypothetical wormholes rather than Alcubierre drives but still somewhat relevant. It is
here. However, it still is awfully vague and doesn't suggest anything approaching the Alcubierre drive negative energy density requirements discussed previously.
There's another aspect to this, even in regard to the basic idea working in the first place. It is deviating vastly, vastly from physics actually directly explored with experimentation. If one looks at the history of science, a fundamental trend is the great body of past experimental evidence is never disproven. Sometimes a particular assumption of a theory not directly proven with experiments will turn out to be not quite universally right, but the body of experimental evidence remains true. Even Newton's laws had to be modified by relativity to a noticeable degree only when eventually tested at many orders of magnitude different speeds from the many tests that verified relationships like P = MV, KE = 0.5 M*V^2, and so on. However, for this, there are assumptions but not exactly any experiment directly providing favorable evidence.
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Considering the preceding, there doesn't seem to be much benefit in a story using the Alcubierre drive in particular since its plausibility is so miniscule.
If a story must have FTL, one might as well adopt a soft sci-fi approach to the technology, a "show, don't tell" approach, and avoid giving drive details unnecessarily, as implausible technobabble wouldn't help. Vagueness is better in that case.
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If instead the goal was fully hard sci-fi, that does require no FTL. However, STL doesn't have to be totally restrictive of a story.
One doesn't strictly need a galactic civilization. Unless one is writing about a large portion of the ~ 0.4 trillion star systems within a galaxy
like the Milky Way, a mere billionth of a galaxy is enough star systems for almost any story.
For example, while sci-fi may depict "galactic" civilizations with their most major fleet battles involving anywhere from a handful of ships to a few thousand ships, that's really far less than the industrial potential of a single star system, let alone hundreds of billions of star systems. Probably the story doesn't require quintillions of ships in space battles and doesn't require a galactic civilization.
A single star system has on the order of around a trillion trillion tons (~ 1E27 kg) of usable
material, trillions of times more than the historical metal production of earth.
There probably wouldn't be enough pages in a story to have description of events on more than dozens of star systems, at least certainly not the hundreds of billions of star systems in a galaxy.
A star cluster is enough for a vast civilization.
Some star clusters have much more density than earth's stellar neighborhood, such as on the order of a star per cubic light-year. Using that as a random illustration, that would be around 300 stars within a 4 light-year radius, not excessively far apart even with slower-than-light transport and no FTL. Such is more than enough for a vast interstellar society.
For perspective, in the Star Wars galaxy, the total number of worlds directly seen in the six movies is 13, counting large moons (Yavin IV and Endor). They may be described as a galactic civilization, but one doesn't see trillions of worlds, not that a story could show that many. They have a few million planets according to canon, but even a hundred or a thousand worlds is plenty for a story in comparison to the preceding 13 worlds depicted onscreen.
Even one star system like the solar system can naturally contain quite a number of large bodies, a number of worlds. Moons should sometimes be counted, such as Saturn's moon Titan of 5150-km
diameter that is larger than the
diameter of the planet Mercury, particularly when they can be utilized much, such as terraformed. In fact, the solar system contains 25
major bodies of 1000-km and greater diameter, not counting whatever number exists farther out in the Kuiper Belt and Oort Cloud ... probably quite a high number if
an estimated 35000+ bodies of 100-km and greater diameter out there is any guide.
The preceding was for natural bodies. Under some assumptions like a past civilization before the era of concern, much more is possible. For example, the trillion trillion tons of
suitable material in and around a star system like the solar system is enough for artificial worlds with ground meters to tens of meters thick having a total area on the order of 10 to 100 quadrillion square kilometers, at ~ 10-100 tons/m^2.
Since under a million square kilometers is enough for a substantial artificial world, that would correspond to up to on the order of 10 to 100+ billion artificial worlds per star system.
So the 4-light-year-radius cluster of 300 star systems can contain effectively up to trillions of different worlds, up to many quintillions of population if such is desired.
Even aside from the possibilities in artificial worlds like the preceding, potentially tens of natural bodies of 1000+ km diameter per star system would alone correspond to thousands of worlds in the 4 light-year radius cluster.
With no FTL, travel time all the way across the star cluster is a few years, perhaps subjectively mostly equivalent to a few months with periodic hibernation, life extension, or the like. The net effect could be an interstellar society not vastly less cohesive than the British Empire of the 18th century, which took months to travel between some parts. STL travel between star systems can potentially sometimes be as little as months since some stars would be closer to each other than the average. An analogy is the Alpha Centauri
star system where Alpha Centauri C is 0.2 light-years from the other two stars in the triple star system that are close together. Between worlds around a particular star in the cluster, transit time can be either hours, days, weeks, or months depending upon the destination and the speed of the transport ship.
Of course, the preceding is only one example out of many possibilities, but the general idea is that a lot is possible without unrealistic FTL, if the goal is fully hard sci-fi.
Steering. As I understandit, the warping of space-time should basically act the way a sci-fi shield or cloak should - nothing would get through it. This is why some suggest it be a prelaid course. But could it not also be controlled internally simply based off timing the creation and release of the warp, given that the orbital physics of the galaxy are already well known, and the velocity and time of the ship would be known?
