Back in 1994, the Mexican physicist Miguel Alcubierre proposed a way for spacecraft to travel faster than the speed of light. Instead of accelerating the spacecraft across a region of spacetime, his idea was to contract spacetime ahead of it and expand spacetime behind it. In this scenario, the spacecraft sits in a flat bubble of spacetime that experiences little acceleration.
The laws of physics that govern this behavior are Einstein’s field equations for general relativity. And they are fearsomely complex. Nevertheless, Alcubierre came up with solutions that allow this kind of faster-than-light motion in an imagined device now known as an Alcubierre Drive.
This solution came with significant caveats, however. For example, it requires negative matter that can support a negative energy density. The absence of any evidence that negative matter can exist throws some doubt on whether such a device could ever work. Another problem is maintaining such a bubble, which would tend to collapse with unfortunate complications for the spaceship and its crew.
Bubble Mechanics
It is this last problem that has caught the attention of Katy Clough at Queen Mary University of London and colleagues. “There is (to our knowledge) no known equation of state that would maintain the warp drive metric in a stable configuration over time,” they say. Instead, the warp bubble will disperse or collapse to a central point. “This instability gives rise to the possibility of generating gravitational waves,” they point out.
What makes this interesting is that physicists now have detectors that can spot gravitational waves moving across the universe. So Clough and co set themselves the task of understanding such a collapse and what it would look like to a distant observer.
It also raises the, albeit speculative, possibility that we could observe the catastrophic collapse of warp drive bubbles generated by some distant but advanced civilization. “Physically, this could be related to a breakdown in the containment field that the post-warp civilization (presumably) uses to support the warp bubble against collapse,” say the researchers.
Clough and co begin by simulating a warp drive bubble, allowing it to collapse, and then observing its effect on the surrounding spacetime. They have even created videos showing how the warp drive collapses and how this creates gravitational waves that propagate away.
Death Signature
The results reveal the signature of the catastrophic death of a warp drive bubble. The collapse blasts matter into space—both positive and negative matter in this scenario. And it also produces gravitational waves of the kind we know exist. “We see a burst of gravitational-wave radiation leaving the collapsed remnant of the warp bubble,” say the researchers.
That makes this kind of event potentially observable. Unfortunately for physicists, the gravitational waves created by a spaceship-sized warp drive would have a frequency that would be too high to be observed by the current generation of detectors. These can see lower frequency waves generated by much bigger events, such as the collision between black holes and neutron stars. “For a 1km-sized ship, the frequency of the signal is much higher than the range probed by existing detectors, and so current observations cannot constrain the occurrence of such events,” they say.
Of course, Clough and co acknowledge that the idea of detecting advanced civilizations by listening for the echoes of warp drives that have disastrously collapsed is highly speculative. But there’s nothing wrong with good science driven by dreams, even fanciful ones.
Curiously, a properly functioning warp drive would not necessarily produce gravitational waves and so would not be observable in this way. Which means the only other way to confirm their existence would be if their owners drive one here. Now that would be an Earth-shaking event.
Ref: What no one has seen before: gravitational waveforms from warp drive collapse : arxiv.org/abs/2406.02466
