A new study suggests that by using the slingshot effect to propel self-replicating probes through interstellar space, an advanced extraterrestrial civilization should be able to visit every corner of the galaxy in a startlingly short amount of time. The Fermi Paradox, it would seem, is alive and well.


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Before we get to the new study, let’s quickly review what we mean by self-replicating probes and their relation to the Fermi Paradox.


Exponential Rate of Expansion

The hypothetical self-replicating probe (SRP) is an idea that’s been around since the 1940s. Devised by the brilliant mathematician John von Neumann (which is why they’re also called Von Neumann probes), it’s a non-biological system that can replicate itself. Von Neumann wasn’t thinking of space exploration and colonization at the time, but other thinkers, like Freeman Dyson, Eric Drexler, and Robert Freitas, have since extended his idea to exactly that.

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Once launched into space, an SRP could travel to a neighboring star system, and through the application of robotics, molecular assembly, and AI, seek out resources to build an exact replica of itself. Really, all it would need to do is find an asteroid with the right material components.


Image via cg-masters.

And based on the sophistication or purpose of the probe, it could establish colonies on suitable planets (either by spawning biological organisms or robots imbued with either AI or uploaded minds). More simply, an SRP could spawn Bracewell communication probes, which could make contact with a resident (or future) alien civilization.


Once its mission is complete, it would spawn daughter versions of itself, which would be sent towards the nearest star system. Lather, rinse, repeat.

And indeed, the power of the SRP lies in its ability to replicate at an exponential rate. The initial rate of exploration would be slow, but after producing potentially millions upon millions of offspring, the rate of expansion would increase by an order of magnitude. So even at a speed of about a tenth the speed of light, these probes could cover a huge amount of territory in a relatively short amount of time from a cosmological perspective.


Needless to say, the concept of the SRP has fueled much of the Fermi Paradox, the suggestion that we should have seen signs of extraterrestrials by now.

Slingshot Dynamics

And now, thanks to a new paper by Arwen Nicholson and Duncan Forgan from the Institute for Astronomy at the University of Edinburgh, the Fermi Paradox has just gotten considerably worse.


Potential problems or inhibitors for SRP propagation include the energy and time required to travel from star to star. A self-constructing probe requiring propulsion engines and a fuel source would not only be complicated, it would also be very time-consuming.

But according to the new study, which was published in the International Journal of Astronomy, aliens (or future humans for that matter) could use the the slingshot effect to propel SRPs from star to star. And in fact, this is precisely the same phenomenon that was used to move the Voyager spacecrafts through our solar system as they hopped from planet to planet. But for it to work on a galactic scale, the SRPs would use slingshot maneuvers around stars, gaining a boost in velocity by extracting energy from each star’s motion around the galactic center.



These maneuvers would carry little-to-no extra energy cost. And, as shown in previous work, a single Voyager-like probe exploring the galaxy could do so 100 times faster when carrying out these slingshots than when navigating purely by powered flight.

Self-replication on the Fly

Interestingly, Nicholson and Forgan assume that the probe collects matter (like dust and gas) from the interstellar medium as it travels through space. It literally builds a replica of itself as it’s traveling — so it doesn’t have to stop.


“[The] parent probe reaches the new destination star, and before it slingshots around the star it releases the replica probe,” they note in the study. “Both the parent and replica use the slingshot to boost their velocity. As the velocity boost from a slingshot trajectory depends on the angle between the stars, the parent and the replica will achieve different velocity boosts as they will have different destination stars.”


The researchers put this model to test by using a computer simulation. What they discovered was that, by using this technique, an alien civilization could send probes traveling no faster than 10% the speed of light to every single solar system in the galaxy in only 10 million years. Which is incredible — that’s an amount of time that’s significantly less than the age of the Earth.

So Where the Hell Are the Probes?

This means that an alien civilization could (and should) have arrived in our solar system by now.


So where are the probes? Or the colonies?

The grim possibility is that we’re alone, and no alien civilization exists to send out the probes. But that’s weird and highly improbable given that intelligence could have emerged in our galaxy about 5 billion years ago. But it’s also conceivable that the probes are in fact here, but invisible to us. Either we don’t have the technology to detect them, or they’re sitting idle waiting for us to pass some kind of test or technological threshold.


But as the researchers conclude, the "nearest-star slingshot" strategy remains "the most time effective way to explore a population of stars" and that "a fleet of self-replicating probes can indeed explore the Galaxy in a sufficiently short time to warrant the existence of the Fermi Paradox."

Read the entire study at the International Journal of Astronomy: “Slingshot Dynamics for Self Replicating Probes and the Effect on Exploration Timescales.”