Breaking the 'cosmic speed limit' with the Casimir effect
To make a warp drive, first you need to master the Casimir effect. The Casimir effect, for the uninitiated, is a very small attractive force that exists between two uncharged but conductive parallel plates that are held very, very close together. It used to be a purely hypothetical offshoot of relativity. However, now that we've seen it happen in real life, it's high on the list of real-world phenomena scientists are investigating to figure out how we might finally crack the problem of interstellar space exploration.
The idea of an Alcubierre warp drive rests on the principle that under the right conditions, spacetime itself can contract. That's where the Casimir effect comes in. It would drag a spacecraft forward by contracting space itself in front of the vessel's nose. This would mean that a hypothetical vessel could break the "cosmic speed limit" by arriving at their destination faster than the speed of light, without ever having to accelerate its squishy human passengers to relativistic speeds.If it wasn't a warp bubble, what did they build?
The paper that describes the structure in question is quite dense. It reports an analysis of the distribution of quantum vacuum energy around a spherical bead of silver-plated silicon, fixed in the middle of a cylinder held between two parallel plates. In the abstract, it says that a "micro/nano-scale structure has been discovered that predicts negative energy density distribution that closely matches requirements for the Alcubierre metric." Later, it more closely characterizes its specific target as the "predicted toroidal Casimir energy density distribution for [a] sphere-cylinder system comprised of a 1 µm diameter sphere suspended in the middle of a 4 µm diameter cylinder."
Fig. 1 (above) shows a couple extreme close-ups of parallel-plate structures proposed for use in studying the Casimir effect. (Image: study authors.)
Superluminal travel is an attractive goal, to be sure. But the cost is steep. To move faster than light, ships like the USS Enterprise have to open up a warp bubble around themselves. They do this by applying amounts of energy that require things like dilithium and antimatter. Here on Terra Prime, the best of human understanding still requires exotic matter to create the conditions for a warp bubble.
There's no such thing as a free lunch. But scientists have learned how to produce the Casimir effect without having to resort to exotic matter, and if we want to make space itself contract, then we'd best figure out everything we can about spontaneous attractive forces in nature.'We have not built a real warp bubble in the lab'
The study was conjectural, and in the paper(Opens in a new window) they explicitly acknowledge it as such. Nevertheless, several initial write-ups of the paper framed things incorrectly. This resulted in various articles claiming the new discovery represented "the creation of an actual, real-world warp bubble." The quote below is representative of the initial confusion:"To be clear, our finding is not a warp bubble analog, it is a real, albeit humble and tiny, warp bubble," White told The Debrief(Opens in a new window), quickly dispensing with the notion that this is anything other than the creation of an actual, real-world warp bubble. "Hence the significance."But this statement is not supported by the paper, as Dr. White made plain when we contacted him for an interview. "One thing I want to make sure I'm very clear on here," he explained to ExtremeTech, "is that we did some numerical analysis to identify a real structure that one could manufacture that is predicted to manifest a real warp bubble. We have not built a real warp bubble in the lab. The whole premise of real and analog is meant in the context of the physics parlance."
Will it blend?
Being an analog of another phenomenon means there's an unavoidable disconnect. Research results may not generalize between the two. This is the same reason that a drug that's "demonstrated to work in mouse models" doesn't always make it out of human clinical testing. Mice, pigs, and monkeys are useful biological analogs to humans, but the overlap isn't perfect.
Fig. 10(b): Notional micro/nano structure depicting an array of sphere-cylinder units, proposed for use studying the Casimir effect. Image: study authors
There are ways of learning about the physics of one thing by looking at another. Dr. White explained that phenomena exist called "acoustic black holes," which behave like black holes in useful ways that we can study, without actually being black holes themselves. Acoustic black holes can trap sound waves within a volume of fluid. It's not unlike the way a black hole traps light waves within a volume of space. Furthermore, acoustic black holes emit a kind of energy analogous to the Hawking radiation emitted by black holes.
Obviously it's less work to make a region of differential fluid flow than it is to make an actual singularity. But the reduced labor comes with a price: accuracy suffers. Because light and sound are so different, only some of the results generalize from one case to the other."Blue-sky" research on Skylake CPUs
The researchers did the modeling of the parallel-plate structure, and their subsequent numerical analysis, using a cluster of 660 Intel Skylake CPUs. But this isn't even the biggest cluster they've used for their research on this topic. "We routinely do analysis today on the custom Casimir cavities as we prepare for some of the empirical work we're doing to measure Casimir force, with some of these custom topologies," said Dr. White, "and we have to spin up clusters of this size on a regular basis."
This work looking into the Casimir effect is one among several other ambitious research projects at White's nonprofit, the Limitless Space Institute (LSI). Dr. White compared the organization's basic "blue-sky" research to modern GPS systems, in that they must account for both relativity and quantum effects in order to produce the highest possible accuracy. Needless to say, that requires an awful lot of computing power.
Even so, he added, "We would like a lot more, because even with a thousand CPUs, we have to kind of hold the details of the model down, to a point, so that it can get results for us."Ambitious alphabet soup: DARPA, NASA, LSI
For a long time, NASA has been synonymous with space exploration. Recently humans have begun to look again toward putting boots on the moon. But that's not the limit of our ambition; we've got satellites and Voyagers and landers and solar probes, and even a plucky little rover that's hanging out on the surface of Mars doing all sorts of science in tandem with its companion drone, trying to scout and prepare the way for humans to finally land there.
LSI works with DARPA's Defense Sciences Office, awards grants and research fellowships, and does its own internal research at NASA's Eagleworks lab. (Eagleworks is the lab that built the EmDrive, an experiment that studied a carefully shaped cavity in hopes that it could capture and direct microwave energy, creating thrust with no propellant.) But the nonprofit has also funded academic research into other exotic ways of getting from A to B. Their research targets include beam-energy propulsion and relativistic solar sails, fusion propulsion, and even traversable wormholes.
Fig. 8 - Plot of York Time. This image shows the expansion and contraction of space associated with the Alcubierre metric. The plot also includes a simple representation of craft overlayed to illustrate the alignment of the exotic matter ring (T00) with the expansion and contraction of space. The flat region at the middle of the York Time plot represents the region of flat spacetime in the middle of the warp bubble where coordinate time is the same as proper time, and proper acceleration α is zero. This would be where sensitive spacecraft systems/instruments and possibly crew would be located.
'Nuclear-electric propulsion gives us the solar system'
Suppose you wanted to do something like send a human being to the rings of Saturn, on a timescale of hundreds of days, comparable to current travel times between here and Mars. We could harness fission, or even fusion, to get us a long way out into the reaches of the outer solar system. "Nuclear-electric propulsion, where you've got a nuclear reactor that's coupled with some form of electric propulsion, Hall thrusters, what have you -- that type of architecture can take human beings to every destination in the solar system," said Dr. White. "It's well explored in the literature; all it requires is the dedication to build and field those systems."
It's an order of magnitude more energy to get a payload to Saturn than it takes to get that same payload to LEO. But the journey's megawatt-scale power demands aren't beyond the reach of nuclear-electric, or even chemical propulsion. As design sizes shrink, we could potentially meet power requirements with a micro-reactor on the scale of the boxcar-sized designs under research elsewhere in the public sector.
We can muster the energy to get to incredibly distant places; what we can't do yet is get there fast enough to be worth the trip. "Nuclear-electric propulsion gives us the solar system," said Dr. White. "But if we want to get to another star system within some fraction of a human lifetime, that's where we need to look on the frontiers of physics."
Note: The Debrief has updated its story(Opens in a new window) since publication to remove the phrase "anything other than the creation of an actual, real-world warp bubble," but the headline still implies that a specific, reproducible phenomenon has been observed / discovered. This is inaccurate. This paper describes a model that predicts a phenomenon. Scientists have yet to observe or create an actual warp bubble, whether in nature or in a lab.
