The universe is filled with flashy pyrotechnics, such as exploding stars or flaring black holes, which spew out unfathomable amounts of light and energy. These luminous objects have helped humans understand our cosmic surroundings for centuries, but a new study suggests they could have a more futuristic function—spaceship propulsion.
Given how much energy these sources pump out, it’s worth considering if any spacecraft could harness their power, argue the co-authors of a study that proposes such an approach: Manasvi Lingam, an astrobiologist at the Florida Institute of Technology, and Abraham Loeb, a theoretical physicist at Harvard University.
Lingam and Loeb propose that astrophysical objects could accelerate spacecraft close to the speed of light, which is the fastest pace possible in the universe, in their paper, which was posted this month on the preprint site arXiv and is not yet peer-reviewed. In other words, spaceships could plug themselves into these enormous energy outlets.
However, the researchers emphasize that their results are preliminary and focused on ideal circumstances.
“This was mostly a conceptual study,” Lingam said in a phone call. “This is not something that is within the ability of human beings in the near future or even something that we can foresee in the long-term future.”
“In the very distant future, perhaps,” he added, but that would probably be “millions of years down the line.”
While it’s a shame that we won’t live to see supernovae blasting spaceships to relativistic speeds, the study paints a fascinating picture of how such a system might work. Lingam and Loeb focus on two spacecraft concepts that already exist: light sails, which use photons to generate thrust, and electric sails, which are propelled by high-energy subatomic particles.
The team studied what would happen if both types of sail were paired with energetic celestial objects, such as massive stars, supernovae, and pulsar wind nebulae, which are X-ray emitting environments that can emerge in the aftermath of supernovae. Microquasars and active galactic nuclei (AGN), which are flares caused by matter falling into black holes, were also considered as potential fuel sources.
Though each match-up produced different variations, the basic idea was always the same. Humans, or any advanced space-faring species, would send a bundled sail to an existing high-energy source, or to a region where a star or galactic flare is predicted to erupt.
“That’s when you would unfurl the sail, just as on a sailboat, and you would take advantage of that boost coming from all these photons or particles being emanated,” Lingam explained. “You would achieve the kind of speed you want, but then shortly afterward, you would close up the sail, because as it enters the interstellar medium, it’s subjected to a lot of impact from dust particles and from neutral gas.”
“The space between stars is not really empty,” he continued, “so it would actually undergo quite significant damage over long distances.”
The team found that supernovae could propel both types of sails to about 10 percent the speed of light over a period of about a month. The most energetic blasts, such as those emitted by AGN, could push the sails all the way to the brink of that cosmic speed limit within years.
The invention of such high-speed travel would unlock exploration of the entire Milky Way on relatively short timescales, enabling the sails to zip across the galaxy within tens of thousands of years. It would be an astronomical adventure, but there are naturally a lot of caveats and unknowns tangled into this hypothesis.
First, you could not buy a ticket to sail the galactic seas yourself. The sails would probably have to be controlled remotely or by artificial intelligence, as they could not safely carry fragile lifeforms like humans. To that point, it’s not even clear what type of weird effects relativistic speeds might have on robotic instruments, though it is a good premise for a science fiction story.
“What happens to an instrument moving at 10 percent the speed of light or 50 percent the speed of light over a long period of time, say thousands of years?” Lingam said. “We don’t really have any data on what would happen even to instruments, so that remains an open question.”
The structural properties of the sails would also be pushed to their limits, both due to interstellar weathering as well as radiation and particle damage from the propulsion source itself.
“Because particles impact the sail, in some cases, they are going to literally burn a hole in the sail,” Lingam noted, adding that “while it is accelerated, and the sail is unfolded, you have so much radiation striking the scale it could also cause the sail to buckle or warp in some way.”
These structural instabilities are already being investigated as part of Breakthrough Starshot, a $100 million project aimed at designing light sails that could reach the nearest star system, Alpha Centauri. It would be exponentially more difficult to design sails, powered by supernovae or microquasars, that zoom beyond our local galactic neighborhood, which is why the study concludes that this concept will effectively out of reach for humans for the foreseeable future.
A super-sophisticated alien species, however, may have figured out how to bypass all these challenges, which offers a potential application in the search for extraterrestrial intelligence (SETI).
“If there are some other technological species that are more advanced than we are, then this could be a viable means of transportation for them,” Lingam said. “Maybe we could search the vicinity of these [energetic] environments for some kind of technosignatures, whether it’s radio signals or some kind of artifacts that might be present there.”