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The Intergalactic Fermi Problem

by Paul Gilster | Jun 24, 2026 | Astrobiology and SETI | 25 comments

The headwaters of the Fermi Paradox channel directly through Michael Hart and Frank Tipler, and it’s a testament to the power of their arguments that this remains true today. It was Hart who in "An Explanation for the Absence of Extraterrestrials on Earth" (published in the Quarterly Journal of the Royal Astronomical Society in 1975) pointed out something blindingly obvious once stated. Moving at one-tenth of the speed of light, a civilization could send its probes throughout the galaxy in as little as 650,000 years.

Hart set an upper limit on this at 2 million years, but either way the point resounded in the astrophysics community because these are tiny time spans compared to the age of the universe. Hart even factored in a pause after each leap to a new star to found a ‘colony,’ or whatever such a probe would do there. Our Sun being a relatively youthful 4.6 billion years old, that was a vast amount of time for earlier civilizations to have mastered technologies opening up trips to the stars, but we have yet to find evidence of them.

The ‘Where are they?’ question resonated with Tipler when he picked up John von Neumann’s idea of self-replicating probes. Tipler pointed out that this wave of replication would be unstoppable. The fact that we saw no evidence of it led to the title he chose for his paper: "Extraterrestrial Intelligent Beings Do Not Exist," which was published in 1980 in the Quarterly Journal of the Royal Astronomical Society. It quickly led to spirited argument in the pages of Physics Today and continues to motivate debate.

It would be fun sometime to go through that early back and forth, which included Frank Drake, Carl Sagan, Gregory Benford and William Newman, but I’ll fight off my digressive instincts to home in on the paper I want to talk about today. It’s from David Kipping, and takes Hart and Tipler’s ideas a logical step further. If we can extrapolate a ‘filled’ galaxy within 650,000 years (and Kipping points out that this number continues to look viable), then what about galactic expansion? After all, intergalactic travel times should be endurable for machine intelligence. Should we expect signs that other galaxies – perhaps all galaxies — should have been ‘infected’ by self-replicating technologies by now?

Image: Could it be that entire galaxies are infested with self-reproducing technologies? This one is the barred spiral galaxy NGC 1365, split diagonally in this image: The James Webb Space Telescope’s observations appear on bottom right, and the Hubble Space Telescope’s at top left. David Kipping’s new paper examines how we can extend the Hart-Tipler argument on the expansion of technologies through one galaxy into cosmological realms. Credit: NASA, ESA, CSA, STScI, PHANGS Team, Janice Lee (STScI), Thomas Williams (Oxford).

All of this raises the question of what a self-reproducing probe would be likely to do to a planet it encounters. It is striking that we don’t have to assume bad intent on the part of the builders. If self-reproducing probes built by civilizations far ahead (technologically) of our own are simply sent out as scouts and explorers, over the course of aeons some may begin to spawn destructive offspring simply because of the gradual introduction of errors into their programming. These in turn reproduce. From this we get the concept of the ‘berserker’ probe that destroys worlds.

Or perhaps, as Kipping muses, they simply go about converting planets into computational substrate. Modern developers pay no attention, for example, to the survival of small creatures in the landscape they ravage to build new apartment houses. Whether such a probe would notice a fledgling technological civilization or not is a matter of debate. But let’s look at that idea of infection. It is not intended to imply the malignant spread of anything. From the paper:

We use the term “infection” in a mathematical sense only: a self-propagating transition from a habitable/untransformed state to an uninhabitable or observer suppressing state. No biological analogy is intended. The infection fronts are mathematically modeled as spherical wave fronts, which can be interpreted either as literal isotropic expansion or as an effective envelope for a sufficiently dense directed-probe strategy (e.g. Crick & Orgel 1973). In this way, the model could be considered to encompass a variety of infection modes. Indeed, our intention here is to avoid conditioning the model upon a specific mechanism because any assumptions of “advanced” behavior often age poorly (e.g. Martian canals; Chambers 1999), since we cannot reliably predict what new technological paradigms might arise.

Although there have been several papers looking into cosmological expansion, in particular a 2015 title by S Jay Olson and a 2013 paper by Stuart Armstrong and Anders...