Key Takeaways:

  • Stars travel through space, and their gravity can be used by civilizations to travel between star systems.
  • Hitchhiking on stars can be a faster way to colonize the galaxy compared to traditional methods that involve traveling between stars with spaceships.
  • The center of galaxies might be a good place to search for extraterrestrial intelligence (SETI) because stars are closer together there, making it easier to hitchhike between them.
  • The concept of star hitchhiking helps explain Fermi’s Paradox, which questions why we haven’t encountered extraterrestrial life yet. It’s possible such civilizations existed in the past but are gone now, or they haven’t reached us yet.
  • While star hitchhiking has advantages, it also relies on waiting for stars to come close enough. We may need to develop faster spaceships or send probes to nearby stars to jumpstart colonization efforts.

You can keep your Millennium Falcon, Jupiter 2, and USS Enterprise. The fastest way to see the universe is to star hitchhike.

According to University of Rochester astronomer Jonathan Carroll-Nellenback, “a fast-moving star is as good as a spaceship.”

The concept is simple. As a star system passes by, let yourself be drawn in by its gravitational pull and let its space travel take you to other systems. Perhaps you will disembark at one of those systems and establish yourself on some bizarre new planet. Eventually, though, another star will pass near by, and you’ll be able to board a flight to a new location. And so on.

Carroll-Nellenback has created a computer simulation that illustrates how a space-faring civilization could colonize vast regions of the galaxy by hitching rides with stars.

The story starts with a civilization that is just starting to spread throughout the universe, deep in the vastness of the spiral arms of the galaxy. 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. The civilization quickly launches itself into orbit after a faster-moving star passes by, traveling across the galaxy and coming across an increasing number of star systems for settlement.

“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 tens of millions to billions of years (William Newman and Carl Sagan) to five million years (Ian Crawford, Birkbeck College, University of London). 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. Even though these seem like unachievable lengths of time to us, any space-faring civilization with enough durability may be able to achieve them.

This raises a question that Enrico Fermi famously wondered back in 1951—

“Whereis everybody?”

Fermi’s simplest response was that interstellar travel must be exceedingly difficult, if not impossible, in order for ET to not exist. We know, however, that this is not the case. There are many ways to reach the stars as long as one is patient and has the required tools and power source, such as generation ships, slow robotic probes, hitching rides on interstellar comets or passing star systems, or Breakthrough Starshot’s nano-sized StarChips that ride on laser beams.

Geoffrey Landis tried to use his percolation theory to explain why extraterrestrials appeared to be missing from the framework of interstellar travel. He emphasized that not all settlements will produce daughter settlements, and that some or maybe all settlements would eventually fail and put an end to further growth. 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.

He speculated that Earth might be in one of these voids.

Everything is in Motion

Except that stars actually move—as we have seen. Every 220 million years, for instance, the Sun completes one orbit around the Milky Way. With a distance of 4.2 light-years, Proxima Centauri is currently the star closest to the Sun, but it hasn’t always been and it won’t be 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.

We might have taken a ride on Scholz’s Star 70,000 years ago if there had been human space travelers. Scholz’s Star is currently 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. On the basis of this, Carroll-Nellenback and Wright collaborated with Columbia University’s Caleb Scharf and the University of Rochester’s Adam Frank to create 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 are going to be gaps that may remain unfilled for several million years. This could be the response to Fermi’s query; although they might not be present right now, they might have existed in this area long ago—possibly even before the dinosaurs.

Could we ever establish such a case? Under the guise of their Silurian hypothesis, Frank and Gavin Schmidt posed this exact question: is it possible that aliens settled on Earth millions or billions of years ago? The geological record of Earth is far from complete, and many of the traces of past human habitation would be eliminated by weathering and tectonic turnover. The isotopes and compounds found in deep ocean sediments would hold any remaining evidence.

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.

Kardashev Civilization Patterns

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 the three classifications of technological civilizations developed by Soviet astronomer Nikolai Kardashev. All of the energy on a planet is accessible to a Kardashev Type 1 (K1) civilization. A Dyson swarm (or Dyson sphere; it would not be a solid object) of solar panels collecting all the light that the star emits is most commonly depicted as being necessary for a Type II (K2) civilization to be able to harness the entire energy output of a single star. A civilization of Type III (K3) could harvest the energy output of an entire galaxy if it traveled to every star and encircled 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. Therefore, even though a Dyson swarm may block a star’s light, it could still be identified by its own thermal infrared emission. The entire galactic light would be transformed into thermal infrared emission by a K3 civilization. Therefore, by carefully examining data from NASA’s Wide-field Infrared Survey Explorer (WISE) mission, Wright and his colleagues set out to search for K3 civilizations by looking for galaxies with anomalous excess infrared emission.

The search turned up nothing. It appeared as though there were no K3 civilizations in existence. Wright emphasized that it did not, however, rule out civilizations that were in the process of evolving into K3 civilizations. They may be categorized as K2.9, or 2.8, civilizations since they have colonized most, but not all, of the stars in their home galaxy.

The sub-K3 civilization depicted in Carroll-Nellenback’s computer simulation is based on the hypothesis that a significant portion of the spiral disc may remain unresolved while the galaxy’s core is fully populated. A galaxy exhibiting this pattern of infrared excess could be a dead giveaway for a star-faring civilization.

Time to Wait

There are disclaimers. Jason Wright notes that a lot of conjecture and assumptions are made in any model involving hypothetical aliens, as he discusses Carroll-Nellenback’s computer simulation.

“It makes a lot of assumptions, of course,” he says. “This particular animation was intended for ships that launched at a specific frequency and had a specific range.”

The rate of interstellar settlement would alter if a civilization developed starships capable of traveling farther at a faster speed or if they possessed low-tech starships that could not travel very far. 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.

The longevity of entire civilizations and settlements also plays a significant role. 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. However, we will have to wait a lot longer for a star to approach us, even though we might try to send missions to the closest stars in the near future. According to data from the European Space Agency’s Gaia satellite, which has tracked the positions and motions of over a billion stars, in roughly 1.3 million years, a red dwarf star by the name of Gliese 710 will travel through the Oort Cloud at a distance of 2.3 trillion km (~16,000 astronomical units).

Within the next 5 million years, 16 stars and 97 stars will pass within 60 trillion kilometers (roughly 6 light-years) and 150 trillion kilometers, respectively, of the Sun.

It is impossible for us to know if humans will still be around in 1.3 million years. Therefore, in addition to star hitchhiking, we’ll need to get a head start by sending spacecraft to our nearest stars in order to settle the galaxy.

Technological obsolescence

Carroll-Nellenback recognizes the balance between spacecraft technology and a fortunate stellar rendezvous. In light of this, the next task for him and his group is to investigate how modifications to spacecraft technology, both upgrades and downgrades, impact the galactic expansion pattern.

“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. “Until the entire galaxy is colonized, it creates a selection pressure on technology, and then [the technology level] drops back a little, to where it is needed to maintain a steady state.”

Another possible outcome of technological advancement is the Incessant Obsolescence Postulate, as described by Marc Millis of the Ohio Aerospace Institute. This shows how technology could advance at such a rate that, centuries or even millennia after the first explorers set out, those setting out for the stars could surpass the first travelers, only to find, upon reaching their destination, that their species has already established itself.

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 our robotic avatars go ahead of us, or will we be the ones setting out? Moving across the galaxy to settle is certainly one way to contribute to the survival of our species. Once we were spread across multiple worlds, it would be more challenging for disasters to destroy us all. Certain settlements might fail on different planets, but others would persist, at least temporarily. There is some appeal to safety through galactic settlement—not putting all your eggs in one basket, so to speak—in a time when existential threats hang heavy over all of us.

The diaspora can start if we can transport ourselves to the closest stars and leave them to finish the job.

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