Will future humans use warp drives to explore space? We are not in a position to rule out this possibility. But if our distant descendants ever do, it won’t involve dilithium crystals, and Scottish accents will have evaporated into history by then.
Warp mechanics have their roots in one of the most popular sci-fi franchises ever, but they have a scientific basis. A new paper examines the science behind them, asking whether a warp drive containment failure would emit detectable gravitational waves.
The documentary is called “What No One Has Seen Before: Gravitational Waves from Warp Drive Collapse”. The authors are Katy Clough, Tim Dietrich and Sebastian Khan, physicists from institutions in the UK and Germany.
There is room for warp drives in General Relativity, and Mexican physicist Miguel Alcubierre described how they could theoretically work in 1994. It is known in space and physics circles for its Alcubierre propulsion.
Everyone knows that no object can travel faster than the speed of light. But warp units can offer a solution. By warping space-time itself, a warp drive spacecraft would not violate the faster-than-light (FTL) rule.
“Despite their origins in science fiction, warp drives have a concrete description in general relativity, with Alcubierre first proposing a spacetime metric that supported faster-than-light travel,” the authors write.
There are clear scientific obstacles to actually creating a warp drive. But it is possible to simulate how one would work and how they might be detectable via gravitational waves in the event of a failure. Warp drives warp spacetime itself, as do binary mergers of compact objects such as black holes and neutron stars. It is theoretically possible that they emit a gravitational wave signal in the same vein as fusion. “To search for such signals and correctly identify them in measured data, it is important to understand their phenomenology and properties,” the authors explain.
It starts with understanding how warp drives can work, so we need to dive deep into the physics.
“The basic idea behind warp drive is that instead of exceeding the speed of light directly in the local frame of reference, which would violate Lorentz invariance, a ‘warp bubble’ could travel distances faster than the speed of light (as measured by some distant observer) by contracting space-time in front of it and expands the space-time behind it,” the paper states.
The first hurdle is that warp drives require a zero energy state (NEC). Physics says that a region of space cannot have a negative energy density. There are theoretical solutions for this, but none of them are practical for now.
“Other issues with warp drive metrics include the potential for closed time-like curves and, more practically, difficulties for those on board in controlling and deactivating the bubble,” the authors explain. The crew would not be able to send signals to the front of the ship. As this article explains, it is difficult for events inside the warp bubble to affect events outside the warp bubble.
“From a dynamic warp drive simulation perspective, stability is a key challenge,” the authors explain. The equations show that the Alcubierre Drive can initiate a warp bubble using Einstein’s equation, but no known equation can sustain it. “There is (to our knowledge) no equation of state that keeps the warp drive metric in a stable configuration over time. Therefore, while the warp bubble may initially be required to be constant, it will quickly evolve from this state, and in most cases the deformation of the warp fluid and spacetime will dissipate or collapse to a central point.”
While instability is the main obstacle to warp drives, it is also what could make them detectable. If the Alcubierre drive reaches a constant speed, it is not detectable. It generates no gravitational waves and has no ADM mass. ADM stands for Arnowitt-Deser-Misner, named after three physicists. I will leave it to curious readers to read more about the ADM Mass.
But warp drive is only undetectable if it is constant and stable. Once it breaks down, speeds up or slows down, it could be detectable. The authors deflate the warp drive bubble in their work. “Physically, this could be related to a fault in the containment field that the post-warp civilization (presumably) uses to support the warp bubble against collapse,” they write.
In their formulations, the nature of the ship itself is not important. Only the warp bubble and the warp fluid inside are significant.
Scientists simulated the collapse of a warp bubble. They found that the collapse generated gravitational waves with characteristics different from those generated by fusion. “The signal arrives as a burst, initially devoid of gravitational wave content, followed by an oscillating period with a characteristic frequency of the order of 1/[R],” they write. “Overall, the signal is very different from typical compact binary coalescences observed by gravitational wave detectors and more akin to events such as the collapse of an unstable neutron star or the head-on collision of two black holes.”
The authors point out that although the warp drive produces a GW signal, it is outside the frequency range of our current ground-based detectors. “Proposals have been made for detectors with a higher frequency, so that it will be possible to limit the existence of such signals in the future,” they write.
The ship itself could also emit some type of multimessenger signal, but it’s hard to know how the ship’s matter would interact with regular matter. “Because we don’t know the type of matter used to construct the warp ship, we don’t know if it would interact (besides gravity) with normal matter as it travels through space,” the researchers explain.
This is a fun thought experiment. It is possible that one day in the distant future there will be some type of solution for FTL travel. If so, it may be related to a better understanding of dark matter and dark energy. If any ETIs exist, they may be able to use basic knowledge of the universe that we don’t yet have.
If they could figure out how to construct and use a warp drive, despite all its apparent impossibilities, their activities could create gravitational waves that our future observatories could detect in other galaxies. But for now it’s all theoretical.
“We note that the obtained waveforms are likely to be highly specific to the model used, which has several known theoretical problems as discussed in the Introduction,” the authors write in their conclusion. “Further work would be needed to understand how general the signatures are and to properly characterize their detectability.”
No doubt some curious physicists will continue to work on it.