Warp drives can generate gravitational waves

Will future humans use warp drives to explore the cosmos? 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 the Scottish accents will have faded into history by then.

Warp drives have their roots in one of the most popular science fiction franchises ever, but they do have a scientific basis. A new paper explores the science behind it and questions whether a warp drive malfunction would emit detectable gravitational waves.

The article is titled “What No One Has Seen Before: Gravitational Waveforms from Warp Drive Collapse,” and is posted on the Internet. arXiv preprint server. The authors are Katy Clough, Tim Dietrich and Sebastian Khan, physicists from institutions in Britain and Germany.

There is room for warp drives in general relativity, and Mexican physicist Miguel Alcubierre described in 1994 how they could theoretically work. He is known in space and physics circles for his Alcubierre Drive.

Everyone knows that no object can travel faster than the speed of light. But warp drives could provide a solution. By warping spacetime itself, a warp-drive spacecraft would not break the faster-than-light (FTL) rule.

“Despite having 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 barriers to actually creating a warp drive. But it is possible to simulate how they would work and how they could be detected via gravitational waves in the event of a disturbance.

Warp drives warp spacetime itself, just like 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 vein as mergers.

“To look for such signals and correctly identify them in the measured data, it is important to understand their phenomenology and properties,” the authors explain.

It starts with understanding how warp drives might work, and for that we need to delve deep into the physics.

“The main idea behind a warp drive is that instead of exceeding the speed of light directly in a 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 the spacetime in front of it and expanding the spacetime behind it,” the paper said.

The first barrier is that warp drives require a Null Energy Condition (NEC). Physics states that a region of space cannot have negative energy density. There are theoretical solutions for this, but for now none of them are practical.

“Other issues with the warp drive metric include the potential for closed time-like curves and, from a more practical perspective, the difficulties for those on the ship in controlling and deactivating the bell,” the authors explain.

This is because the crew cannot send signals to the front of the ship. It is difficult for events inside the bubble to influence events outside the warp bubble, as explained in an earlier article.

“From the perspective of dynamically simulating the warp drive, the main challenge is stability,” the authors explain. Equations show that the Alcubierre Drive can initiate a warp bubble using the Einstein equation, but no known equation can support this.

“There is (to our knowledge) no known equation of state that would keep the warp drive metric in a stable configuration over time. Therefore, although one may require the warp bubble to be initially constant, it will soon evolve from that state, and in most cases the distortions of the warp fluid and spacetime will spread out or collapse into a central point.

While instability is a major obstacle to warp drives, it is also the reason they can become detectable. When an Alcubierre Drive reaches a constant speed, it is undetectable. It does not generate gravitational waves and has no ADM mass. ADM stands for Arnowitt-Deser-Misner, named after three physicists.

But the warp drive is only undetectable if it is constant and stable. Once it breaks down, speeds up or slows down, it can be detectable. In their work, the authors collapse the warp drive bubble.

“Physically, this could be related to a malfunction in the containment field that the post-warp civilization (presumably) uses to prop up 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.

The researchers simulated the disintegration of the warp bubble. They found that the collapse generated gravitational waves with different characteristics than those generated by mergers. “The signal comes as a burst, initially without gravitational wave content, followed by an oscillatory period with a characteristic frequency of order 1/[R],” they write.

‘Overall, the signal is very different 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 drive creates a GW signal, this is outside the frequency range of our current ground detectors. “Proposals have been made for higher frequency detectors, so in the future one may be able to put limits on the existence of such signals,” they write.

The ship itself could also emit some kind of multi-messenger signal, but it’s difficult to know how the ship’s matter would interact with ordinary matter. “Since we don’t know what type of matter the warship is made of, we don’t know whether (apart from gravity) it would interact with normal matter as it propagates through the universe,” the researchers explain.

This is a fun thought experiment. It’s possible that some sort of solution for FTL travel will exist sometime in the distant future. If so, it could be related to a better understanding of dark matter and dark energy. If ETIs exist, they may be in a position to exploit fundamental knowledge of the universe that we do not yet possess.

If they have discovered how to construct 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 it’s all theoretical.

“We caution 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 generic the signatures are and to properly characterize their detectability.”

No doubt some curious physicists will continue to work on this.