SETI vs the possibility of interstellar exploration

Science News has a short article discussing a calculation someone has done showing how small the volume of space examined by SETI (Search for Extraterrestrial Intelligence) is relative the overall size of the galaxy.

With no luck so far in a six-decade search for signals from aliens, you’d be forgiven for thinking, “Where is everyone?”

A new calculation shows that if space is an ocean, we’ve barely dipped in a toe. The volume of observable space combed so far for E.T. is comparable to searching the volume of a large hot tub for evidence of fish in Earth’s oceans, astronomer Jason Wright at Penn State and colleagues say in a paper posted online September 19 at

“If you looked at a random hot tub’s worth of water in the ocean, you wouldn’t always expect a fish,” Wright says.

I have no doubt that the amount of stars SETI has examined so far is a minuscule slice of the population of the Milky Way galaxy.  And if SETI’s chief assumptions are correct, it’s entirely right to say that we shouldn’t be discouraged by the lack of results so far.

But it’s worth noting what one of those chief assumptions are, that interstellar travel is impossible, or so monstrously difficult that no one bothers.  If true, then we wouldn’t expect the Earth to have ever been visited or colonized.  This fits with the utter lack of evidence for anything like that.  (And there is no evidence, despite what shows like Ancient Aliens or UFO conspiracy theorists claim.)

But to me, the conclusion that interstellar travel is impossible, even for a robotic intelligence, seems excessively pessimistic.  Ronald Bracewell pointed out decades ago that, even if it is only possible to travel at 1% of the speed of light, a fleet of self replicating robot probes (Bracewell probes) could establish a presence in every solar system in the Milky Way within about 100 million years.  That may sound like a long time, but compared to the age of the universe, it’s a fairly brief period.  Earth by itself has existed 45 times longer.

NASA image via Wikipedia

People sometimes respond that the Earth may be in some type of backwater.  The problem here is, if you know about where the Earth is in the Milky Way, in the Orion Spur off the Sagittarius Arm, about halfway between the center and rim of the galaxy, you’ll know that we’re not really in a backwater.  The backwater theory might be plausible if we were thousands of light years off the galactic plane, beyond the rim, or in a cluster far removed from the main galaxy, but we’re not.  Even then, the nature of the self replicating probe propagation is pretty relentless and would still eventually reach backwater stars.

Of course, if there is only one or a few other intelligent species in the galaxy, then it’s entirely possible that their Bracewell probe is here, just lying low, observing us, possibly waiting for us to achieve some level of development before it makes contact.  (Or maybe it has been making contact 2001: A Space Odyssey style.)

But if the number of civilization is in the thousands, as is often predicted by people speculatively playing with the numbers in the Drake equation, then we should have hundreds of those probes lying around.  Given their diverse origins, we shouldn’t expect them to behave with unanimity.  Even if one probe, or coalition of probes, bullied the others, the idea that such an arrangement would endure across billions of years seems implausible.

And the Earth has been sitting here for billions of years, with an interesting biosphere for most of that time.  The idea that none of these self replicating probes would have set up some kind of presence on the planet, a presence we should now be able to find in the geological record, again seems implausible.  Indeed, if they existed, we should expect to have at least some of them in front of us now.

Now, maybe they are in front of us, and we’re just not intelligent enough to realize what we’re seeing.  Monkeys, after all, likely have no understanding of the significance of the buildings and machinery they climb over.  It seems like something we have to keep in mind, but historically it’s never been productive to just assume we can’t understand something, and taking this principle too much to heart seems like it would make it impossible to ever dismiss any dubious notion.

So SETI largely depends on interstellar travel being infeasible.  This is actually the conclusion a lot of radio astronomers have reached.  Could they be right?  I don’t think we know enough to categorically rule out the possibility.  If they are right, then SETI will be our best chance to someday make contact with those other civilizations, even if it’s only composed of messages across centuries or millenia.

As I’ve written here before, my own conclusion is that some form of interstellar exploration is possible, and that life is probably pervasive in the universe, although most of it is microscopic.  Complex life is probably far rarer, although I wouldn’t be surprised if there aren’t thousands of biospheres, or more, in our galaxy that have it.

But intelligent life capable of symbolic thought and building a civilization?  The data seems to be telling us that this is profoundly rare, so rare that the nearest other intelligent species is probably cosmically distant.  If we’re lucky, they might be close enough that we can encounter them before the expansion of the universe separates us forever.  If we’re not lucky, we’ll never have a chance for that encounter.

Unless of course, I’m missing something?

19 thoughts on “SETI vs the possibility of interstellar exploration

    1. Good point.

      That reminds me of another assumption, that they’d communicate with patterns of electromagnetic radiation. It seems like a decent bet since the only other long distance force (gravitation) would require titanic amounts of energy to manipulate. Of course, this assumes that we’ve discovered all the fundamental forces.

      Liked by 1 person

      1. I’ve thought that maybe a form of quantum communication would be possible.

        If the Big Bang is correct, all matter in the universe would have had to have been near to all other matter at some point. Therefore, all matter should be entangled with other matter. If we knew where or how to look, it would seem possible to detect quantum signals from advanced civilizations even in other galaxies.

        Probably more of an idea for science fiction than real science but still the Earth is 4.5 billion years old and life on it has only been to understand EMR in the last two hundred. So the idea that is best or only way to communicate seems short-sighted.

        Liked by 1 person

        1. Hey James,
          Physicists generally throw cold water on the idea of communicating with quantum entanglement, at least faster than light. Apparently you can’t manipulate the quantum state and change the state of its partner, that is other than causing a wave-function collapse / decoherence, which doesn’t covey information. (Well, given a proper set up, you can eventually change the state of its partner, but that involves using electromagnetism.)

          Entanglement is a read-only affair, similar to both of us opening the first page of a previously published book at a prescribed time. In both cases, we know what the other person will see, but can’t use that to communicate.

          It’s kind of a bummer.

          Liked by 1 person

  1. I don’t think anybody is out there unless you’re talking alternate dimensions. Before you relegate me to the crazys here me out. I believe in science and evolution. Sad I have to say that in our modern world. But I think were missing something when it comes to human consciousness and self awareness. I believe consciousness is rooted in particle physics particularly Quantum Physics. While I don’t believe in any mythical view of a creator I do think consciousness is pervasive in the universe but not necessarily how we think. I think the very either, elements, and particles in the universe are conscious. The very universe its self I believe might be conscious. In the multiverse theory that might mean multiple universes might be conscious. In fact the universe might be living and experiencing through us for our consciousness might be a gift from the universe which it goes back to upon our bodily death. Consciousness I believe is a particle


    1. Hey Matt,
      It’s been a while. I can’t say I buy either panpsychism or quantum consciousness. Panpsychism because I don’t see it as a productive outlook, and quantum consciousness because I just don’t see the evidence. But I’m a skeptic and I know they are popular outlooks.


  2. I see from Wikipedia that self replicating spacecraft are actually called “von Neumann probes”, with spacecraft specifically set up to seek and make contact with other life instead called “Bracewell probes”. But then as that wiki article mentioned, if you’ve got von Neumann capabilities then a Bracewell objective could certainly be added. Beyond the realm of science fiction however, I’m having a hard time imagining successful von Neumann probe propagation.

    Given physics and chemistry, I can conceptually grasp the possibility of living propagation for a coincidence such as Earth. This stuff is utterly amazing, and yet causally plausible to us (or at least now that science has come along to illuminate associated dynamics). But then for a creature like the human to use elements of its environment to build something which survives space for hundreds or millions of years by finding ways to maintain its function out there, and even uses space stuff to reproduce itself to thus widely propagate these sorts of machines. . . Is this not the height of an “If it’s dream-able, then it’s possible” mentality? Consider all of the symbiosis that’s required in order for speedy generations of life here to propagate. This lesson does not suggest that something could ever build machines able to go out into the depths of space and propagate themselves.

    Am I being too pessimistic here Mike? Any thoughts about how von Neumann probes might not be entirely sci-fi? (And of course I mean this in practice. Unfortunately we see lots of sci-fi in modern human “science”.)

    Liked by 1 person

    1. In terms of the self replication, I do suspect you’re being too pessimistic. We already have machines construct other machines today. They’re just not building copies of themselves in a self directed manner. So the real question is whether a machine could be intelligent enough to build copies of itself.

      Note that this doesn’t have to happen the way it does in biology with an initial zygote, spore, or seed reproducing into a complex multicellular organism. (Although the fact that nature does it with modest energy requirements tells me that it’s possible in principle.) It can simply be machines that build a factory to make copies of other machines like it.

      The real question, in my mind, is how likely is it we could construct a machine that could work for centuries in interstellar space. We’ve constructed probes that have worked for decades (see the Voyager probes), but there are difficulties.

      The first is power. It would need a power source that could last for the duration. Once in interstellar space, it would have no other sources. The Voyager probes used plutonium cores but those are only good for a few decades. I’ve seen Americium-241 thrown out as a possible fuel source, but it releases more ionizing radiation and has a lower power density than plutonium.

      The second is entropy. The probe would receive a constant bath of radiation from the interstellar environment, in addition to whatever its power source emits, and that would eventually cause issues. We might imagine self repairing systems to some degree, but again, it wouldn’t have access to any external materials or energy.

      These problems get ameliorated if we can accelerate the probe to a substantial fraction of the speed of light. The Breakthrough Initiative is talking about accelerating probes to 20% of light, although they don’t have a way to slow them back down later. The power requirements to accelerate and then later decelerate the craft could end up being the biggest obstacle.

      Looking long term, there would also be the possibility of errors creeping into the replication process, which implies mutations. Self replicating machines would essentially be a type of life that over long enough periods of time might start to evolve. (I say “might” because the population may never be high enough for natural selection to kick in. Unless of course one of the mutations alters the programming about when to reproduce.)

      Liked by 2 people

      1. There might be other difficulties to creating viable Von Neumann probes.

        For the probe to be able to create new versions of itself (or factories to create new probes), it first would need to be able to find planets or other bodies with the necessary raw materials, navigate to them, and probably land on them. Eventually deceleration would be necessary.

        If it had the strategy of collecting material from small bodies, such as asteroids or comets, it probably would need to do this multiple times before it would ever have enough material to construct even one other version of itself. Each time it collected material it would need to re-accelerate to go to the next body. But each time the next acceleration would require more power because the probe would need to accelerate itself and the additional materials it collected on the body. It wouldn’t be guaranteed it would have sufficient initial power built into it to reach a critical mass to construct a factory or additional probe.

        If it had the strategy of looking for planets, it might have a better chance but then the probe could find itself in a potentially deep gravity well it would have to accelerate both itself and its child probe out of. In addition, it some key ingredient was unable to found on the planet, it might be trapped forever.

        And, of course, the power source for navigating, accelerating, and decelerating would be very problematic. Certainly chemical wouldn’t work. I doubt radioactive power could power a device off the surface of the planet. Some sort of solar sail might be work for navigating within planetary systems but also would not be viable for leaving a planet or moving between systems.

        A more logical strategy would be to emulate the mushroom/spore model. Send out billions of small probes with limited (maybe no) self-contained power with the hope that some might land in an environment where it could replicate but probably not be able to leave that environment.

        Oh, wait. Wouldn’t that be something like panspermia?

        Liked by 2 people

        1. I think you’re right that the probe’s work at its destination is far from easy. But once in a solar system, it has a lot more to work with. For instance, assuming what it needs is close enough, it would have access to the star’s solar energy. But it would definitely need to find raw materials, and that could take years, possibly decades. That might not be too bad if it already spent decades or centuries in transit.

          All that said, if we can find a solution to the deceleration problem, the power needed for even extensive in-system navigation would be a tiny slice of that solution. And sending billions of small probes still leaves us with the problem of deceleration. A tiny probe slamming into a planetary atmosphere at interstellar speeds probably wouldn’t hurt the planet too much, but it’s hard to see how the probe itself survives.

          On spores, I don’t know if you’re a Greg Egan reader, but in one of his stories, he describes a matter-antimatter firework that accelerates a microscopic particle to near light speed, which after crossing interstellar space, slams into the destination star. As it’s slowing down and being destroyed in the star, the microscopic particle creates an orchestrated shockwave in the plasma that exits the far side of the star as a cloud of nanomachines, that then intercept a planet and fall to a site on its surface, where the nanomachines begin constructing stuff. (First three pages: ) Definitely sci-fi level stuff, but thought provoking.


  3. Mike and James,

    I think that you’re right to worry about the crazy amounts of energy associated with this scenario, though consider my deeper problems as well. Yes we may build some of our machines so that they can build machines. But how autonomously? They seem to require humans to provide processed inputs that they’re designed specifically to use. And even then note that they don’t fix themselves when they break. It would be better if we could build machines that make machines without any human help at all — abandoned in a desert or whatever. And note that for the actual thought experiment here, such machines would need to build themselves. This is to say build things capable of interstellar travel which fix themselves when they break, and replicate themselves under the domains of various other stars to thus expand this sort of function indefinitely. Yes, to me this scenario seems like pure science fiction.

    Note that life manages to do what it does through symbiosis rather than alone. One thing feeds another. In this robotic space scenario however we’re talking about machines which do everything. And even if possible, why build them? We’d know going in that it should take millions years for any positive reports, given how slow they would be.

    My boring perspective is that we should progressively be able to do more and more space stuff in our own solar system, though everything else should simply be too far away. There should be countless cradles of life out there, though the distances should mandate that we never actually receive any evidence of them.

    The only potential should come through the signature electromagnetic radiation of advanced civilizations, though I’m not hopeful. Let’s say that some advanced civilization sets itself up to detect our radiation. Good plan… except that the Earth didn’t produce any until just recently. So dividing 100 years of this by our planet’s age of 4.5 billion years nets a minuscule chance (.0000022%) that they happen to pay attention at the right time. How long would we need to produce such radiation to increase our signals to 1% of the Earth’s duration? 45 million years. Thus I’m also not hopeful that we’ll happen across any such radiation from other advanced life.

    I suspect that within a few thousand years we’ll progressively hook ourselves up to pleasure giving machines. This should be great for the Earth’s ecosystem, and even more so because these machines should be the means of our demise. The sedentary existence associated with perfect pleasure should make our species vulnerable to the technology that our lives would thus depend upon. We would fail once it fails. I suspect that it’s the same for intelligent life elsewhere. Thus I doubt that we’ll ever be detected by any of them, or them us.

    Note however that new advanced civilizations should continue to evolve on Earth. Thus our planet should produce instant bursts of undetected signature radiation between extended periods of evolution.

    Liked by 1 person

    1. Eric,
      As I mentioned above, aside from the power requirements that we all agree are an issue, it sounds like your actual skepticism is toward artificial intelligence, that you don’t think we’ll ever be able to build machines intelligent enough to build copies of themselves. You seem to see a crucial obstacle that we won’t be able to overcome.

      Now, for some things, I think that’s a warranted stance. For example, I doubt we’ll ever be able to travel or communicate faster than light, because it’s not in accord with physics as we currently understand it, and nothing in nature has been shown to do so. (At least not in any way that would help.) I could also see skepticism toward the idea that we’d ever be able to observe the insides of a black hole, or rearrange galaxies, or many other things that are either contrary to understood physics or would require cosmological amounts of energy and coordination.

      But the intelligence we need for our machines violates no laws of nature. Indeed, we ourselves are examples of it happening in nature. And those examples happen with quite modest energy requirements. (The human brain reportedly operates on 20 watts.) In other words, intelligent machines already exist, just evolved ones. So the only question is whether we can find a way to design and build them ourselves.

      So my question is, what leads to your skepticism? What crucial barrier do you see that would prevent us from building an engineered system with enough intelligence to build copies of itself, including manage facilities, repairs, etc?

      Of course, if we all eventually end up hooking ourselves to pleasure machines, we might be extinct before anyone has the chance to develop intelligent machines.

      But here I think it’s worth noting something. We already have the ability to put ourselves in a pleasure loop. Anyone who wants to badly enough can obtain drugs that will intensely fire the reward centers of the brain, and many people do. It’s why there are so many addicts. It’s not unheard of for people to take those drugs until they’re dead.

      But most people don’t, and society throws up barriers to doing it, barriers that, despite what many libertarians say, do reduce usage of those drugs. What dis-incentives exist that prevent the whole population from taking those drugs, that cause society to condemn doing so, that would not exist for pleasure machines?

      For that matter, we’ve had the ability to wire animals into similar machines for decades. We talked before about the experiments where rats can hit a lever to trigger a reward release, which typically leads to them doing so until they drop. But I’ve never seen a single human pine to be wired up in that way. Most people find the idea deeply disturbing. What do you see changing that would lead to everyone doing it?

      I suspect what you may be overlooking is that most of us viscerally reject short term hedonism. We can imagine a future and have ambitions about that future, an anticipation that gives us hope of future pleasures, that we know would be destroyed by drugs or pleasure machines. I don’t doubt for a minute that a certain portion of the population will do it, if society permits it, but the fact that the whole world didn’t become an opium den in the 19th century leads me to conclude that it won’t become a giant pleasure machine.

      Liked by 2 people

  4. Yes Mike, I’m bringing up two issues here — an initial smaller one, and then a more substantial larger one. For the first, it’s not only my skepticism that we won’t or can’t build intelligent enough robots. Even if we do, and somehow manage to send them safely to distant stars, they simply should not be designed to survive well enough in places that are merely predicted in general. They should not begin with anything like the benefits that we’ve inherited. Temperature extremities alone might often be a dealbreaker, not to mention gravity on planets that they weren’t optimized for, and on and on and on. We’d be attempting to do something far more ambitious than what evolution has done with us.

    Note that the human doesn’t function here by accident, but rather it evolved to reside specifically on this planet. The vast chain of symbiosis which resides below us evolved into such a role as well. Furthermore our modern abilities (such as building machines) often depend upon a wide range of human specialists who have required time and experience to master their skills. Apparently we take for granted the vast community which permits our species to do what it does, or something which will not be granted for these machines. Even if they were technically far more intelligent than the human, the (eco?) system that we’d need to send along to support such intelligence should simply not be designed well enough for what would be faced.

    So yes, there are many ways in which I’m skeptical of the propagation of these robots, even granting a tremendously advanced conscious intelligence, as well as a free travel pass. I’m not saying that such amazing machines are outright impossible (since life awes me regarding what is possible). Instead I’m very skeptical that we humans will ever build anything marginally close to what would be required for machines to replicate themselves under the domains of other stars.

    Then moving to the other issue, apparently evolution did not specifically design us to seek good looking mates, or to be socially well thought of, or to succeed in business, and so on. It designed us, and all conscious life, to seek happiness, or theoretically all that’s valuable to anything anywhere. Then it’s from this premise that various specifics arise regarding what individual conscious entities find to taste good, to be sexually appealing, and so on. Therefore when it becomes sufficiently understood that we are merely pawns of evolution, or that “value out there” is simply a potential means to the ends of receiving value within our heads, what’s to stop us from eventually circumventing the outside in order to go straight to the source?

    Today the human has nothing close to pleasure machines, and even though we can wire rats up to them. Apparently it feels so good for these animals to work a pleasure switch, that they’ll continue doing so rather than relieve tremendous thirst, hunger, and so on, until they physically just can’t continue working the switch.

    Conversely humans only have drugs — instruments which dangerously alter and degrade conscious function, as well as bring chemical dependency. We fight tremendous battles against the effects of alluring nicotine, alcohol, and so on. I needn’t get into the widely understood problems associated.

    What will happen, however, when a simple pleasure machine becomes invented for the human? This is to say, a machine with the potential to give a person the greatest sensations that he/she has ever felt, for extended periods of time, and without side effects. Here’s an educated guess:

    To be sure, non-government production, sale, and use of these machines should quickly be banned. Given the implications, all such matters should face social opprobrium. But that shouldn’t stop some of the rich from giving them a go. Certain countries should permitt tourists to use them at least. And why not permit them for people who are terminally ill and suffering? Though I expect tremendous political battles, these machines should progressively become a more and more pervasive element of human life, and specifically because they would directly provide the end goal which (theoretically) all conscious life seeks. People should find themselves working very hard in normal life for the opportunity to take such “trips”, and perhaps even perpetually “retire” in such a way.

    We’re speculating that this road would eventually lead to the extinction of humanity, though that’s not entirely certain. People on it should have great worries about such an end as well, and so set up policies to the contrary. This is one of countless uncertainties associated with that path.

    Massimo once gave me a wonderfully hip retort. It went like this, “Sorry Eric, but I’ll take the red pill”. (For any non-sci-fiers out there, this was from The Matrix, where the blue pill is associated with a false but very happy existence). In the end his line simply reflected his hope however. He was unable to provide a reasonable argument to counter my clearly plausible scenario. This is to say, clearly plausible once these machines emerge.

    Liked by 1 person

    1. Eric,
      I agree that the difficulties of recreating Earth’s biosphere are often not appreciated. I think this will put a serious constraint on the ability of biological humans to colonize space.

      But will it put any such constraint on robotic systems? It’s possible that in order to achieve human level intelligence, inevitable engineering trade-offs may require that the substrate of the intelligence be similar to organic brains, in other words, wetware. If so, that would definitely complicate the idea of intelligent interstellar probes.

      But it’s worth remembering that the initial probe has the option of building a basic receiver and having more intelligent systems transmitted to it, enabling it to be a step in a bootstrapping process. It does complicate the picture, but at least to me, it doesn’t make it impossible.

      Of course, as I noted in the post, it’s always possible that the various difficulties add up enough to make the overall endeavor impossible. We can’t rule that out. And the probability of it being true is largely a judgment call at this point. The answer may not be known until long after we’re gone.

      I like Massimo’s response. I’d take the red pill myself. I think any skeptic might. But would most of humanity? I don’t know. They might if they don’t like the long term prospects of anyone who takes the blue pill. Again, I don’t doubt some portion of the population will go for the blue. But the size of that portion is another judgment call that may not be testable in our lifetime.

      Liked by 1 person

  5. Have you had the chance to read about Omuamua the recent “object” that was seen in our solar system? After reading into it, I’m convinced it was in fact a constructed object and not just space debris. A study done at Harvard concluded if a space object were to use the suns radiation for extended space travel, the object would have to be “wide and narrow”. Omuamua was 400m x 40cm. Along with the dimensions that pointed toward interstellar life, the path at which it traveled was also very irregular based on the human understanding of physics. What’s your opinion?

    Liked by 2 people

    1. Hi Brad, hope you don’t mind me inserting my thoughts, but where did you get those dimensions? That’s an aspect ratio of 1:1000, which sounds really extreme and would strongly suggest an artificial structure. Wikipedia states a length of 100 m–1,000 m and a width of 35 m–167 m, giving an aspect ratio more like 1:3 to 1:6, which seems more in line with a natural object.

      Liked by 1 person

      1. According to Alan Boyle, a recent updated estimated length was said to be 1,300 ft wide(about 400m). Along with Harvard’s Smithsonian center for Astro physics Smithsonian center for astrophysics study that determined a object of those dimensions would be needed to use the suns radiation


        1. I have to admit that it is tempting to think of ʻOumuamua as something constructed. It’s elongated shape made me think of Arthur C. Clarke’s Rama object in ‘Rendezvous with Rama’, although it’s not nearly as big as the object in that story. But it being an alien artifact would be an extraordinary conclusion, one we shouldn’t jump to without extraordinary evidence, which I don’t think we have. All we really have right now is an object that is possibly elongated. It’s a pity we can’t rendezvous with it like the characters in Clarke’s story.

          Liked by 2 people

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