Forget building spaceships.
You can keep your Millennium Falcon, Jupiter 2, and USS Enterprise. Star Hitchhiking is the quickest way to see the cosmos.
“A fast-moving star is as good as a spaceship,” says Jonathan Carroll-Nellenback, an astronomer at the University of Rochester.
The idea is simple. Allow yourself to be captured by the gravity of a passing star system as it draws near, and allow its motion through space to carry you to new systems. Maybe you’ll disembark at one of those systems, and set up your settlement on some strange new world. Then, eventually, another star will pass close by, and you’ll catch a ride to somewhere new. And so on.
Carroll-Nellenback has developed a computer simulation that shows how hitching rides with stars would help a space-faring civilization settle large chunks of the galaxy.
It begins with a civilization in the boondocks of the galaxy’s spiral arms, just beginning to reach out into the Universe. The civilization settles a few of its nearest star systems, a bit like if Earth-based humans settled on planets orbiting Proxima Centauri or Wolf 359. Then, a faster-moving star comes by, and the civilization quickly hops into orbit, riding through the galaxy, encountering more and more star systems to settle.
“This fast mover comes through and picks up life,” says Carroll-Nellenback. Eventually, when the expanding wavefront reaches the center of the galaxy, “it just explodes with settlements because the distances between the stars is so small in the center, that the colonizing wavefront grows at the fastest rate possible.”
This is another reason why the Search for Extraterrestrial Intelligence (SETI) should focus its efforts towards the center of our Milky Way Galaxy, and the centers of other galaxies, too. The simulation “shows the pattern of interstellar expansion for any species, no matter how much energy it uses,” says Jason Wright, a professor of astrophysics and director of the Penn State Extraterrestrial Intelligence Center at Penn State University.
The hitchhiking concept was developed as a reaction to older, perhaps more entrenched ideas about how life might spread across an entire galaxy, settling around each one of its stars. Previous estimates for how long this might take have assumed that stars remain fixed in position and that we’ll use spacecraft to cross the expanse between each and every one.
These estimates range from 5 million years (Ian Crawford, Birkbeck College, University of London), to tens of millions to billions of years (William Newman and Carl Sagan). What’s notable about most of these estimates, however, is that they are mostly shorter than the lifetime of the galaxy, which is about 13 billion years. Sure, these are unimaginably long timescales to us, but to any long-lasting space-faring civilization, perhaps they are achievable — and there has been enough time for them to do so.
This raises a question that Enrico Fermi famously wondered back in 1951—
“Where is everybody?”
For Fermi, the simplest answer was that ET isn’t here because interstellar travel must be extremely difficult, if not impossible. We know, however, that this is not the case. Whether it be slow robotic probes, generation ships, hitching rides on interstellar comets or passing star systems, or Breakthrough Starshot’s nano-sized StarChips that ride on laser beams, there’s a multitude of ways to travel to the stars as long as one is patient and has the necessary resources and power source.
Geoffrey Landis tried to explain the seeming absence of aliens in the framework of interstellar travel through his percolation theory. He made the point that not every settlement will launch daughter settlements, and that some and perhaps eventually all settlements would fail, calling a halt on further expansion. He assigned a probability to each settlement as to whether its inhabitants would go on to settle more star systems or not, and those that failed would lead to voids in the galaxy that don’t become occupied by a settler.
Earth could be in one of these voids, he suggested.
Everything is in Motion
Except that in reality, as we’ve seen, stars move. The Sun, for example, completes one orbit of the Milky Way every 220 million years. Proxima Centauri is currently the closest star to the Sun, at a distance of 4.2 light-years, but it hasn’t always been the closest, and will not be the closest in the future. For example, 70,000 years ago a red dwarf called Scholz’s Star passed within 1.9 light-years of the Sun, skirting the outer regions of the Oort Cloud.
If there had been space-faring humans alive 70,000 years ago, we might have hitched a ride on Scholz’s Star. Today, Scholz’s Star is 22 light-years away.
So with all the stars in motion, settlements voids will not always be voids—settled systems will come and go, and life would spring eagerly from them into virgin territory. Based upon this, Carroll-Nellenback and Wright joined forces with astrophysicists Adam Frank at the University of Rochester and Caleb Scharf of Columbia University to develop a new model of interstellar settlement and expansion.
They show that once a settlement expansion-front forms, all the empty star systems behind it also become settled, but because the lifetime of settlements and civilizations is likely going to be finite, the galaxy is not going to be settled all at the same time. There will be gaps that could be unsettled for many millions of years. This could be the answer to Fermi’s question—they might not be here now, but they may have been here a long time ago, perhaps even before the dinosaurs.
Could we ever prove such a scenario? Frank and Gavin Schmidt asked this very question, in the guise of their Silurian hypothesis—the possibility that aliens could have settled Earth many millions or billions of years ago. Earth’s geological record is far from complete, and weathering and tectonic turnover would remove many of the signs that someone else had once settled Earth. Any surviving evidence would be found in the isotopes and compounds contained within deep ocean sediments.
Frank and Schmidt are not saying that aliens did live on Earth millions or billions of years ago, only that we cannot rule out the possibility. The Solar System may have existed in a void since then but may be resettled one day in the future.
Patterns of Kardashev Civilizations
Carroll-Nellenback’s simulation can also tell us about what interstellar civilizations in other galaxies might look like. In 2014–15, Wright published a series of papers under the umbrella title of ‘Glimpsing Heat from Alien Technologies’, called G-HAT for short, or Ĝ for even shorter. It was an ambitious search of about a million galaxies for Kardashev type III civilizations.
This refers to Soviet astronomer Nikolai Kardashev’s three categories for technological civilizations. A Kardashev Type 1 (K1) civilization has access to all the energy available to it on a planet. A Type II (K2) civilization is able to harness the entire energy output of a single star, and this is most often depicted as requiring a Dyson swarm (or Dyson sphere), but it would not be a solid object) of solar panels collecting all the light that the star emits. A Type III (K3) civilization would be able to collect and utilize the energy output of an entire galaxy, by journeying to each and every star and surrounding them with Dyson swarms.
The thing about Dyson swarms, or anything that collects energy, is that they get hot, and have to radiate that heat away or else melt. So while a Dyson swarm may block the light of a star, the swarm would be detectable through its own thermal infrared emission. A K3 civilization would transform all the light of its galaxy into thermal infrared emission. So Wright and his colleagues set about looking for K3 civilizations by scrutinizing data from NASA’s Wide-field Infrared Survey Explorer (WISE) mission, looking for galaxies with anomalous excess infrared emission.
The search drew a blank. There just didn’t seem to be any K3 civilizations out there. However, Wright cautioned that it didn’t rule out civilizations that were on their way to becoming K3 civilizations. Maybe they could be considered K2.9, or 2.8 civilizations, where they’ve settled many of the stars in their home galaxy, but not all.
Carroll-Nellenback’s computer simulation gives us an idea of what a sub-K3 civilization may look like, with the core of the galaxy completely inhabited, while large chunks of the spiral disc may be unvisited. A galaxy with infrared excess in this pattern could give away the presence of a star-faring civilization.
Time to Wait
There are caveats. With any model about hypothetical aliens, there’s a lot of guesswork involved, and a lot of assumptions made, something that Jason Wright acknowledges when discussing Carroll-Nellenback’s computer simulation.
“Of course, it makes a lot of assumptions,” he says. “In particular this specific animation was for ships with a certain range that launched at a particular frequency.”
If a civilization could build starships that could travel farther faster or had limited technology that meant they couldn’t get very far, then the speed with which interstellar settlement takes place would change accordingly. If planets that are habitable and can be settled are less frequent in the galaxy, this too will also affect the rate of expansion, with settled star systems being sparser.
A lot also depends on the lifetime of settlements and entire civilizations. The drawback of the model is that the emphasis is placed on waiting for stars to come to us, rather than the other way around. Yet whereas we might try to launch missions to the closest stars in the near future, we will have to wait much longer for a star to come close to us. Results from the European Space Agency’s Gaia satellite, which has measured the motions and positions of more than a billion stars, indicate that a red dwarf star called Gliese 710 will pass through the Oort Cloud at a distance of 2.3 trillion km (~16,000 astronomical units) in about 1.3 million years time.
In the next 5 million years, 97 stars will come within 150 trillion kilometers, and 16 stars will come within 60 trillion km of the Sun (about 6 light-years).
We have no way of knowing whether humans will still exist in 1.3 million years. So if we want to settle the galaxy, then in addition to stellar hitch-hiking, we’ll need a head-start by traveling to our nearest stars in spacecraft first.
Carroll-Nellenback recognizes the balance between spacecraft technology and a fortunate stellar rendezvous. To that end, he and his team’s next step is to look at how changes to spacecraft technology—both improvements and downgrades—affect the pattern of galactic expansion.
“Over time, you’ll get technology that will advance enough to allow you to start settling systems, and that gives you a competitive advantage,” says Carroll-Nellenback. “It creates a selection pressure on technology until the entire galaxy is colonized, and then [the technology level] drops back a little, to where it is needed to maintain a steady state.”
Improving technology could also lead to what Marc Millis, of the Ohio Aerospace Institute, has called the Incessant Obsolescence Postulate. This describes how technology might improve at such a rate that explorers setting forth for the stars centuries or even millennia after the first travelers set out might overtake those first travelers, who would discover that their species has already settled at their destination by the time they reach it.
What does this all mean for the future of humankind? Expanding across the galaxy is going to be difficult, one way or the other. Will we be the ones setting forth, or will our robotic avatars go in our stead? Settling across the galaxy is certainly one way to help ensure the longevity of our species. Once settled on many worlds, it would become increasingly difficult for catastrophes to wipe us all out. On individual planets, individual settlements might fail, but others would remain, for some time at least. In an era in which existential dangers loom large over us all, safety through galactic settlement—not placing all your eggs in one basket, as it were—has some appeal.
If we can take ourselves to the nearest stars, we can let the stars themselves do the rest of the work, and the diaspora can begin.