Billions of Worlds

I’m old enough to remember when we had nine planets in the Solar System, and zero outside. The news since then has been mixed. Here in our neighborhood we’re down to only eight planets; but in the wider galaxy, we’ve obtained direct evidence for about a thousand, with another several thousand candidates. [Thanks to Peter Edmonds for a correction there.] Now that we have real data, what used to be guesswork gives way to best-fit statistical inference. How many potentially habitable planets are there in the Milky Way, given some supposition about what counts as “habitable”? Well, there are about 200 billion stars in the galaxy. And about one in five are roughly Sun-like. And now our best estimate is that about one in five of them has a somewhat Earth-like planet. So you do the math: about eight billion Earth-like planets. (Here’s the PNAS paper, by Petigura, Howard, and Marcy.)


“Earth-like” doesn’t mean “littered with human-esque living organisms,” of course. The number of potentially habitable planets is a big number, but to get the number of intelligent civilizations we need to multiply by the fraction of such planets that are home to such civilizations. And we don’t know that.

It’s surprising how many people resist this conclusion. To drive it home, consider a very simplified model of the Drake equation.

x = a \cdot b.

x equals a times b. Now I give you a, and ask you to estimate x. Well, you can’t. You don’t know b. In the abstract this seems obvious, but there’s a temptation to think that if a (the number of Earth-like planets) is really big, then x (the number of intelligent civilizations) must be pretty big too. As if it’s just not possible that b (the fraction of Earth-like planets with intelligent life) could be that small. But it could be! It could be 10-100, in which case there could be billions of Earth-like planets for every particle in the observable universe and still it would be unlikely that any of the others contained intelligent life. Our knowledge of how easy it is for life to start, and what happens once it does, is pretty pitifully bad right now.

On the other hand — maybe b isn’t that small, and there really are (or perhaps “have been”) many other intelligent civilizations in the Milky Way. No matter what UFO enthusiasts might think, we haven’t actually found any yet. The galaxy is big, but its spatial extent (about a hundred thousand light-years) is not all that forbidding when you compare to its age (billions of years). It wouldn’t have been that hard for a plucky civilization from way back when to colonize the galaxy, whether in person or using self-replicating robots. It’s not the slightest bit surprising (to me) that we haven’t heard anything by pointing radio telescopes at the sky — beaming out electromagnetic radiation in all directions seems like an extraordinarily wasteful way to go about communicating. Much better to send spacecraft to lurk around likely star systems, à la the monolith from 2001. But we haven’t found any such thing, and 2001 was over a decade ago. That’s the Fermi paradox — where is everyone?

It isn’t hard to come up with solutions to the Fermi paradox. Maybe life is just rare, or maybe intelligence generally leads to self-destruction. I don’t have strong feelings one way or another, but I suspect that more credence should be given to a somewhat disturbing possibility: the Enlightentment/Boredom Hypothesis (EBH).

The EBH is basically the idea that life is kind of like tic-tac-toe. It’s fun for a while, but eventually you figure it out, and after that it gets kind of boring. Or, in slightly more exalted terms, intelligent beings learn to overcome the petty drives of the material world, and come to an understanding that all that strife and striving was to no particular purpose. We are imbued by evolution with a desire to survive and continue the species, but perhaps a sufficiently advanced civilization overcomes all that. Maybe they perfect life, figure out everything worth figuring out, and simply stop.

I’m not saying the EBH is likely, but I think it’s on the table as a respectable possibility. The Solar System is over four billion years old, but humans reached behavioral modernity only a few tens of thousands of years ago, and figured out how to do science only a few hundred years ago. Realistically, there’s no way we can possibly predict what humanity will evolve into over the next few hundreds of thousands or millions of years. Maybe the swashbuckling, galaxy-conquering impulse is something that intelligent species rapidly outgrow or grow tired of. It’s an empirical question — we should keep looking, not be discouraged by speculative musings for which there’s little evidence. While we’re still in swashbuckling mode, there’s no reason we shouldn’t enjoy it a little.

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60 Responses to Billions of Worlds

  1. Hi, Sean. I had to lol at your teaser. I’m getting ready to publish my book this month on Create Space (The Multiple You Universe: Sacred Science for the 21st Century) and it is proposing that #SacredScience is a valuable way to better understand multidimensionality because consciousness is not studied enough in science, whereas mystics have been utilizing dimensional bands of consciousness to communicate with nonlocal beings for a very long time. The unified energy field (as I experience it is intelligence), and the book goes on to share many of my personal experiences and draws proving lines into quantum physics experiments illustrating my point that there are more similarities than most scientists like to consider between metaphysics and physics. Have a great day. If you would like to write on my blog about Dark matter I would be delighted. If you would like to talk about dark matter as it might be consider from a mystics pov, let’s do. Cassandra

  2. Gordon D. Munro says:

    Thank you for inspiring me to expand my vocabulary with 1-“EBH” for cosmological speculation and 2-“behavorial modernity” to avoid contemplation of the Tea Party.

  3. Lab Lemming says:

    Is this the doey-eyed optimist version of the “they wipe themselves out” argument? If you go for abundant life and progressive evolution, then you need a very small history/evolution ratio to keep the neighbours scarce.

  4. Niall says:

    I would agree that the discovery of exo-planets is probably the most important scientific discovery and advancement of the last two decades, but that is still no excuse to take the Drake equation seriously.

  5. Joseph Brisendine says:

    It seems like another reason that “b” might be small is that we don’t know how many conditions we’re placing on the direction of evolution when we require that alien life be “intelligent.” Don’t we really mean “walks and talks and furthermore, wants to know about us too” Considering the image of aliens that occupies the popular imagination, I find it hard to convince myself that we’re only looking for something intelligent, we’re looking for something that can talk back in a way we recognize. And we do have some data to get statistics on how rare that is, because we have some estimates for the total number of species on the planet and some idea about how many of them we consider “intelligent.” None of them except humans would fit the popular image of “intelligence,” yet we know that animals are quite obviously intelligent. Rather than life being rare in the universe, it might instead be the social features that make us communicate and desire to communicate with one another in specific ways that is more unique than we imagine, or alternately that alien intelligence might be transmitted in ways that are more alien than we have imagined (the sentient ocean in Solaris is a pretty far-out example of an imagined intelligent life-form that we would have a very hard time recognizing as such.)

  6. Rahim says:

    Dear Sean,
    Do you think that suicide in a typical society is an example of this general hypothesis?

  7. Michael Mansberg says:

    “life is kind of like tic-tac-toe” …

    Sounds like something Forrest Gump might say!

  8. Pingback: Sean Carroll’s Fermi resolution: the “Enlightentment/Boredom Hypothesis” | Gordon's shares

  9. Joe Bloggs says:

    While fun to think about, the EBH seems impossible as defined! What definition of boredom results in inaction? Whether we’re talking about restlessness or existential ennui, boredom is a problem to be solved. It is the unique trait that _triggers_ creativity and exploration.
    More likely is a future where we are satisfied enough so as not to need a space exploration programme. Off the top of my head, I can only think of two ways this could happen: 1. Our capacity for creativity/boredom is forcibly removed (c.f. Brave New World) or 2. The entire capitalist world grinds to a halt in favour of a quiet life resembling monasticism. Option 1 is a generic doomsday scenario, and option 2 is essentially John Lennon’s “Imagine” — neither disturbing, nor plausible.

  10. Antonio Sanchez says:

    Sean, lets enjoy the ride. There is no purpose o meaning after all, but I suspect that we will reborn ad infinitum in the multiverse.

  11. Jens says:

    I seem to recall that even if a civilization, such as ours, was reduced to sending out probes with our “seed” in every direction, with a maximum speed of 1% of light, we could still colonize our entire galaxy in a ridiculously short period of time, something like 100’s of millions of years. I therefore don’t immediately see the EBH theory as likely. I mean, even if a civilization discovered everything there was to discover, and thus realized there still was no way around Einstein’s speed limit, surely the knowledge that they could at least colonize their galaxy and discover answers to many “minor” things in the process, would prevent a EBH?

  12. Jens says:

    Fermi’s question, though, is still very valid.

  13. Brent Meeker says:

    If you consider how quickly intelligent life evolved on Earth, arguably only 65Myr after the KT extinction, and unarguably within 3Byr, the implication is that b is not small. If you consider that the evolutionary precursors to intelligence, large animal, grasping limbs, social organization, speech, have all evolved separately more than once the implication is that b may be near 1

  14. Dan says:

    It’s interesting to know that there are other planets somewhat similar to Earth. It’s interesting, too, to consider that some might harbor intelligent life, or any life for that matter. But given the incredibly vast distances and time involved, I can’t imagine there ever has been or ever will be any visiting going on in either direction. We’re not that interesting and I’m not sure we’ll ever be that smart.

  15. Pingback: Billions of Worlds | Wonder Planet X

  16. Charon says:

    “there could be billions of Earth-like planets for every particle”

    I’m going to rule this possibility out as unphysical* 😉

    *Yeah, yeah, yeah, I caught the “observable universe” bit…

  17. I don’t want to Live on these Mutiple Universes Anymore! Professor Farnsworth’s next logical conclusion.

  18. John says:

    And meanwhile, an ancient civilization not so distance from our own has been checking for changes of spins of particles in signals to determine if there is any pattern or logical sequence to the changes in their spin…

  19. Meh says:

    I didn’t know what to call it at the time, but I think I hit the EBH paradox when I was 19 or 20.

  20. James Cross says:

    I wrote this earlier this year:

    “One explanation for the Fermi Paradox is simply that the conditions for the first 8-9 billion years in our universe and our galaxy were not conducive to life. Conditions were too chaotic. If we assume that life requires about 4-5 billion years to reach intelligence and consciousness at our level, as was required on Earth, then intelligent life on other planets might be roughly at our level – some more advanced, some less so – but perhaps very few with an ability to colonize the galaxy. Of those with ability to colonize the galaxy, some (maybe even most) may have had little interest in it. At any rate, the extended period of time for any of these civilizations to have colonized the galaxy might not have existed.”

    My phrase about “little interest” is something like EBH though I would lean more to the E part of it.

    When we look at the broad trends in evolution of life, we can see the tendency for simpler to become more complex through combination and assimilation. That is not to say, however, that more complex often becomes simpler too. The trend toward greater complexity is likely, maybe somewhat inevitable, given the right rates species creation. even if most new species are simpler than there predecessor species.

    This is somewhat counter intuitive but let me explain. If we begin with some small number of species (let’s put aside the origin of life question for the moment), then in a given time period each species could remain the same, evolve into a new spcies with the same level of complexity, evolve into a more complex species, evolve into a less complex one, or some combination of the four (for example, a new species breaks from the original but the original survives). Even if the rate of development of more complex species is low as long as the rate of development of new species in general is relatively high, we will eventually develop increasingly complex organisms. This is because the small number of species we began with initially grows increasingly larger. As the number expands, there is now a larger pool of species that is eligible to evolve into a more complex species.

    Since this is exactly the process that happened on Earth (although it took about 3+ billion years to get to us), this would likely be occurring on other worlds also.

    Life as it becomes more complex builds new structures by combining and consolidating old ones. Multicellular organisms arise by single celled ones combining. This turns out to be surprisingly easy. Scientists have created multicellular yeast in just a few generations. I read recently a multicellular algae created experimentally in just a few generations. Solitary organisms evolve into social organisms – ants, bees, birds, and most mammals. Human evolved from small groups to large groups – now a global society. Language and culture are the new structures built on the old structures that enable this complexity.

    The same must be happening on other worlds.

  21. Greg says:

    Suppose most members of most intelligent species eventually get bored of life. Those members will die out, or at least fail to conquer the galaxy, leaving only those individuals who aren’t bored of life. So there is natural selection in favor of not getting bored, which makes the EBH unlikely in my view.

  22. Gordon Munro says:

    Gump only qualifies as intelligent by the standards of the average American voter.

  23. jim says:

    Even if the EBH hypothesis were true they would leave behind some evidence of their existence. Also even if they destroyed themselves they would leave behind the same evidence of their existence.

    The most obvious answer is the evidence hasn’t reached us yet.

    This is assuming our definition of intelligence – that they’ve reach the level we have – broadcasting EM. If they’ve surpassed our level (perhaps by skipping right over it) then they’re likely invisible to us which means another obvious idea: we’re really missing something about reality.

  24. I think this hypothesis says more about those positing it than about the cosmos itself — it strikes me as a statement of personal and civilizational exhaustion and lack of imagination. There are so many ways this universe can stay interesting without creating cosmically detectable structures or physically conquering the galaxy. For example, LSD was only discovered in the 1940s, well after Copernicus, Einstein and Hubble, yet it has opened up inner vistas for exploration that were scarcely imagined by Western man previously. So I think the great frontier for science going forward is the inner cosmos, which scientists seem to have little to say about, but which mystics have been exploring for millennia with no sign of having done more than scratched the surface.

    It may indeed be the case that the Enlightenment, as conceived by 17th century European rationalists, is approaching some asymptotic limit to its novelty, but that just means that it is time for an “Endarkenment”, to re-discover the magic and mystery which kept mankind from despairing at having figured it all out for millennia before modern science came on the scene. And in a Darwinian sense, those who find life interesting will propagate themselves, while those who are bored will die out, so the problem is presumably self-correcting.

    An example of what is possible is Jack Parsons, sorcerer, scientist and co-founder of the JPL, who said:

    “If we wish to unite with the universe, let us court the whole of nature, all experience, all truth and the splendour of the awesome cosmos itself. For ‘out there’ lies the great campaign that comes first and last; the ultimate adventure of the individual into himself. He must go down like Moses into his unknown self, out into the new dimension, out with Orpheus and the barque of Arthur, with Tammuz and Adonis, with Mithra and Jesus, into the labyrinths of the Dark Land. There he will meet The Mother and hear Her final question: “What is man?”. Thereafter, close by the heart of the cryptic Mother, he may find the Graal; ultimate consciousness, total remembrance, instinct made certain, reason made real. For it is he, wonderful monster, embryo god who has swum in the fish, shed the skin of the crocodile, peered from the eyes of serpents, swung with the apes and shaken the earth with tramp of the tyrannosaur’s hoof. It is he who has cried out on all crosses, ruled on all thrones, grubbed in all gutters. It is he whose face is reflected and distorted in all heavens and hells – he, the Child of the Stars, the son of the ocean; this creature of dust, this wonder and terror called MAN.”

  25. Student says:

    I’m okay with the fact that our species will probably not exist forever (unlike a lot of people, apparently: But I find the EBH, according to which a species wouldn’t event want to exist forever, stunningly depressing. That’s interesting to realize.