Will humans of the future use warp drives to explore the cosmos? We are unable to eliminate this possibility. But if our distant descendants ever do it, it won’t involve dilithium crystals, and Scottish accents will have faded into history.
Warp Readers has its roots in one of the most popular science fiction franchises of all time, but it has a scientific basis. A new paper examines the science behind them and questions whether a failure of the warp engine’s containment would emit detectable gravitational waves.
The article is titled “What No One Has Seen Before: Gravitational Waveforms from Warp Engine Collapse.” The authors are Katy Clough, Tim Dietrich and Sebastian Khan, physicists from institutions in the United Kingdom and Germany.
There is room for warp engines in general relativity, and Mexican physicist Miguel Alcubierre described how they could theoretically work in 1994. He is well known in space and physics circles for his Alcubierre engine.
Everyone knows that no object can travel faster than the speed of light. But warp drives can offer a workaround. By warping spacetime itself, a spaceship with a warp drive would not break the faster-than-light (FTL) rule.
“Although they originate in science fiction, warp drives have a concrete description in general relativity, with Alcubierre having been the first to propose a space-time metric supporting faster-than-light travel,” write authors.
There are obvious scientific obstacles to creating a warp drive. But it is possible to simulate how they work and how they could be detected via gravitational waves in the event of a failure. Warp engines warp spacetime itself, as do binary mergers of compact objects like black holes and neutron stars. It is theoretically possible that they emit a gravitational wave signal in the same spirit as mergers. “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 engines might work, and for that we need to dig deeper into the physics.
“The main idea behind a warp engine is that instead of exceeding the speed of light directly in a local reference frame, which would violate Lorentz invariance, a “warp bubble” could travel distances further faster than the speed of light (as measured by a certain distance) by contracting the space-time in front of it and expanding the space-time behind it,” the paper states.
The first hurdle is that warp engines require a zero energy condition (NEC). Physics states that a region of space cannot have a negative energy density. There are theoretical solutions to this problem, but as of yet, none of them are practical.
“Other issues with the warp metric include the potential for closed time curves and, from a more practical point of view, difficulties for people on board the ship to control and deactivate the bubble,” the authors explain. This is because the crew would have no way of sending signals to the front of the ship. It is difficult for events inside the bubble to influence events outside the warp bubble, as this article explains.
“From the perspective of dynamic simulation of warp drive, the main challenge is stability,” 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 known equation of state that would maintain the warp engine metric in a stable configuration over time. Therefore, although we can require that the warp bubble initially be constant, it will quickly evolve out of this state and, in most cases, the warp fluid and spacetime warps will will disperse or collapse into a central point.
Although instability is a major obstacle to warp drives, it is also what could make them detectable. If an Alcubierre Drive reaches a constant speed, it is not detectable. It does not generate any gravitational waves and has no ADM mass. ADM stands for Arnowitt-Deser-Misner, named after three physicists. I leave it to curious readers to find out more about the ADM mass.
But distortion is only undetectable if it is constant and stable. Once it breaks down, accelerates or decelerates, it may be detectable. In their work, the authors let the bubble of distortion collapse. “Physically, this could be related to a breakdown of the containment field that the post-warp civilization uses (presumably) to protect the warp bubble from 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.
The researchers simulated the rupture of the warp bubble. They found that the collapse generated gravitational waves with different characteristics than those generated by mergers. “The signal appears as a burst, initially having no gravitational wave content, followed by an oscillatory period with a characteristic frequency of order 1/(R),” they write. “Overall, the signal is very distinct from the typical compact binary coalescences observed by gravitational wave detectors and more closely resembles 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 engine creates a GW signal, it is outside the frequency range of our current ground detectors. “Proposals for higher frequency detectors have been made, so in the future one may be able to set limits on the existence of such signals,” they write.
The ship itself could also send some type of multimessenger signal, but it is unclear how the ship’s matter would interact with ordinary matter. “As we do not know the type of matter used to construct the warp ship, we do not know whether it would interact (except gravitationally) with normal matter as it propagates through the Universe,” the researchers explain.
This is a fun thought experiment. It’s possible that some type of workaround to FTL travel will exist one day in the distant future. If so, it could be linked to a better understanding of dark matter and dark energy. If ETIs exist, they could be able to exploit fundamental knowledge about the Universe that we do not yet possess.
If they could build and use a warp drive, even with all its apparent impossibilities, their activities could create gravitational waves that our future observatories could detect, even in other galaxies. But for now, everything is theoretical.
“We caution that the resulting waveforms are likely very specific to the model used, which has several known theoretical problems, as noted in the introduction,” the authors write in their conclusion. “Further work would be needed to understand how generic the signatures are and properly characterize their detectability.”
There is no doubt that some curious physicists will continue to work on this.