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 Sandberg, Kipping finds them laced with complexities that complicate the discussion. In response, this paper is much in the spirit of Hart and Tipler in that the model is pared down to its essentials. The key parameters are spawn rate (λ) – the rate of the change of state from an ‘uninfected’ galaxy to an infected one. The second is propagation speed (u) and the third is the start time for when probes begin to appear in the cosmos. In other words, when in the 13.8 billion year history of the cosmos do self-reproducing probes begin to be produced?
Too simple a model? Deliberately so, and I think this is an important point:
We certainly welcome more sophisticated treatments, such as adding additional parameters to account for probabilistic spreads, behaviours, probe mutations, etc. However, we firmly believe that complexity must first build upon a simple baseline model to make it easily interpretable. Every new parameter adds potential confusion to what drives simulation outcomes, as well representing new points of logical vulnerability.
Simple model or not, work the numbers and the results will make any SETI optimist edgy. For waves of infection could well have spread across the cosmos by now, from one galaxy to another, from cluster to cluster, in just the way Hart and Tipler assumed, although now involving waves of probes on a cosmological scale rather than just the confines of our galaxy. Given the age of the universe, even the classic 0.1 of lightspeed makes such expansion possible for machine probes.
Assume 0.1 c as the propagation speed and calculate the point at which half the universe has been filled with technology. The calculations show that if only 1 in 240,000 galaxies, or equivalently 1 in 24 quadrillion stars, becomes infected, that is enough to have filled the universe to the point where half has been infected by our era. We can adjust the start time for the era of self-replicating probes from the 7.3 billion years after the Big Bang used here to a more likely 4.5 billion years (which is the amount of time Earth has had to support life). That allows for more expansion: The figure now becomes 1 in 100 quadrillion stars.
Let’s pause on that. This is saying that it would take only 1 in 100 quadrillion stars to have mounted a wave of self-replicating probes to get to the point where half of the visible universe is infected by this time in our existence. It only gets worse, of course, if we move past that figure of one-tenth of light speed. Push up closer and closer to light speed and everything compresses, as you might expect. All it takes is for 1 in a billion galaxies to have started the expansion wave of self-replication for the cosmos to be half filled. That’s one in 100 quintillion stars. Are these long odds or what? All civilizations except one in 100 quintillion can decide not to build such probes, but all it takes is that one.
This is what David Brin, in a key paper in 1983, called the Exclusion Principle. Even a single civilization out of a vast number of them is all it takes for waves of self-reproducing probes to gradually infest the galaxy. When we do not see these, we must ask what factors have excluded this from occurring. Do civilizations always destroy themselves before they can build such devices? That’s bad news for us, because in a century or two and perhaps sooner, we look to be capable of making self-reproducing probes of our own.
The odds that Kipping’s calculations come up with are stunning. A universe of galaxies half of which are ‘infected’ with self-replicating probes seems a rational extrapolation, and perhaps a bit less because we are not (yet) infected. But here we have to face a major point. I’ll quote the paper first and then riff on it. The italics are mine:
One might argue that any scenario for which half the Universe is filled poses no logical contradiction to our existence. We would simply live in the other half. We remind the reader though that f½ represents a tipping point of a rapid phase transition, and even small positive perturbations to the fiducial parameters quickly fills the cosmos. To show this, we repeated the grid of calculations shown in Figure 1 but solving for f = 99.9% instead. The results, presented in Figure 2, reveal a broadly similar set of solutions, with a modest shift in the contours in logarithmic space.
Remember that Kipping’s term f stands for the fraction of galaxies that are infected. In the paper’s Figure 1, the author graphs solutions that produce a cosmos half-filled with infected galaxies. Pushing the f figure up to 99.9 percent illustrates how swiftly a cosmos almost completely filled with infected galaxies can occur. The point here is that we don’t get to 50% saturation and then assume an equally lengthy future period gradually closing on 100%. Instead, we are dealing with a phase transition – think what happens when water goes from liquid to steam. The teapot doesn’t linger in a threshold condition for long. In cosmic terms, the 50% is itself the threshold of instability, leading to a runaway condition. Push past that threshold and the cosmos is rapidly transformed.
Image: This is Figure 1 from the paper. Caption: A grid of solutions that produce a cosmos precisely half-filled by an infection that has some spontaneous spawn rate within galaxies and then emanates an infection wavefront propagating at a speed given by the y-axis. The x-axis varies the earliest time for which we allow infection seeds to spawn. The contours denote the solved spawn rate to produce half-filling, framed in terms of the mean number of galaxies required to produce one infection seed. Credit: David Kipping.
Why, then, do we not see evidence of this in the night sky? Simply saying that we live in a part of the universe that hasn’t yet been filled seems like extremely wishful thinking. Kipping digs into the anthropic principle, specifically its weak version which suggests that we by necessity live in a part of the universe that is uninfected because otherwise we would not be here to observe.
I lack the ability to present the math involved at this point in the paper (extended into its equation-laden appendix), so I will send those better qualified to the text. Working through models of anthropic reasoning, Kipping finds that it’s possible to construct a universe (or multiverse) in which we observers do not yet detect such an infected cosmos, but note this “important nuance”:
Presumably, the probability of a technological species developing is proportional to the spawn rate of artificial infections. Accordingly, universes with f → 0 may not be so conducive to our emergence after all, since their low spawn rate implies that their intrinsic parameters are tuned to somehow greatly inhibit the development of complex life. This re-framing leans on what is known as the Self Indication Assumption (SIA) in anthropic reasoning (Bostrom 2013).
The paper is arguing that to be consistent with our own existence and observations, the spawn rate (λ) has to be tuned to an extraordinarily small number, ∼10−20 per Gyr per star. Like the cosmological constant, among other parameters, the spawn rate seems to be “enigmatically fine-tuned.” But we needn’t get too far into fine-tuning problems given that models of anthropic reasoning vary, and as the author points out, the definitive theory of anthropic reasoning has yet to be achieved. Which leaves ample scope for the cosmological Hart-Tipler problem to swim into focus as a new problem fit for discussion not only by physicists but philosophers, as surely it will.
Is the possibility of self-replicating probes so far beyond the realm of reality that we can rule them out? Clearly not. It’s interesting to see that even in recent years (and here I’m thinking about a paper Kipping cites, Alex Ellery’s “Self-replicating probes are imminent–implications for SETI” – citation below – which makes the case that self-replication is not far away from the capabilities of our own civilization. Here’s a snip from the abstract of that paper:
We are developing the ability to 3D print entire robotic machines from extraterrestrial resources including electric motors and electronics as part of a general in-situ resource utilization (ISRU) capability. We have 3D-printed electric motors which can be potentially leveraged from extraterrestrial material that should be available in every star system. From a similar range of materials, we have identified a means to 3D print neural network circuitry. From our industrial ecology, self-replicating machines and indeed universal constructors are feasible.
If feasible for us, how much more so for civilizations whose lifetimes take in millions of years? Many of the proposed explanations for the Fermi Paradox have sociological roots that often veer into anthropocentrism. Just how we are to model the ‘ethics’ of extraterrestrials is a worthy question, but explanations moving in this direction and applying to *every* extraterrestrial civilization fail to convince. If self-reproducing probes can be built by even a species not yet at Kardashev Type 1 status, and if we are forced to say that it would only take one in inconceivably vast numbers of stars to produce a builder civilization of these probes, we are left with questions that are more perplexing that ever.
Where are they?
The paper is Kipping, “The Cosmological Hart-Tipler Conjecture,” submitted to Astrobiology (preprint). The Ellery paper I refer to above is “Self-replicating probes are imminent – implications for SETI,” International Journal of Astrobiology, 21(4) (2022), 212–242 (abstract). The Armstrong and Sandberg paper is “Eternity in six hours: Intergalactic spreading of intelligent life and sharpening the Fermi paradox,” Acta Astronautica Volume 89 (August–September 2013), pp. 1-13 (abstract). The Olson paper is “Homogeneous cosmology with aggressively expanding civilizations,” Classical and Quantum Gravity Vol. 32, No. 21 (15 October 2015) 215025 (abstract).
On reading this article, a couple of beings belonging to a technologically advanced extraterrestrial civilization turn to look (or the exobiological equivalent) at each other and say:
ET1: “Why would we do that?”
ET2: “I don’t know.”
Occam’s razor says: false premise.
Papers like this, by design, cannot delve into the question of what it would take to build an actual real-world device, out of actual real-world materials. We have no idea whether it’s physically possible to create a probe that will handle the stresses of interstellar travel and overcome the copy-machine effect to keep the expansion going.
And no, you cannot just say that it would work if the probe was Sufficiently Advanced. Part of what we don’t know is if Sufficiently Advanced is possible!
I wonder, is Earth the results of infection?
The assumption is that it is not.
Interesting!
Maybe James Blish to that one first.
The Seedling Stars
https://en.wikipedia.org/wiki/The_Seedling_Stars
A wonderful memory, that one, Al. Thanks.
My thoughts exactly. What if we cannot detect the “infection” because we are the infection? What if our previous copy is expecting us to self-replicate to some other star? He, she, or it might be disappointed in our progress so far.
A measure of how difficult a project it is to populate an entire galaxy with any given intelligent species is the fact staring us in the face: nobody is here yet. It may in fact be an impossible task for any ETI, even allowing for machine assistance. Until we can conduct an in-depth survey of a significant portion of our galaxy we will continue to have no idea what may actually be going on. Let’s continue the discussion every thousand years or so. Unless you believe we are the ones that are going to accomplish it. Personally, I am at a location that is experiencing yet another heat dome which is primarily (in its intensity) due to human induced climate change. Yet somehow despite our inability to conduct our affairs in a sane, environmentally appropriate manner some may believe we are destined to become masters of our galaxy. Amazing hubris indeed.
“If self-reproducing probes can be built by even a species not yet at Kardashev Type 1 status, and if we are forced to say that it would only take one in inconceivably vast numbers of stars to produce a builder civilization of these probes, we are left with questions that are more perplexing that ever.
Where are they?”
It’s only perplexing when you make a whole bunch of assumptions about what “they” are and what they will do, without any data other than what is missing, then jump to either “where are they” with a perplexed look, or even further, we must be alone, perhaps in the entire universe.
The answer to the question of where are they, assumes we have some idea of what ‘they’ are, in other words technological and willing to create mindless destruction. The whole concept of a civilization building these probes is an extrapolation of our current technology, and it assumes that our understanding of the universe is complete, or almost complete, and assumes that our own destructive behavior won’t change as our civilization advances. The fact that we have no evidence of a single builder of these probes, just means either there hasn’t been a single builder of them (so far) or we haven’t detected them yet or no advanced civilization would ever do such a thing.
So when we look up at the stars, what do we expect to see if ETI is common? Shining megastructures, starships zipping about, star systems so transformed that they look artificial even to our eyes? Maybe our expectations are very wrong.
Whether self-reproducing machines can be built or not, life is certainly self-reproducing. Suppose rather than self-reproducing probes, a single probe deposits samples of life on each suitable world and then falls silent, exhausted after N worlds are seeded. That takes more resources, so perhaps only a fraction of systems is visited, perhaps in a feasible bubble of some average radius?
Humans have assumed that evolution leads to us. There is a certain “manifest destiny” implied by this, a march of evolution until humans are eventually arrived at. But that took perhaps 4 bny, during which most life was prokaryotic. For all but the last few tens of millennia, technology was largely absent, and life on Earth was not technological. Because we are here, that does not mean that this is a common evolutionary path. Perhaps it is premature, possibly hubristic, to believe we will be the explorers and perhaps the next seeders of the galaxy or the universe.
If human-level intelligence is not a survival trait, then perhaps there are many living worlds, but none are seeded; they naturally spawn life, which remains self-limiting technologically, i.e., a level that prevents global destruction of the species.
If so, when we look up at the stars, what we should expect to see are some systems with life on worlds, but none with advanced technology. Most planets will have only prokaryotic life, and a small percentage will have complex life. But none will have advanced to the stage where they are about to launch starships, which would be such a fleeting stage that we would be very lucky to detect any before they disappear.
Perhaps, as Stan Clark above implies, life on Earth might be the result of a seeding experiment, or possibly even by natural panspermia.
Are we an example of the “Rare Earth” hypothesis? I would go further and suggest that the “Great Filter” is still in front of us, and it is this filter that precludes the universe from appearing developed. This needn’t mean that humans become extinct, just that our civilization will be thrown back to a less technologically developed stage, and over time, our species will spawn new species in our clade, but that none will ever settle the galaxy, let alone the universe. We don’t even need some opposing force to keep us down, no predatory berserkers in the “Dark Forest”.
Are we so lonely that we must keep inventing reasons why there is no one else to party with?
One option is that we are in the infected zone, but we think of it as normal. A wild idea I can’t quite seem to shake is a suspicion that cosmic dust might contain a swarm of self-designing probes with hidden computational and VLBI capability. If not that, maybe some of the electromagnetic phenomena in our sun and other stars are capable of thinking. Those places are where almost all the mass, energy, and physical space is; Earth is less than a hood ornament by comparison. We look at a galaxy and see stars and dust.
Modeling only preclusive expansion is equivalent to assuming expansion must be preclusive or that preclusive expansion has a competitive advantage and displaces non-preclusive expansion. Allow stable boundaries between preculsive and non-preclusive expansions and the model’s only productive argument against observer selection effect, phase transition to virtually total coverage, evaporates.
To paraphrase Netwon’s formulation of Occam’s Razor; a model must be sufficiently complex and no more. Allowing for non-preclusive expansion forces our models to be far more complex. We would have to model variation in boundary dynamics between different modes of expansion, variation in the order modes emerge, modes transitioning into a different mode, etc.
Preclusive expansion passes a conceivability threshold and can be modeled to fill the universe. So what? Convince me it is the only game in town. Convince me that Deep Time and intelligence select for it.
It would be just awful to any Great creator if the fermi paradox was solved by interstellar lethargy…i.e. they could not be arsed to go out into the unknown !
Nick Bostrom posited a rock/paper/scissors trilemma based on the idea that humans are constructs in a computer simulation.
1. “The fraction of human-level civilizations that reach a posthuman stage (that is, one capable of running high-fidelity ancestor simulations) is very close to zero”, or
2. “The fraction of posthuman civilizations that are interested in running simulations of their evolutionary history, or variations thereof, is very close to zero”, or
3.”The fraction of all people with our kind of experiences that are living in a simulation is very close to one”.
———————— Questions/Thoughts
• Could the answer to “where are they?” be that the simulation—assuming we’re in one—didn’t include “extraterrestrial” constructs?
• I struggled with why he specified the simulations were of ancestors instead of the future which I would think be of more interest. But I realized that IF we are in a simulation, we haven’t yet invented a computer capable of creating the simulation. Hence we are in the Simulation-creator’s past history line. Still, why bother with the past?
• Is it possible the simulation will disallow the ASI singularity by throwing up Tri-Solarian science roadblocks that are insolvable puzzles —or— could the singularity be the key to the Escape Room and the simulated wool drops from our eyes and we meet our makers? (I don’t really know what that even means!)
• Or is it simulations all the way down because consciousness and computation are a “field” permeating space/time ala panpsychism?
—————–Freely-associated literary tidbits:
Douglas Adams’ “Hitchhiker’s Guide to the Galaxy”: the Earth and presumably its inhabitants, were built and rebuilt in a hyperdimensional planet/computer-building workshop. Ok, artificial, not simulated and the answer is 42 not 47 ;)
John Scalzi’s “Redshirts”: the crew of a starship resembling the Enterprise realize their reality and timeline are under periodic influence of a badly written television show, “Chronicles of the Intrepid,” from the past. It’s one of my favorite short sci-fi novels but it boggled my mind as to how it all worked. Spooky action at a distance? And it reminds me of the movie Galaxy Quest too.
Lem’s “His Master’s Voice”: communication via modulated neutrinos which may be actually be a cosmic life force.
MOST RELEVANT and not often mentioned: Sagan’s book “Contact” ends when Ellie examines the program’s output. She finds a circle formed from 0s and 1s after 10^20 digits in pi’s base-11 representation—evidence of her journey — and either a constructed or simulated universe.
————————
I doubt we are in a simulation but the chance is >0
—————– Across the unsimulated universe, I think:
• life is common
• sentience in such life, if not consciousness, is universal
• we are rare as a technological species
• the great filter still lies ahead
• it is meaningful to continue the search for signal/signs of off-world life
• either we or our ASI overlords will build self-replicating probes thus answering “Where are they?” for somebody
We still have not determined if it is common for biologically active planets (regardless of how common they may be) to remain stable long enough for their inhabitants to create a space-faring technology. Neither do we know if the societies capable of developing this tech don’t usually self-destruct even before they get an opportunity to deploy them.
The required milestones that lead to high tech:
0) Time, billions of years of relatively stable and benign planetary conditions.
1) highly complex multicellular biologies capable of the required anatomical sophistication to invent or study physics and mathematics
2) a planet with the physical resources to allow for the development of electronics, optics, metallurgy and all the other sciences needed to build space ships or radio telescopes
3) The organization and culture required to successfully recognize and avoid all the potential self-destructive and polluting consequences of rapid technological expansion.
3) A psychological component in the inhabitants that leads them to a compulsive obsession to study and perfect natural science and engineering.
We simply don’t know whether these characteristics are common or rare in the universe, we simply assume they (or some of them) must be enough like us to do these things, BUT THERE IS NO WAY WE CAN KNOW THAT. We, as tech-savvy inhabitants of this present space and time have decided THEY must be “just like us”. But we are the only known example, and even we haven’t yet actually achieved the ability to routinely conduct extrasolar communication and travel–we just assume we are on the verge of doing so.
We know we can get this far, because we did. But we don’t know if we are the first civilization to have gotten this far, or the last. That there are other cultures out there capable of the kind of technology we are discussing here certainly cannot be ruled out (we have one example!). But if such achievements are common or rare is simply something we cannot say.
Our speculations of what ETI must be like are created by individuals like us, and our intellects and logic have been selected for by the nature of our interests and hobbies (yes, we are all well-educated physical science nerds who read a lot of science fiction).
The fact we have no evidence we have been visited by extraterrestrials means absolutely nothing. The Fermi Paradox means nothing. The universe may be crawling with advanced civilizations, or we may be the only one. The truth is probably somewhere between the two extremes, but until we get more evidence, we simply have no way of knowing where.
We obviously haven’t been able to examine enough of the universe to have collected enough data to make any assumptions or estimates as to the distribution of other technical species. Until we do that, we have no justification to speculate on their capabilities and motivations.
We are space groupies, and our thinking is determined, to a very great extent, by our prejudices and interests. In spite of our familiarities with the sciences and history of our species, we are not really objective enough to speak authoritatively on these issues. To put it another way, if we want to ask about the nature of God, the last person you want to consult is a priest. He already has an agenda.