The idea of one day travelling to another star system and seeing what is there has been the fervent dream of people long before the first rockets and astronauts were sent to space.
But despite all the progress we have made since the beginning of the Space Age, interstellar travel remains just that – a dream. While theoretical concepts have been proposed, the issues of cost, travel time and fuel remain highly problematic.
A lot of hopes currently hinge on the use of directed energy and lightsails to push tiny spacecrafts to relativistic speeds. But what if there was a way to make larger spacecraft fast enough to conduct interstellar voyages?
According to David Kipping – the leader of Columbia University's Cool Worlds lab – future spacecraft could rely on a Halo Drive, which uses the gravitational force of a black hole to reach incredible speeds.
In it, Kipping addressed the single-greatest challenges posed by space exploration, which is the sheer amount of time and energy it would take to send a spacecraft on a mission to explore beyond our Solar System.
As Kipping told Universe Today via email:
"Interstellar travel is one of the most challenging technical feats we can conceive of.
"Whilst we can envisage drifting between the stars over millions of years – which is legitimately interstellar travel – to achieve journeys on timescales of centuries or less requires relativistic propulsion."
As Kipping put it, relativistic propulsion (or accelerating to a fraction of the speed of light) is very expensive in terms of energy.
Existing spacecraft simply don't have the fuel capacity in order to be able to get up to those kinds of speeds, and short of detonating nukes to generate thrust – à la Project Orion (video above) – or building a fusion ramjet – à la Project Daedalus – there are not a lot of options available.
In recent years, attention has shifted towards the idea of using lightsails and nanocraft to conduct interstellar missions. A well-known example of this is Breakthrough Starshot, an initiative that aims to send a smartphone-sized spacecraft to Alpha Centauri within our lifetime.
Using a powerful laser array, the lightsail would be accelerated to speeds of up to 20 percent of the speed of light – thus making the trip in 20 years.
"But even here you are talking about several terra-joules of energy for the most minimalist (a gram-mass) spacecraft conceivable," said Kipping.
"That's the cumulative energy output of nuclear power stations running for weeks on end (which by the way we have no way of storing so much energy either)! So this is why it's hard."
In the 1963 book, Interstellar Communications (Chapter 12: "Gravitational Machines"), Dyson described how spacecraft could slingshot around compact binary stars in order to receive a significant boost in velocity.
As Dyson described it, a ship that would be dispatched to a compact binary system (two neutron stars that orbit each other) where it would perform a gravity-assist maneuver.
This would consist of the spaceship picking up speed from the binary's intense gravity – adding the equivalent of twice their rotational velocity to its own – before being flung out of the system.
While the prospect of harnessing this kind of energy for the sake of propulsion was highly theoretical in Dyson's time (and still is), Dyson offered two reasons why "gravitational machines" were worth exploring:
"First, if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future where engineering on an astronomical scale may be both feasible and necessary.
"Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kind of observable phenomena a really advanced technology might be capable of producing."
In short, gravitational machines are worth studying in case they become possible someday, and because this study could allow us to spot possible extra-terrestrial intelligences (ETIs) through the technosignatures such machines would create.
Expanding upon this, Kipping considers how black holes – especially those found in binary pairs – could constitute even more powerful gravitational slingshots.
This proposal is based in part on the recent success of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has picked multiple gravitational wave signals since the first was detected in 2016.
According to recent estimates based on these detections, there could be as many as 100 million black holes in the Milky Way galaxy alone.
Where binaries occur, they possess an incredible amount of rotational energy, which is the result of their spin and the way they rapidly orbit one another.
In addition, as Kipping notes, black holes can also act as a gravitational mirror – where photons directed at the edge of the event horizon will bend around and come straight back at the source. As Kipping put it:
"So the binary black hole is really a couple of giant mirrors circling around one another at potentially high velocity. The halo drive exploits this by bouncing photons off the "mirror" as the mirror approaches you, the photons bounce back, pushing you along, but also steal some of the energy from the black hole binary itself (think about how a ping pong ball thrown against a moving wall would come back faster).
"Using this setup, one can harvest the binary black hole energy for propulsion."
This method of propulsion offers several obvious advantages. For starters, it offers users the potential to travel at relativistic speeds without the need for fuel, which currently accounts for the majority of a launch vehicle's mass.
There is also the many, many black holes that exist across the Milky Way, which could act as a network for relativistic space travel.
What's more, scientists have already witnessed the power of gravitational slingshot thanks to the discovery of hyper-velocity stars.
According to research from the Harvard-Smithsonian Center for Astrophysics (CfA), these stars are a result of galactic mergers and interaction with massive black holes, which causes them to be kicked out of their galaxies at one-tenth to one-third the speed of light – ~30,000 to 100,000 km/s (18,600 to 62,000 mps).
But of course, the concept comes with innumerable challenges and more than a few disadvantages.
In addition to building spacecraft that would be capable of being flung around the event horizon of a black hole, there's also the tremendous amount of precision needed – otherwise the ship and crew (if it has one) could end up being pulled apart in the maw of the black hole.
On top of that, there's the simple matter of reaching one:
"[T]he thing has a huge disadvantage for us in that we have to first get to one of these black holes. I tend to think of it like a interstellar highway system – you have to pay a one-time toll to get on the highway, but once your on you can ride across the galaxy as much as you like without expending any more fuel."
The challenge of how humanity might go about reaching the nearest suitable black hole will be the subject of Kipping's next paper, he indicated. And while an idea like this is about as remote to us as building a Dyson Sphere or using black holes to power starships, it does offer some pretty exciting possibilities for the future.
In short, the concept of a black hole gravity machine presents humanity with a plausible path to becoming an interstellar species. In the meantime, the study of the concept will provide SETI researchers with another possible technosignature to look for.
So until the day comes when we might try something like this out for ourselves, we will be able to see if any other species has already taken a stab at it and made it work!