Centauri Dreams
Imagining and Planning Interstellar Exploration
What the Comments Taught Me: A Reply on Self-Replicating Probes
Self-replicating probes continue to be a controversial subject, just as they were when Frank Tipler up the ante on Michael Hart by invoking them as a way of further tightening the tension of the Fermi Paradox. After all, Tipler had discovered an economic edge. Any civilization that wants to colonize the galaxy is going to expend vast resources, but if self-replication is available, that culture need only create the first probe, and let subsequent ones harvest resources as needed. Self-replication or not, the galaxy gets filled up in only a fraction of the current age of the Milky Way, but the economic stimulus provides yet another tightening of the Fermi knot. I hadn’t thought about all of this in connection with actual probe designs, but Peter Marinko’s article on the matter clearly touched a nerve, judging from the messages I’ve been getting about it. When Peter wrote recently with his thoughts on reader reactions, I asked him for permission to run it as a regular post rather than a comment, because I think this is a lively question and would like to see us continue to explore it.
by Peter Marinko
My previous post here, “A Metallurgist’s Doubts About Self-Replicating Probes,” argued that von Neumann probes are constrained less by physics than by process-chain closure and materials aging. The discussion that followed sharpened my thinking more than the original post did, and I want to begin by paying some of that debt.
On carbon: Adam Crowl supplied the numbers I should have had at hand: carbonaceous chondrites run some 3–5% organics, dormant comets are thought to be coated in hydrocarbon-rich asphalt, and Freitas’s study considered Titan — an organics-drenched world — alongside the Moon. Alex Tolley added the wider inventory: aromatics, CO, CO₂, CH₄, hydrogen-rich giant atmospheres, and tholins — the reddish-brown organic polymers, first named by Sagan, that form when ultraviolet light and charged particles work on simple molecules like methane and nitrogen, and that coat Titan, Pluto, and many cometary surfaces.
I accept the correction. Carbon is not scarce in the cosmos. But availability is not accessibility: 3–5% organics dispersed through a chondrite is a feedstock concentration problem, and concentration is exactly the step that has no gravity, no water, and no atmosphere to help it. What the correction really did was promote carbon from an afterthought to a criterion — as the reader will see below, three independent process chains now demand it.
On refractories: Alex Tolley and John both pointed to the same escape: induction heating with magnetic levitation, melting metal without touching a crucible at all. This is a genuinely good answer, and I concede it for the class of operations it covers — melting and casting nickel-iron, which asteroids supply free of charge. But it is not free, and the price is paid in the currency this study cares most about. A levitated melt has its entire surface exposed, and it radiates as T⁴. A crucible is not merely a container; it is insulation. Remove it and the induction coil must continuously replace radiative losses that a lined furnace would simply have prevented. In an exergy ledger, that is a permanent tax on every kilogram melted.
Nor does levitation cover reduction. Extracting metal from oxide requires a hot, chemically aggressive, contained environment, and containment is where linings live. Here I owe the discussion a nuance from my own field that I should have raised myself: melting is not the only route. Iron oxide can be reduced in the solid state, producing iron powder which is then pressed and sintered to finished shape — this is how Höganäs in Sweden has made metal powder for decades, and it is how tungsten-carbide drill inserts are produced. Powder metallurgy skips the melt entirely, which is a real advantage for a probe: no crucible, no tapping, no casting. But it does not escape the problem. Solid-state reduction needs long tunnel furnaces, a reducing atmosphere of hydrogen, carbon, or carbon monoxide, and sustained high temperature — and those furnaces need linings too. The refractory bootstrap survives every route I know how to draw.
Image: PG: I sometimes wonder what John von Neumann would say if he could see the length and depth of the debate over self-replicating interstellar probes. To my knowledge, he never considered self-replication in the context of star systems and certainly not colonizing an entire galaxy. We could use his insights today: What are we missing? Credit: Physics Today, although this old photo is widely available and I don’t know its origin.
Abelard Lindsey, writing from inside the industry, put the honest bound on it: difficult, but perhaps a decade or two of development. That is a fair estimate, and it belongs in the ledger as a research task rather than a wall.
On semiconductors: Lindsey and I are in violent agreement: this is the hard one, and he notes it must be solved on Earth as well before AI becomes cheap. “Building chips rather than carving them,” as he puts it, is the holy grail. I have no better idea, and I am not going to pretend otherwise.
On the mother-factory: Tolley’s most interesting move was to propose a way around closure rather than through it: a factory ship that travels star to star, spawning non-replicating probes from local resources, populating the galaxy more slowly but without ever needing full self-reproduction. I want to be clear that this is a good idea — and also that it concedes my central point. A mother-factory does not close the loop; it carries the fraction it cannot produce, and expands until that stock is exhausted. It converts an infinite-generation architecture into a finite one, and the number of generations it buys is precisely the vitamin inventory divided by the vitamins per copy. That is not a refutation of the closure argument. It is the closure argument, written as a mission design. The right question becomes: how many nodes does the stock buy? That number is computable, and Hephaistos is built to compute it.
On life: The deepest challenge came from Henry Cordova and Elisee Reclus, and it deserves more than a nod, because it attacks my framing rather than my arithmetic: robust self-replicating systems demonstrably exist, cover the Earth, and arose without design. If biology can do it, why not machines?
Here is my answer, and it is the reason I am not moving the goalposts. Life is not a counterexample to the closure problem; life is what closure looks like when you pay its actual price. A cell does not manufacture bearings to micron tolerance, does not need vacuum, does not require phase-pure silicon or reference metrology, and above all does not need to specify its output. It tolerates enormous error, discards most of its offspring, and lets selection curate the survivors — over billions of years, in a medium (liquid water, at moderate temperature, with an atmosphere and a gravity well) that supplies concentration and transport for free. Biology bought replication by abandoning precision, determinism, and speed, and by spending geological time as its currency.
A von Neumann probe cannot make that trade. It must arrive at a specified place, build a specified artifact to specified tolerances, and do so in decades. The moment we relax those requirements enough for a biological strategy to work — accept vast error, accept mostly-failed offspring, accept deep time — we no longer have an engineering project; we have seeded a biosphere and lost the ability to say what it will become. That may be a defensible thing to do. It is not the thing anyone is proposing when they invoke probes crossing the galaxy in a few hundred thousand years.
So I take the biological objection seriously, and my conclusion from it is not optimism but a sharper statement of the problem: self-replication is cheap if you can pay in error and time, and murderously expensive if you must pay in precision and schedule. The exergy ledger below is an attempt to price the second option honestly.
Where this goes next:
Skepticism is cheap. The honest next step, for a critic who spent a career in industrial process engineering, is to try to make the thing work — on paper, with real process chains and mass balances — and see exactly where it breaks.
That is what I am now attempting, in a study I am calling Project Hephaistos, after the god who forged automata for Olympus: a virtual self-replicating probe, audited line by line, where every assumption and every capitulation is logged in public. Two ledgers run through it. The Vitamin List records every component the probe cannot make for itself, with masses attached. The Exception Ledger records every problem I have deliberately set aside. I expect the second document to be the more valuable of the two.
In a future post I will set out the mission architecture, and the first trade study: whether it is better to send a probe slowly, with today’s technology, and let it fight fifty millennia of aging — or quickly, at a tenth of light speed, and let it fight the interstellar medium. Both, it turns out, are running the same race against the same opponent. Only the costume changes.
My thanks again to everyone who wrote in. Keep it coming — the ledger has room.
A Metallurgist’s Doubts About Self-Replicating Probes
Frank Tipler jolted the astrophysics community in 1980 when he introduced self-replicating interstellar probes into discussion of the Fermi Paradox. The mathematical model of self-replication came from John von Neumann, and was codified in 1966 (after von Neumann’s death) by Arthur Burks in Theory of Self-Reproducing Automata (1966). SF fans will also know of Fred Saberhagen’s berserker novels and short stories (the first appeared in 1963). I’ve found an even earlier SF reference but will leave that for a future post. Right now I want to introduce Peter Marinko, who today weighs in on self-replication and the problems therein. Based in Uppsala, Sweden Peter holds an M.Sc. in metallurgy and has a career background in industrial process engineering. He has studied SETI under Erik Zackrisson at Uppsala University, and his current work explores the thermodynamics of technological civilizations — including a manuscript on high-exergy technospheres and the longevity of detectable civilizations, currently under peer review at the International Journal of Astrobiology. A preprint is available on Zenodo.
by Peter Marinko
Discussions of von Neumann probes — here and elsewhere — tend to treat replication as a systems problem: the probe arrives, mines local material, and builds a copy of itself. The hard part is usually assumed to be propulsion, navigation, or AI. As someone who has spent a career in metallurgy and industrial process engineering, I would like to suggest that the hardest part is the one that gets a single sentence: “mines local material and builds a copy.”
Let me raise four concrete problem areas, in increasing order of difficulty.
1. Beneficiation without gravity, water, or atmosphere
“Asteroid mining” is a misleading phrase. Mining is the easy part; the problem is beneficiation — concentrating useful elements out of undifferentiated regolith. Every terrestrial concentration process relies on things an asteroid lacks: gravity-driven sedimentation, water-based flotation, density separation in fluids, atmospheric combustion. Electrostatic and magnetic separation in microgravity are conceivable in principle, but neither has been demonstrated at industrial scale, and both work poorly on the fine, cohesive, electrostatically charged dust that dominates regolith.
2. Reduction metallurgy without an industrial hinterland
All terrestrial metal production rests on an invisible foundation: carbon or hydrogen as reducing agents, fluxes, and — critically — refractory materials for the furnaces. Refractories are the forgotten enabling technology of civilization. A furnace lining must itself be manufactured, at high temperature, in a furnace. Bootstrapping this loop from raw regolith, with fully closed chemical cycles (no atmosphere to vent to, no water to waste), is a chicken-and-egg problem that no study I am aware of has worked through at the level of actual process flowsheets.
3. The closure problem, honestly accounted
The classic NASA study (Freitas et al., 1980) assumed ~90–96% “closure” — the fraction of its own components a system can reproduce — with the remainder supplied as “vitamins” from home. But the missing few percent are not marginal; they are precisely the hardest items: semiconductors, precision bearings, sensors, and insulation. Consider something as unglamorous as wire insulation. Virtually all electrical insulation on Earth is organic polymer, resting on a petrochemical industry, resting in turn on a biosphere that spent hundreds of millions of years concentrating carbon. Inorganic alternatives (glass fiber, ceramics, mica) exist but are brittle, heavy, and require entirely different process chains to apply to fine conductors. A modern semiconductor fab is arguably the most complex artifact humanity has built, drawing on tens of thousands of specialized inputs. Shrinking that into a 500 kg seed — or even Freitas’ original 100-ton seed — is not an engineering detail. It may be the entire problem.
4. Aging over interstellar timescales
Even a probe that could replicate must first arrive functional after a voyage of tens of thousands of years. We have essentially no empirical data on machine longevity beyond ~50 years (Voyager, surviving on redundancy and switched-off instruments). Over interstellar timescales, materials face cumulative radiation damage and lattice defects, embrittlement and transmutation; creep and solid-state diffusion (solder joints, thin films and interfaces are only kinetically frozen, not thermodynamically stable); tin and zinc whisker growth; outgassing and cold welding in vacuum. The repair systems age too. Replication must outrun degradation — and degradation never sleeps.
A thermodynamic framing
These four problems share a common structure. A self-replicating probe is, in effect, a miniaturized high-exergy technosphere that must rebuild its entire exergy cascade — from raw, unconcentrated feedstock to precision components — at every node, before its own irreversible degradation catches up. The feasibility question is then not “does physics forbid it? (it does not) but “can accessible exergy per node sustain full process closure faster than irreversible losses accumulate?
This is the same ratio, I would argue, that governs the longevity of detectable civilizations generally — a question I explore in a recent preprint on the thermodynamics of technological civilizations. But the probe case is a cleaner test, because the system boundary is sharp and the accounting is (in principle) tractable.
Questions for discussion
1. Has anyone attempted an actual process flowsheet — not a block diagram — for closing even a simple metallurgical loop (say, iron from chondritic material to finished machine parts) without terrestrial inputs?
2. Is there a credible inorganic-only pathway for electrical insulation and semiconductor packaging?
3. What is the realistic closure fraction if “vitamins” are disallowed — and does the seed mass then grow beyond anything launchable?
4. Are there materials strategies (amorphous metals? self-annealing designs?) that could plausibly survive 10,000+ years of transit?
My suspicion, as a practitioner, is that von Neumann probes are constrained not by the laws of physics but by process-chain closure and materials aging — both, at root, thermodynamic limits. If that is right, it bears directly on the Fermi paradox: the galaxy may be quiet not because nobody tried, but because replication is harder than arithmetic suggests.
I would be glad to be proven wrong on any specific point above — ideally with a flowsheet.
New Horizons: Pushing Toward the ‘Termination Shock’
Mission planning for any future star probe will adjust not only for conditions in the interstellar medium but also the Solar System’s outer reaches. Let’s confine ourselves for now to conditions in the outer heliosphere. Currently we have precisely one spacecraft operating here. New Horizons has only reached 65 AU from the Sun, while Voyager 1 exited the heliopause in 2012 at 121 AU, and Voyager 2 crossed in 2018 at about 119 AU. New Horizons won’t have sufficient power to keep taking data as it makes its own crossing in the 2040s, but from its current position in the Kuiper Belt we can look back at what the spacecraft has reported so far about the solar wind and the local interstellar medium.
New Horizons’ Solar Wind Around Pluto (SWAP) instrument is the key here, examining how the solar wind slows as we leave the inner system behind. A new study from Southwest Research Institute (SwRI) points out what happens as this stream of hot ionized hydrogen and helium nuclei fills the heliosphere. The wind’s speed varies, some 300 to 500 kilometers per second from sources near the solar equator and up to 600-800 km/s from regions near the corona.
You would expect this ‘wind’ to cool as it begins to push against the interstellar medium, and indeed it does, forming the termination shock that both Voyagers have penetrated and crossed, and toward which New Horizons now moves. It’s at the termination shock that we see a sharp drop in the solar wind speed that indicates the outer boundary, the heliopause, is approaching. New Horizons should still be functional when it reaches the termination shock, conceivably as early as the end of this decade. Voyager 1 found it at 94 AU, Voyager 2 at 84 AU, reminding us how malleable the heliosphere is as its outer boundaries adjust to the onset of interstellar plasma.
Image: An SwRI-led study sheds light on the deceleration of the solar wind as it journeys away from the Sun and interacts with and picks up interstellar material. NASA’s New Horizons spacecraft measured the solar wind as it traveled from just beyond Uranus’ orbit into the outer Kuiper Belt (red shaded region), detailing the gradual slowdown caused by interactions with interstellar materials (red line). Credit: SwRI.
We can learn a great deal as we accumulate data on solar wind interactions in the outer heliosphere. SwRI’s Heather Elliott led the study. Says Elliott:
“Eventually, the solar wind reaches the outer boundaries of the heliosphere — the sphere of influence where the solar wind affects the space environment — where it interacts with incoming interstellar material. The shape and properties of these heliospheric boundaries control the amount of Galactic Cosmic Rays (GCRs) that can enter our solar system and reach Earth. Therefore, the data from New Horizons combined with observations from other missions, such as IBEX, IMAP and Voyager will enhance our understanding of the edge of the solar system.”
So far, the data have been useful as New Horizons keeps moving outward. Along the way, the solar wind begins to run into neutral gas particles that have entered the heliosphere from the outside interstellar medium. The interaction with the solar wind, in which these atoms become ionized, adds mass to the solar wind, Elliott adds. And that is the mechanism for slowing the wind down.
In previous years, we have learned that between 30 and 43 AU, the solar wind has slowed 5 to 10 percent in comparison to its value near Earth. This is from data not only from New Horizons but also Voyager 2. Assuming New Horizons is still operational when it hits the termination shock, we would expect to see a sharp drop in the speed of the solar wind. In fact, Voyager 2 found a 46 percent drop in speed at the termination shock at its distance of 84 AU.
And note this from the paper:
The drop in speed in the Voyager 2 TS [termination shock] measurements was dramatic. At the Voyager 2 TS crossing, the speed went from ∼320 down to ∼140 km s−1 a few days after the crossing, corresponding to a ∼ 56% speed reduction across the TS (J. D. Richardson & E. C. Stone 2009). A sudden speed drop of 56% would be large and steep enough to readily confirm that NH crossed the TS. Unlike Voyager 2, the SWAP instrument on NH also measures interstellar hydrogen pickup ions, such that the modification of the TS by the interstellar pickup ions will be measured at the upcoming NH TS crossing.
As a sidenote, it’s worth remembering that there is no clear boundary here. Indeed, the shape of the entire heliosphere flexes and churns in response to ambient conditions and thus is partially dependent on the clouds of interstellar material the Sun is moving through at the time. At present, we are in the whimsically named ‘Local Fluff,’ part of the Local Interstellar Cloud, and near or perhaps already edging into a region called the G-Cloud, a prominent citizen of which is the system called Alpha Centauri. In any case, we’ve learned from the IBEX (Interstellar Boundary Explorer) satellite that the interactions on the heliosphere paint a picture of a dynamic, changing shape as opposed to the smooth ‘bubble’ that is often depicted in artist renderings of the heliosphere.
IBEX and its successor satellite IMAP (Interstellar Mapping and Acceleration Probe) carry an interesting message of their own: We can continue to learn without having an actual set of instruments on the scene. In sharp contrast to New Horizons, these two spacecraft work by remote sensing, detecting energetic neutral atoms (ENAs) produced in the interaction of the solar wind with neutral atoms at the heliopause. So we have one satellite in a highly elliptical Earth orbit (IBEX) and another at the L1 Lagrange point, both of them helping us to understand conditions at the termination shock and beyond.
As Elliott pointed out in that first quote above, conditions in the heliosphere’s boundary with the LISM matter if for nothing else because of the dangers posed by Galactic Cosmic Rays (GCRs), leading to issues of spacecraft design both for manned as well as unmanned missions. It’s good to know that New Horizons is on the case and will remain so, but for how long? What I’m hearing is that the spacecraft’s Radioisotope Thermoelectric Generator (RTG) should be able to keep observations and return of data robust through the end of this decade, but as with the Voyagers, we’re moving toward the end of active life.
What will replace our one source in the outer heliosphere? The need for resources in and beyond the Kuiper Belt should have us moving toward mission designs and propulsion options that go beyond chemical methods. Sail missions like the Solar Gravitational Lens mission now being developed at the Jet Propulsion Laboratory continue to intrigue me, particularly as we begin to explore assembly options enroute to deliver the largest possible payload. We will need precursor ‘sundiver’ missions as we test out these technologies.
The paper is Elliott, “The Gradual Slowing of the Solar Wind in the Outer Heliosphere,” The Astrophysical Journal, Vol. 1001, Number 1 (3 April 2026). Full text.
The Intergalactic Fermi Problem
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).
HD 39474: A Brown Dwarf Shapes a Planetary System
With well over 6,000 exoplanets now confirmed and a continuing flow of data containing new detections, it has been clear for some time that our own Solar System’s model is hardly a template. I enjoy dipping into the bewildering variety of new systems and pondering the contingencies that have led to their architecture. Science fiction is an intensely visual genre, so I naturally try to imagine the more extreme systems. But more than most, today’s catch at HD 39474, an F-class star in Pictor some 360 light years out, is just begging for a gifted SF writer to go to work on it. Here we have, in addition to the central star, a long-period transiting brown dwarf with a planetary system, coplanar and aligned with the brown dwarf, packed inside its orbit.
HD 39474 is also, at least for now, known as TOI-201, TOI standing for TESS Object of Interest, an indication that while the Transiting Exoplanet Survey Satellite’s photometry has found what looks like a planetary transit, that result has not yet been confirmed. Various things can mimic a transit, including stars in an eclipsing binary system, so confirmation through radial velocity methods or additional transits is necessary. Nonetheless, a new study in Nature looks solid, and the system it points to is of exceptional interest. The work describes a ‘mono-transit’ in TESS data sets that is tentatively identified as a massive brown dwarf designated TOI-201c.
A single transit can indicate a planet or brown dwarf whose orbit greatly exceeds the observational period, which is why such a transit is not enough to confirm the detection. But there is a lot more going on here. In fact, according to Alessandro Sozzetti (INAF-Astrophysical Observatory of Turin), TOI-201c has been characterized by transit timing variations of an inner planet as well as the photometric transit and radial velocity measurements, with upcoming confirmation through GAIA astrometric data. Being characterized through four different methods appears to be a first.
The work, led by the European Southern Observatory (with strong involvement from Italy’s National Institute for Astrophysics (INAF) reminds us of the blurred star/planet distinction. Brown dwarfs can have planetary systems of their own, warmed by their exceedingly faint light. TOI-201c has the longest orbital period, some 2,881 days, for which a mass has been confirmed, in this case through radial velocity readings.
Within the brown dwarf’s orbit are two further transiting objects that are aligned with it. Getting into the dynamics of system formation here is going to be interesting work. TOI-201d has a period of 5.8 days and appears to be a rocky super-Earth, while the gas giant TOI-201b is in a 53-day orbit. With an orbital eccentricity of 0.622, the brown dwarf is a significant perturber. According to the researchers, anything much farther from the star than the orbit of Mars around the Sun would be dynamically unstable.
Luca Naponiello (INAF), second author of the paper on this work, takes note of the brown dwarf’s impact:
“The presence of the brown dwarf on such an elliptical orbit forced the planets to form and survive by occupying the innermost and hottest edges of the primordial disk. Furthermore, the data show that during the close approach of the brown dwarf, the warm Jupiter undergoes strong and sudden variations in its transit timing, bearing witness to an intense and vigorous dynamic interaction currently underway between the two giants,”
Image: Close-up artistic representation of the TOI-201 system. In the foreground is the massive brown dwarf TOI-201 c, followed by the hot Jupiter TOI-201 b (subject to strong gravitational perturbations), the star TOI-201, and finally the super-Earth TOI-201 d. Credits: INAF / generated with AI Gemini.
ESO spectography from its FEROS and PLATOSPEC instruments complemented the TESS data to offer up this extremely stressed system, which makes the case that even in environments as challenging as these, planets find a way to form. How long they last is another question, and I assume future work may give us some thoughts on the survival of the gas giant here. In any case, finding an inner gas giant in these circumstances draws into question theories of gas giant formation that assume distances beyond several AU from the central star. We should be hearing a lot more about the system at TOI-201 given the stress it puts upon earlier formation models.
The paper is Jones et al., “A distant brown dwarf coplanar to a warm Jupiter and a hot super-Earth,” Nature 654 (17 June 2026), 614-618 (abstract).
The Physics of Interstellar Travel
Coryn Bailer-Jones’ The Physics of Interstellar Travel fills a need which has become apparent only in the last twenty years. Indeed, going back to the turn of the century, one would find the idea of traveling to another star discussed only in relatively isolated pockets, often presented at the tail end of conferences devoted to other astronautical topics. Papers, though, were being written at an increased rate, building on early work begun in the 1950s through the efforts of luminaries such as Les Shepherd and Eugen Sänger and continuing into the era of Robert Forward. By the year 2000, a number of mission designs had been created, still very much on the back burner but of high interest to specialists.
In today’s landscape, interstellar travel has become a vibrant topic. The wave of interest that energized the field incorporated high-visibility projects like NASA’s 100 Year Starship and in 2016, the emergence of the Breakthrough Starshot Initiative, which focused directly on the design of a probe that could reach a nearby star, presumably Proxima Centauri, within a human lifetime. Public interest in starflight has likewise been galvanized by the fast pace of exoplanet discovery, and growing attention to the question of studying such worlds through actual missions. Cementing the enthusiasm has been a stream of Hollywood depictions that offered viewers enticing imagery of such journeys.
The surge in papers discussing interstellar flight has exposed the lack of a college- and graduate-level treatment, a textbook wholly devoted to this topic. The Physics of Interstellar Travel meets that need with the precision of a key clicking home in a lock. It is a thoroughly researched analysis that presents travel to a star within the context of known physics, validating the perception that such journeys are within the realm of future engineering. Early chapters on orbital mechanics and the mathematics of rocketry illustrate the fact that each section can stand on its own in specialized classes at higher levels, while the quantitative analysis offered here will be of use to any student who has mastered college physics and is ready for the next educational step.
Although Bailer-Jones accepts the idea that star travel violates no physical laws, he is careful to acknowledge the challenges that emerge and the direction of future work that will eventually meet them. The principles of rocketry lead him to present fusion and beamed lightsail concepts as the likeliest paths forward, with the clarity provided by mathematical analysis applied to options including ion engines and antimatter. The nature of the interstellar medium is considered in terms of dust mitigation as well as the possibility of ramjet solutions. Communications and navigation receive thorough treatment but so do the essentials of orbital mechanics and relativistic motion.
The Physics of Interstellar Travel is, in short, a comprehensive extension of current textbooks in astronautics into the realm of missions once thought to be impossible. This book’s mathematical rigor should clarify for rising students the realization that steps we take today can result in practical outcomes, with the goal of reaching another star conceivable by the end of this century. Bailer-Jones advocates a step-by-step approach in which precursor work always tests new ideas to avoid the problem of future missions making earlier ones obsolete before they have reached their target. Where science and engineering have not yet taken us, this textbook illustrates the direction of steps forward, aiding the community in the construction of the needed roadmap.
