SETI: Looking for von Neumann Probes

by Paul Gilster on October 2, 2013

In a recent paper outlining a novel strategy for SETI, Michael Gillon (Université de Liège) makes a statement that summarizes what Robert Forward began saying back in the 1970s and even earlier. Interstellar flight is extraordinarily difficult, but not beyond the laws of physics:

Our technology is certainly not yet mature enough to build a probe able to reach one of the nearest stars in a decent time (i.e. within a few decades), but nothing in our physical theories precludes such a project. On the contrary, the constant progress in the fields of space exploration, nanotechnology, robotics and electronics, combined with the development of new possible energy sources like fusion reactors or solar sails, indicate that interstellar exploration could become a technological possibility in the future, provided that our civilization persists long enough.

That last issue about the survival of our society is the L variable in the Drake equation, referring to the lifespan of any technological civilization. We don’t, alas, have any idea what its value is, which means we have no assurance going forward that any civilization can expect to have a lifetime long enough to explore the stars. Finding another functioning technological society would give us hope that such entities don’t necessarily destroy themselves.

But Michael Gillon is after other game in this new paper, titled “A novel SETI strategy targeting the solar focal regions of the most nearby stars.” After running through the Fermi question and noting that self-replicating interstellar probes of the kind posited by John von Neumann could fill the galaxy within, at most, hundreds of millions of years, Gillon asks whether any such probe in our own Solar System would be detectable. He’s interested in the question of communications and invokes the Sun’s gravitational lens, first discussed by Von Eshleman in terms of astronomy and richly examined by Claudio Maccone, as the key to how any interstellar probes would communicate.

Just how big a difference the use of a gravitational lens could make is outlined in Maccone’s Deep Space Flight and Communications (Springer, 2009) and an earlier paper (see The Gravitational Lens and Communications). Bit error rate, a measure of the quality of a radio signal, becomes problematic (to say the least) if we try to communicate with a probe at Alpha Centauri with existing technologies like the Deep Space Network. But a communications relay at the Sun’s gravitational focus — 550 AU and beyond, depending on the wavelength we want to work at and the need to avoid the Sun’s coronal effects — would radically improve the situation.

In fact, Maccone has shown that at 32 GHz, the combined transmission gain brought by using this kind of link between the Sun and Alpha Centauri is 1016, making it possible to communicate with such a probe using only low power transmitters. A mere forty watts of transmitting power produces an all but flawless bit error rate, and the situation improves even more radically if we assume a relay at Alpha Centauri’s own gravitational lensing distance. Flawless communications then become possible at a power of less than 10-4 watts using the two 12-meter spacecraft antennae Maccone plugs into his assumptions.


Image: Pulled from the cover of Claudio Maccone’s book, this image shows his proposed FOCAL probe with antennae deployed, ready to take advantage of the Sun’s gravitational lensing.

But assuming a communications relay somewhere between 550 AU and 1000 AU in our own Solar System, the work of a self-reproducing interstellar probe, how could we go about finding it? Gillon looks at traditional techniques of optical imaging and stellar occultation but finds that they would probably not be able to turn up so small an object — he assumes an antenna with a diameter of no more than one or two dozen meters — so he suggests looking for ‘leakages’ in any traffic between the two systems (he uses Alpha Centauri purely for illustrative purposes):

Attempts to detect the hypothesized ICD [interstellar communications devices] can still be performed now, basing on the very purpose of the device: not only to receive messages from Alpha Cen, but also to send messages to Alpha Cen and to one or several probes orbiting the Sun. An intense multi-spectral monitoring of the focal region of Alpha Cen with, e.g., the Allen Telescope Array, could in principle detect some leakages in these communications, depending on the used technology, communication frequency, and emission power.

Underlying the search is the hypothesis that self-reproducing probes would be unlikely to communicate with their original stellar system, wherever it happened to be. More likely is that communications would be networked among nearby stars. Gillon goes on to say:

A communication strategy based on direct connexions between neighboring systems would be a much better solution, with the extra-benefit that the information gathered by probes would be spread among their whole network, without any loss even in case of collapse or migration of the original civilization. The first part of our hypothesis is thus that the envisioned probes would use this direct communication strategy.

A galaxy fully colonized by self-reproducing probes should, in Gillon’s view, produce an interstellar relay in the focal region of at least one nearby star, leading to a series of SETI searches looking for incidental radiation from these devices. It’s an interesting notion (and I am much in favor of fully exploring the gravitational lens and its implications), but it’s hard to see how a civilization able to build interstellar probes and the communications tools to support them would be unable to shield its technology from detection if it chose to do so.

The paper is Gillon, “A novel SETI strategy targeting the solar focal regions of the most nearby stars,” accepted for publication in Acta Astronautica (preprint).


Tulse October 2, 2013 at 10:21

Given that the point of using solar focal regions is that they greatly decrease the power needed for communication, I can’t see how detecting any leakage of that power is at all likely. In the case of Alpha Centauri if “flawless communications then become possible at a power of less than 10-4 watts using the two 12-meter spacecraft antennae”, what would be the power of the leaked signal that we would expect to detect?

Solar focus communications seem analogous to laser communications to me — it provides relatively strong signals when aimed directly at the receiver, and very little otherwise. I think we’re more likely to detect omnidirectional broadcast EM (perhaps unintentionally emitted) than we are this kind of emission.

Rob Flores October 2, 2013 at 11:35

I think it’s a good idea to step back from the actuall VN probes and concentrate on the motivations/impetus of any ETI in creating them.

If they have an ounce of sense they will try out the Probe systems in simulators. In a honest test, ladden w/ unforseen (to the designers of the probe, not the simulation programmers), if it can be shown that you can
get a disastrous results from a replicating swarm. That civilization might
choose to scale back the goals of the probe. I can see setting limits ourselves when we gain the tech to produce VN probes.

One you launch the probes you cannot assume that whatever shielding/cloaking systems will 100% undetectable to all civs the probe
encounters, and obcourse give away the fact the VN probe launching civ is somewhere in the galaxy. Esp a very Jr. and naive one.
But this is besides the point, a trully advanced
civ would not need complicated travelling probes, Once it knew it was the most advanced and had no rivals, it would likely coat the galaxy with low cost small entities that could self-repair and be eons lasting. There would not be a significant development enviromental/emerging setience/ETI’s arising in the 200+ billion stars of the Mway that they would miss. And
It would not take too much effort to keep tract of them as even in 200+ billion stars. For an advanced civ keeping tabs on all the stars of the M-way with 1,000 years interval in every report would be quite tractable. And once ETI’s where detected it would show it’s hand Kill/Befriend/Study.

Ron S October 2, 2013 at 16:57

As Tulse says the opportunity to detect RF spillage is very poor. The author recognizes this (in the paper) then goes on to suggest attempting to transmit to it (METI). Except that communications links are reciprocal and, of course, we have no idea where to aim. The author also notes that the visual magnitude of the attached sail is below detection limits with existing or planned instruments.

Another oddity is in the calculation of the solar sail to be used for positioning the station. The calculation is done for radiation intensity at 1 AU, yet the station is at 1,000 AU. That’s not going to work.

torque_xtr October 2, 2013 at 17:16

The destruction of a civilization may be easy, but it is likely much harder to render the tech-capable species themselves extinct. The current high-tech society may be fragile, but blow all the nukes in the centers of the cities, or above the clathrate deposits in Atlantic, smash it with ten-km-asteroid, boil with 10-degrees global warming (the poles remain cool!) – you’ll never kill all humans with anything smaller than Chicxulub impact. The most will surely perish, but some will always survive. From all that billions, the hardy ones that manage to adapt, survive on the insects and carrion, and then live and breed through new stone age would be counted at least in many thousands. And if the bottleneck is behind, a new hi-tech civilization would almost certainy emerge again, a hundred thousand or a million years later – that doesn’t matter on the galactic timescale.
So, how does this affect L-parameter?
What is the L parameter

Rob Flores October 2, 2013 at 18:14

You might not kill all Humans/ETI’s on a planet, but you will leave them at the mercy of the natural forces of the universe. Comets, Meteors, Novae. It takes a lot luck for rising Species to not be intercepted by those forces BEFORE space travel develops. Humans have existed for 40-50,000 years it is not a given that techology is a quick outcome of intelligence arising.

Astronist October 2, 2013 at 19:23

The point of the probes being self-reproducing is presumably so that when one probe arrives, it can send out daughter probes to other stars. But as you noted, interstellar flight is extraordinarily difficult.

Why therefore does Gillon think that small communications systems at the Sun’s gravitational focus (over four times the current distance of Voyager 1) would be easier to detect than the large-scale industrial infrastructure in the inner planetary system that the probe would need to construct and especially to fuel daughter probes?

Stephen A.

CharlesJQuarra October 2, 2013 at 21:09

A stable propellantless orbit at any distance from the Sun (1 AU, or 550 AU, doesn’t matter) requires the probe to have an average aerial density on the miligrams/square meter. A graphene wisp-like sail could do the job:

If the aerial density is higher than that, then the probe needs to be stabilized from a laser coming from the inner solar system.

Probably an advanced civilisation will have no trouble in making a fully functional statite probe. However, maybe they will still find convenient to beam power from the Sun.

How close have we look in the inner solar system? have our observations focused only on the ecliptic plane? how is our monitoring near Mercury orbit at polar orbits/off the ecliptic?

Michael Gillon October 3, 2013 at 3:01


In my hypothesis, the device at the solar focal point of a nearby star is a communication relay that does not only sends/receives interstellar messages through the gravitational lensing technique, but also sends/receives messages to/from the probe(s) exploring our solar system. Thus it could send frequent messages to a probe located in the inner solar system, and we could attempt to detect them.

To Ron S:
This is the whole point of this SETI strategy, we know exactly where to aim, at the solar focal regions of the most nearby stars.
Concerning the calculation of the require solar sail. Gravity and radiation pressure both decrease as the inverse square of distance, so if for a given sail, the force due to radiation pressure equals the solar gravity at 1 AU, it will be the same at 1000 AU.


NS October 3, 2013 at 4:24

Re high-tech civilization re-emerging, it’s been pointed out here and elsewhere that the second time around would be much more difficult than the first. Not only would the survivors of the catastrophe have to deal with a world that was much harsher (and sparser) biologically, but the earlier civilization would have already used many resources that a second industrial civilization would need to get started. Without easily available coal, iron, and crude oil it’s hard to imagine how it could happen.

David Cummings October 3, 2013 at 12:12

NS, what is commonly envisioned in a “second start” is a start from all the way back at stone age. Such a second start would indeed be more difficult than the first start, since as you say the easily available coal, iron and oil are all gone (although I would argue that the iron is actually more available than before, in the form of scrap, than it was originally as ore).

But what if the “second start” is from this scenario:

A devastating war wipes out 99% of humanity and even 99% of all big city structures, but still leaves 1% of humanity alive and — and this is the key thing — huge libraries of printed books still available in suburban or rural libraries.

In that scenario, even if industrial capacity and power generation are reduced to zero, the economy essentially smashed back to barter levels, and the civilization for all intents and purposes stopped… the survivors will still have a big leg up on the first start, just by virtue of the existence of all those books. And that doesn’t even consider the survival of at least some gadgets, whether functioning or not.

In other words, I think a disaster scenario in which all knowledge survives is more likely than one in which no knowledge survives. And if you have all the knowledge of the previous civilization, it doesn’t much matter if you have to dig deeper to get at the coal.

In my opinion.

Ron S October 3, 2013 at 15:34

Michael, thank you for the clarification.

Assuming you only need to compensate for A Cen focal point motion you can remain inertial, once compensating for solar gravity as you say. It’s limiting but possible. Just be sure you have enough surplus motive force to, say, adjust for unexpected encounters with KBO if the probe must operate for some years.

Wojciech J October 3, 2013 at 17:48

Mass self-replicating probes are unnecessary if you have highly advanced hypertelescopes. Alien civilization would know precisely where to aim their probes in search of life, and there would be no need for dangerous experimentation with self-evolving machines beyond contact and control from originating species.

Eniac October 3, 2013 at 20:11


Without easily available coal, iron, and crude oil it’s hard to imagine how it could happen.

Coal and oil were not in use until the last few centuries of the rise of man. Wood, as a renewable resource, does not suffer from permanent depletion, and has been a mainstay fuel in the many millenia before. Even if coal and oil were no longer available (which isn’t true), wood could be carried much further than it has been. Then there is water, sun, wind and geothermal energy to pursue alternative paths. Iron, as has been pointed out, is much more easily won from car wrecks and structural steel than from ore. As has also been pointed out, there will be plenty of information available, saving a lot of hard work and time in discovering and inventing things.

All this leaves very little left of your argument, and I have to respectfully disagree. Completely.

Eniac October 3, 2013 at 20:28


Why therefore does Gillon think that small communications systems at the Sun’s gravitational focus (over four times the current distance of Voyager 1) would be easier to detect than the large-scale industrial infrastructure in the inner planetary system that the probe would need to construct and especially to fuel daughter probes?

An excellent argument, which for me renders the chance of finding something extremely slim even before considering the difficulties of detectability.

You would have to assume that the probes have stopped reproduction, cleaned up thoroughly after themselves, and gone into hiding. Communication would be the only active operations, presumably in the service of building and maintaining the galactic internet and collecting information for Galactopedia. There must also be dormant “seeds” stored somewhere in the solar system, ready to revive the full infrastructure when necessary to regenerate the hardware and avoid the otherwise inevitable decay.

Possible, but a bit far-fetched, in my opinion.

Rob Flores October 4, 2013 at 11:57


“Coal and oil were not in use until the last few centuries of the rise of man. Wood, as a renewable resource, does not suffer from permanent depletion, and has been a mainstay fuel in the many millenia before. Even if coal and oil were no longer available (which isn’t true), wood could be carried much further than it has been. Then there is water, sun, wind and geothermal energy to pursue alternative paths. Iron, as has been pointed out, is much more easily won from car wrecks and structural steel than from ore. As has also been pointed out, there will be plenty of information available, saving a lot of hard work and time in discovering and inventing things.”

Eniac, you miss the entire point. it takes a large population to support the
Rise of High technology. Crippling humanity’s tools to expand food production and industrial production, will lead into a very protracted
dark age.

It took an advanced large advanced society, a substantial portion of it’s productivity to go to the MOON even. Matter of fact it was so expensive that we do not have the social impetus to go back. Now try selling a moon
shot to a de-populated land which has the elemetary knowledge, but by the way you we will have to ration your goods to try this stunt. Pure Fantasy

ljk October 4, 2013 at 14:19

The long, dark search for E.T. (and for ourselves)

By Amanda Alvarez Published October 03, 2013 Inside Science News Service

There’s nothing quite as simultaneously awe-inspiring and humbling as gazing at the starry sky and coming to terms with your own fleeting role in the cosmos.

Science journalist Lee Billings sets the stage for his first book, “Five Billion Years of Solitude,” with this very thought, describing how he, and many of the scientists he interviews, first fell in love with the heavens. But it is a love letter to a place we may never reach. As our telescopes become ever more powerful, Billings writes, the universe appears to be receding before humanity’s outstretched hands, while the pressing problems of life on Earth draw our gaze, and our ambitions, down from the skies.

“Solitude” is a “meditation on humanity’s uncertain legacy,” as the 20th century’s space race and boom years have given way to manmade terrestrial crises that have not only hampered space exploration, but made clear how the only life we know hangs fragilely in the balance. Billings literally brings the stars down to earth, as he connects the dots between geology, biology, astrophysics, engineering, and economics. Fracking, it turns out, has an awful lot to do with searching for E.T. with radio telescopes. Single-pixel measurements of the chemical “color” of alien planets’ atmospheres can tell us a lot about their ability to harbor life, and can also inform us about where our own planet came from – and where it’s going.

Much of the discussion in the book centers on habitability – what makes Earth unique in the solar system and (so far) the galaxy, how planetary conditions have changed, how it will all end billions of years from now (cooked alive by an engorged Sun, followed by darkness and nothingness), and how we can predict the number of other civilizations there might be out there using what is known as the Drake Equation.

The equation’s many terms, Billings explains, can be boiled down to just one: L, or a civilization’s longevity. Possible outcomes seem to be one of two extremes: a (cosmically) relatively short-lived civilization that may succumb to self-annihilation, or a civilization that transcends its squabbles, its planet, and itself, harvesting the energy of entire stars as it travels through the universe, near-immortal.

It is no accident that Billings here carefully dwells on the orchids raised by Frank Drake, a giant in the search for extraterrestrial intelligence, or SETI. Tended correctly, these flowers can live in perpetuity, yet each individual bloom is short-lived, much like the radio frequency visibility window of our planet, which is now largely closed thanks to the adoption of digital communications and fiber optics.

The radio telescope-based search for extraterrestrials, once fueled by the optimism of Drake and the late Carl Sagan among others, has given way to the current en vogue field of exoplanetology, which seems poised to discover habitable Earth-like worlds any day now. That is, says Billings, if it weren’t for infighting, shifting organizational and funding priorities, and other failings that make us human. The dust jacket description and introduction hype this fraught narrative, which the rest of the book doesn’t quite fully deliver. The cutting-edge climate science, optics, and chemical detection techniques being used by the exoplanet hunters, however, are described in thorough and clear detail.

Billings oscillates between character-driven chapters – the personal histories, egos, and rivalries of prominent scientists – and longer narratives on the geologic history of earth and the cosmos. At times “Solitude” reads like a eulogy for the SETI titans of the 1960s and 1970s, while expressing tentative hope for both the current exoplanet boom, and our collective will to keep searching. Space dreams are continually brought back to their roots in earth science; a fairly large chunk of the book is devoted to fostering an appreciation for the “interactions of air, water, rock and sunlight” that created the thermodynamic sweet spot of Earth.

In “Solitude,” Billings uses deft descriptions and dazzling wordplay, though at times the language can appear dense. One chapter in particular is littered with a few too many acronyms to keep track of: a seemingly endless list of ambitious, bloated, and consequently shuttered projects that suffered from the downturn of the early 2000s. The glories of the Space Age are briefly revisited, and those familiar with SETI history will recognize seminal events in the field – the Green Bank conference, the Arecibo message – but will also note the absence of some of its most well-known figures, like former SETI Institute director Jill Tarter.

The timing of “Solitude’s” October 3 release couldn’t be better. Not only does there appear to be renewed public interest in space, with the success of the Curiosity rover, the confirmation of the Voyager 1 probe’s solar system exit, and the impending launch of the James Webb Space Telescope, but one of the book’s protagonists, MIT astrophysicist Sara Seager, just last week received a MacArthur “genius grant.” Seager is introduced relatively late in the book, and in describing her path from canoeing the barren lands of Canada to the study of the barren cosmos, Billings indulges in a triumphalist crescendo that rounds out the book.

The big question – what’s next, not just in space but here on Earth – is, of necessity, left unanswered, as it is unknown to scientist, author, and reader alike. Rather than rousing spirits and making a grand call for renewed vigor in space exploration, “Solitude” succumbs to a denouement similar to that of the shuttle program it laments. The descriptions of setbacks, ignorance, and death are not gratuitous, though. Billings knows that it is only through meditating on these that we can seize this singular moment in human history and become “momentarily eternal.”

Jeff Wright October 4, 2013 at 18:36

There is a part of me that cannont help but wonder if the great daylight fireball of 1972 might have been an aerobraking probe

It stayed in atmosphere for 1,500 km, did not look to break up at on on the super-8 footage, and was to make a resonant return…

torque_xtr October 5, 2013 at 9:56

So, concerning L parameter, the point is that once high-tech-capable civilization emerges, it is extraordinarily difficult to completely destroy both the species and the generated knowledge. At least Chicxulub-grade impact is needed, or the consequent disasters that reduce the civilization to the stone age and then strike again when they are most vulnerable (to render them extinct like other Homo species). With anything less, when at least some population and knowledge survives and no subsequent disasters ensue, the high-tech will emerge again, and again, and again…
So the expected L parameter would be on the order of 100 000 years divided by the (1 – probability that the high-tech would emerge again) and (for the communication-effective L) multiplied by the percentage of time during which the species are able to communicat (the later development isn’t accounted for) I guess the probability is very close to unity (with the assistance of al the remaining knowledge; the archaeology in re-emerging civilization would be a completely different and much more needed science!), so L actually could be measured in millions years…

Tarmen October 5, 2013 at 10:39

Since we were already knocked back so badly once before, 70000 yrs ago, maybe we should think of this age as our Second Start. Maybe we H.Sapiens snuffed out the First Start of the Neanderthal civilization. Hmmm, if we get knocked down again before we we dispense with books altogether, we may have a good chance for a Third Start. But in another century we may not have physical books to fall back on. In fact some ‘rise of the machines’ scenario might super-cede any chance for a human Third Start. Maybe, if any AI archive(s) survived somewhere and were networked to a powersource and a 22nd century 3D printer factory. A civilization on Earth without any humans ?? Perhaps .
Perhaps those speculated VN ‘probes’ long ago committed the same ‘crime’ at their own birth world. Ie snuffed out their organic parents when the choice presented itself.

Leo Serrano October 5, 2013 at 16:08

is it possible to consider that our Biology on planet earth could be the result of a vonn Neumann probe? or at least one of those mass extinctions that occurred on earth at some point in the past?

Tarmen October 5, 2013 at 19:01

Leo, for sure mass extinction of dynosaurs opened up the path for mammals and eventually primates and humans. In dynosaurs’ time, us mammals were just foot sized little jumpy things hiding in the weeds. Dynosaurs’ mass extinction opened up future for us mammals.

Von Neuman probes? Have they been here? Are they still kicking around here ? Dead artifacts or active comm relays ? Maybe ‘nearby’ alien probe(s) ?
Maybe. Lot of water under the bridge. We don’t know.

Eniac October 6, 2013 at 0:59

Rob Flores:

Eniac, you miss the entire point. it takes a large population to support the Rise of High technology.

If you reread NS’s message and my quote of it, you will see that he is talking about resources, not population size. I’ll let you decide who is missing the point after you do.

Now try selling a moon
shot to a de-populated land which has the elemetary knowledge, but by the way you we will have to ration your goods to try this stunt.

The idea is not to ask the “stone-aged” population to do a moon shot. The idea is to ask them to follow a similar path as the original stone-age population, to eventually, again, arrive at a state where a moon shot (and much more) is possible. The presumption has to be that the knowledge of history and readily available recipes for the perished technology will make this easier and/or faster than the first time, but that presumption could be debated.

Eniac October 6, 2013 at 10:33

Leo Serrano:

is it possible to consider that our Biology on planet earth could be the result of a vonn Neumann probe?

It is possible. However, whatever “they” brought to Earth would have to have been very primitive indeed. Phylogenetically, we can trace our ancestry back to a very primitive organism (the Last Universal Common Ancestor, LUCA), which would not likely have survived had there been more sophisticated organisms around, such as you would expect in the case of contamination brought in by a highly developed multicellular organism.

In general, any attempt to explain life on Earth by having it come from somewhere else is suspect. It does not solve the tough problem of explaining how life originated. It just kicks the can down the road.

coolstar October 6, 2013 at 18:22

A very interesting paper that I stumbled across independently. The first thing that occurred to me was that you might look for these when the earth was located on the line between the probe and the system it was in contact with (at inferior conjunction as seen from the probe). This would limit you to looking for probes from systems that lie very close to the ecliptic. Except that surely the probe would be smart enough NOT to broadcast during this time……And with such a huge gain, it would be a very hard detection regardless. As the author pointed out, this might be the one case when actually broadcasting purposely to make contact could be a reasonable strategy.

Andrew Palfreyman October 8, 2013 at 1:45

I think a mission to the grav focus would be very worthwhile indeed, but perhaps we need to wait until we have a web of beams in place before attempting it – otherwise it will take forever to get there. So we need mirrors out there too for braking. The grav focus is actually a spherical shell, so the amount of area to be covered is absolutely vast. The mission would consist of a large swarm of small radio receivers looking for anomalously strong signals.

ljk November 8, 2013 at 10:22

What if Civilisations Act Like Parasites? Implications for SETI

Tuesday, November 5, 2013 / Duncan Forgan

We have no way of knowing how an alien civilisation will act. This is one of the biggest stumbling blocks of the Search for Extraterrestrial Intelligence. After all, it’s very easy to explain away the lack of contact with alien life (Fermi’s Paradox) by simply saying “well, they don’t want to talk to us”, or “they’re not allowed to because of the Prime Directive“. These sorts of arguments are the weakest solutions to Fermi’s Paradox, because they rely on knowledge we don’t have.

So how do we solve this conundrum? Sadly, we can’t – at least until we make first contact, that is. So in the meantime, we are forced to play let’s pretend, and speculate on how alien civilisations will behave. But we can still be sensible, and rein in our wilder ideas. Ideally, our educated guesses should have a basis in something biological – not too Earth-centric, but in processes that we think must occur regardless of where life arises.

So Jonathan Starling and I turned to the concept of symbiosis. Symbiosis describes the relationship between two different species. These relationships can be broadly categorised as

Parasitical – one species (the parasite) uses the other (the host) to its own advantage, having a negative effect on the host.

Mutualist – both species benefit from the interaction.

Commensalist – both species interact, and one benefits, but the host is not affected positively or negatively

Full article here:

ljk November 13, 2013 at 15:41


Today 10:03am

How Self-Replicating Spacecraft Could Take Over the Galaxy

Forget about generation ships, suspended animation, or the sudden appearance of a worm hole. The most likely way for aliens to visit us — whatever their motive — is by sending robotic probes. Here’s how swarms of self-replicating spacecraft could someday rule the galaxy.

Back in late 1940’s the Hungarian mathematician John Von Neumann wondered if it might be possible to design a non-biological system that could replicate itself in a cellular automata environment, what he called a universal constructor.

Von Neumann wasn’t thinking about space exploration at the time, but other thinkers like Freeman Dyson, Eric Drexler, Ralph Merkle, and Robert Freitas later took his idea and applied it to exactly that.

Full article here:

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