The woods are dark and full of terrors

I wanted to "circle back" on a comment I'd made on the "Dark Forest" hypothesis, which is basically the idea that we don't see signs of alien life because everyone's hiding for fear of everyone else.  This will probably be the last I want to say about the "Fermi Paradox", at least until the next time I feel like posting about it ...

I haven't read the books the Dark Forest hypothesis comes from [in March 2024, Netflix release 3 Body Problem, based on the first book in the series, and, um ... I have questions] but my understanding is that the hypothesis is based on the observation that when a relatively technologically developed society on Earth has made contact with a less developed society, the results have generally not been pretty.   "Technology" in this context particularly means "military technology."  The safest assumption is that this isn't unique to our own planet and species, but a consequence of universal factors such as competition for resources.

If you're a civilization at the point of being able to explore the stars, you're probably aware of this first hand from your own history, and the next obvious observation is that you're just at the beginning of the process of exploring the stars.  Is it really prudent to assume that there's no one out there more advanced?

Now put yourself in the place of that hypothetical more advanced civilization.  They've just detected signs of intelligent life on your world.  You are now either a threat to them, or a potential conquest, or both.  Maybe you shouldn't be so eager to advertise your presence.

But you don't actually see anyone out there, so there's nothing to worry about, right?  Not so fast.  Everyone else out there is probably applying the same logic.  They might be hunkering down quietly, or they might already be on their way, quietly, in order to get the jump on you, but either way you certainly shouldn't assume that not detecting anyone is good news.

Follow this through and, assuming that intelligent life in general isn't too rare at any given point in time, you get a galaxy dotted with technological civilizations, each doing its best to avoid detection, detect everyone else and, ideally, neutralize any threats that may be out there.  Kind of like a Hunger Games scenario set in the middle of a dark forest.


This all seems disturbingly plausible, at least until you take scale into account.


There are two broad classes of scenarios:  Either faster-than-light travel is possible, or it's not.  If anyone's figured out how to travel faster than light, then all bets are off.  The procedure in that case seems pretty simple: Send probes to as many star systems as you can.  Have them start off in the outer reaches, unlikely to be detected, scanning for planets, then scanning for life on those planets.  If you find anything that looks plunderable, send back word and bring in the troops.  Conquer.  Build more probes.  Repeat.

This doesn't require listening for radio waves as a sign of civilization.  Put a telescope and a camera on an asteroid with a suitable orbit and take pictures as it swings by your planet of choice.  Or whatever.  The main point is if there's anyone out there with that level of technology, our fate is sealed one way or another.


On the other hand, if the speed of light really is a hard and fast limit, then economics will play a significant role.  Traveling interstellar distances takes a huge amount of energy and not a little time (from the home planet's point of view -- less for the travelers, particularly if they manage to get near light speed).  By contrast, in the period of exploration and conquest from the late 1400s to the late 1700s it was not difficult to build a seaworthy ship and oceans could be crossed in weeks or months using available energy from the wind.  The brutal fact is that discovering and exploiting new territories on Earth at that time was economically profitable for the people doing the exploiting.

If your aim is to discover and exploit resources in other star systems, then you have to ask what they might have that you can't obtain on your home system using the very large amount of energy you would have to use to get to the other system.  The only sensible answer I can come up with is advanced technology, which assumes that your target is more advanced than you are, in which case you might want to rethink.


Even if your aim is just to conquer other worlds for the evulz or out of some mostly-instinctive drive, you're fighting an extremely uphill battle.  Suppose you're attacking a planet 10 light-years away.  Messages from the home planet will take 10 years to reach your expeditionary force, and any reply will take another 10 years, so they're effectively on their own.

You detected radio transmissions from your target ten years ago.  It takes you at least 10 years to reach their planet (probably quite a bit longer, but let's take the best case, for you at least).  They're at least 20 years more advanced than when the signal that led you to plot this invasion left their planet.

You've somehow managed to assemble and send a force of thousands, or tens of thousands, or a million.  You're still outnumbered by -- well, you don't really know until you get there, do you? -- but hundreds to one at the least and more likely millions to one.  You'd better have a crushing technological advantage.

I could come up with scenarios that might work.  Maybe you're able to threaten with truly devastating weapons that the locals have no way to counter.  The locals treat with you and agree to become your loyal minions.

Now what?

Unless your goal was just the accomplishment of being able to threaten another species from afar, you'll want to make some sort of physical contact.  Presumably you land your population on the planet and colonize, assuming the planet is habitable to you and the local microbes don't see you as an interesting host environment/lunch (or maybe you've mastered the art of fighting microbes, even completely unfamiliar ones).

You're now on unfamiliar territory to which you're not well adapted, outnumbered at least a hundred to one by intelligent and extremely resentful beings that would love to steal whatever technology you're using to maintain your position.  Help is twenty years away, counting from the time you send your distress call, and if you're in a position to need it, is the home planet really going to want to send another wave out?  By the time they get there, the locals will have had another twenty years to prepare since you sent your distress call, this time with access to at least some of your technology.


I'm always at least a little skeptical of the idea that other civilizations will think like we do.  Granted, it doesn't seem too unreasonable to assume that anyone who gets to the point that we would call them "technological" is capable of doing the same kind of cost/benefit analyses that we do.  On the other hand, it also seems reasonable to assume that they have the same sort of cognitive biases and blind spots that we do.

The "soft" sciences are a lot about how to model the aggregate behavior of not-completely rational individuals.  There's been some progress, but there's an awful lot we don't know even about our own species, which we have pretty good access to.  When it comes to hypothetical aliens, I don't see how we can say anything close to "surely they will do thus-and-such", even if there are practical limits on how bonkers you can be and still develop technology on a large scale.

In the context of the Dark Forest, the question is not so much how likely it is that alien species are actually a danger to us, but how likely is it that an alien species would think they were in danger from another alien species (maybe us) and act on that by actively going dark.

Our own case suggests that's not very likely.  There may be quite a few people who think that an alien invasion is a serious threat (or for that matter, that one has already happened), or who think that it's unlikely but catastrophic enough if it did happen that we should be prepared.  That doesn't seem to have stopped us from spewing radio waves into the universe anyway.  Maybe we're the fools and everyone else is smarter, but imagine the level of coordination it would take to keep the entire population of a planet from ever doing anything that would reveal their presence.  This seems like a lot to ask, even if the threat of invasion seems likely, which, if you buy the analysis above, it's probably not.

Overall, it seems unlikely that every single technological civilization out there would conclude that staying dark was worth the trouble.  At most, I think, there would be fewer detectable civilizations, than there would have been otherwise, but I still think that as far as explaining why we haven't heard from anyone, it's more likely that whatever civilizations there are, have been or will be out there are too far away for our present methods to detect (and may always be), and that the window of opportunity for detecting them is either long past or far in the future.

Fermi on the Fermi paradox

One of the pleasures of life on the modern web is that if you have a question about, say, the history of the Fermi paradox, there's a good chance you can find something on it.  In this case, it didn't take long (once I thought to look) to turn up E. M. Jones's "Where is Everybody?" an  Account of Fermi's Question.

The article includes letters from Emil Konopinski, Edward Teller and Herbert York, who were all at lunch with Enrico Fermi at Los Alamos National Laboratory some time in the early 1950s when Fermi asked his question.  Fermi was wondering specifically about the possibility that somewhere in the galaxy some civilization had developed a viable form of interstellar travel and had gone on to explore the whole galaxy, and therefore our little blue dot out on one of the spiral arms.

Fermi and Teller threw a bunch of arguments at each other, arriving at a variety of probabilities.  Fermi eventually concluded that probably interstellar travel just wasn't worth the effort or perhaps no civilization had survived long enough to get to that stage (I'd throw in the possibility that they came by millions of years ago, decided nothing special was going on and left -- or won't come by for a few million years yet).

Along the way Fermi, very much in the spirit of "How many piano tuners are there in Chicago?" broke the problem down into a series of sub-problems such as "the probability of earthlike planets, the probability of life given an earthlike planet" and so forth.  Very much something Fermi would have done, (indeed, this sort of exercise goes by the name "Fermi estimation") and very similar to what we now call the Drake equation.

In other words, Fermi and company anticipated much of the subsequent discussion on the subject over lunch more than fifty years ago and then went on to other topics (and presumably coffee).  There's been quite a bit of new data on the subject, particularly the recent discovery that there are in fact lots of planets outside our solar system, but the theoretical framework hasn't changed much at all.

What's a Fermi paradox?

So far, we haven't detected strong, unambiguous signs of extraterrestrial intelligence.  Does that mean there isn't any?

The usual line of attack for answering this question is the Drake equation [but see the next post for a bit on its origins --D.H Oct 2018], which breaks the question of "How many intelligent civilizations are there in our galaxy?" down into a series of factors that can then be estimated and combined into an overall estimate.

Let's take a simpler approach here.

The probability of detecting extraterrestrial intelligence given our efforts so far is the product of:

• The probability it exists
• The probability that what we've done so far would detect it, given that it exists

(For any math geeks out there, this is just the definition of conditional probability)

Various takes on the Fermi paradox (why haven't we seen anyone, if we're pretty sure they're out  there?) address these two factors

• Maybe intelligent life is just a very rare accident.  As far as we can tell, Earth itself has lacked intelligent life for almost all of its history (one could argue it still does, so feel free to substitute "detectable" for "intelligent").
• Maybe intelligent life is hard to detect for most of the time it's around (See this post for an argument to that effect and this one for a bit on the distinction between "intelligent" and "detectable").  A particularly interesting take on this is the "dark forest" hypothesis, that intelligent civilizations soon figure out that being detectable is dangerous and deliberately go dark, hoping never to be seen again.  I mean to take this one on in a bit, but not here.
• One significant factor when it comes to detecting signs of anything, intelligent or otherwise: as far as we know detectability drops with the square of distance, that is, twice as far away means four times harder to detect.  Stars are far away.  Other galaxies are really far away.
• Maybe intelligent life is apt to destroy itself soon after it develops, so it's not going to be detectable for very long and chances are we won't have been looking when they were there .  This is a popular theme in the literature.  I've talked about it here and here.
• Maybe the timing is just wrong.  Planetary time scales are very long.  Maybe we're one of the earlier ones and life won't develop on nearby planets for another million or billion years (basically low probability of detection again, but also an invitation to be more rigorous about the role of timing)

At first blush, the logic of the Fermi paradox seems airtight: Aliens are out there.  We'd see them if they were out there.  We haven't seen them.  QED.  But we're not doing a mathematical proof here.  We're dealing in probabilities (also math, but a different kind).  We're not trying to explain a mathematically impossible result.  We're trying to determine how likely it is that our observations are compatible with life being out there.

I was going to go into a longish excursion into Bayesian inference here, but ended up realizing I'm not very adept at it (note to self: get better at Bayesian inference).  So in the spirit of keeping it at least somewhat simple, let's look at a little badly-formatted table with, granted, a bunch of symbols that might not be familiar:

	We see life (S)	We don't see life (¬S)
Life exists (L)	P(L ∧ S)	P(L ∧ ¬S)	P(L)
No life (¬L)	P(¬L ∧ S)	P(¬L ∧ ¬S)	P(¬L)
	P(S)	P(¬S)	100%

P is for probability.  P(L) is the probability that there's intelligent life out there we could hope to detect as such, at all.  P(S) is the probability that we see evidence strong enough that the scientific community (whatever we mean by that, exactly) agrees that intelligent life is out there.  The ¬ symbol means "not" and the ∧ symbol means "and".  The rows sum to the right, so

• P(L ∧ S) + P(L ∧ ¬S) = P(L) (the probability life exists is the probability that life exists and we see it plus the probability it exists and we don't see it)
• P(S + ¬S) = 100% (either we see life or we don't see it)

Likewise the columns sum downward.  Also "and" means multiply (as long as the two probabilities are independent; they are here, since we allow for false positives), so P(L ∧ S) = P(L)×P(S).  This all puts restrictions on what numbers you can fill in.  Basically you can pick any three and those determine the rest.

Suppose you think it's likely that life exists, and you think that it's likely that we'll see it if it's there.  That means you think P(L) is close to 100% and P(L ∧ S) is a little smaller but also close to 100% (see conditional probability for more details) .  You get to pick one more.  It actually turns out not to matter that much, since we've already decided that life is both likely and likely to be detected.  One choice would be P(¬L ∧ S), the chance of a "false positive", that is, the chance that there's no life out there but we think we see it anyway.  Again, in this scenario we're assuming false positives should be unlikely overall, but choosing exactly how unlikely locks in the rest of the numbers.

It's probably worth calling out one point that kept coming up while I was putting this post together: The chances of finding signs of life depend on how much we've looked and how we've done it.  A lot of SETI has centered around radio waves, and in particular radio waves in a fairly narrow range of frequencies.  There are perfectly defensible reasons for this approach, but that doesn't mean that any actual ETs out there are broadcasting on those frequencies.  In any case we're only looking at a small portion of the sky at any given moment, our current radio dishes can only see a dozen or two light years out and there's a lot of radio noise from our own technological society to filter out.

I could model this as a further conditional probability, but it's probably best just to keep in mind that P(S) is the probability of having detected life after everything we've done so far, and so includes the possibility that we haven't really done much so far. 

To make all this concrete, let's take an optimistic scenario: Suppose you think there's a 90% chance that life is out there and a 95% chance we'll see it if it's out there.  If there's no chance of a false positive, then there's an 85.5% chance that we'll see signs of life and so a 14.5% chance we won't (as is presently the case, at least as far as the scientific community is concerned).  If you think there's a 50% chance of a false positive, then there's a 90.5% chance we'll see signs of life, including the 5% chance it's not out there but we see it anyway.  That means a 9.5% chance of not seeing it, whether or not it's actually there.

This doesn't seem particularly paradoxical to me.  We think life is likely.  We think we're likely to spot it.  So far we haven't.  By the assumptions above, there's about a 10% chance of that outcome.  You generally need 99.99994% certainty to publish a physics paper, that is, a 0.00006% chance of being wrong.  A 9.5% chance isn't even close to that

Only if you're extremely optimistic and you think that it's overwhelmingly likely that detectable intelligent life is out there, and that we've done everything possible to detect it do we see a paradox in the sense that our present situation seems very unlikely.  But when I say "overwhelmingly likely" I mean really overwhelmingly likely.  For example, even if you think both are 99% likely, then there's still about a 1-2% chance of not seeing evidence of life, depending on how likely you think false positives are.  If, on the other hand, you think it's unlikely that we could detect intelligent life even if it is out there, there's nothing like a paradox at all.

My personal guess is that we tend to overestimate the second of the two bullet points at the beginning.  There are good reasons to think that life on other planets is hard to detect, and our efforts so far have been limited.  In this view,  the probability that detectably intelligent life is out there right now is fairly low, even if the chance of intelligent life being out there somewhere in the galaxy is very high and the chance of it being out there somewhere in the observable universe is near certain.

As I've argued before, there aren't a huge number of habitable planets close enough that we could hope to detect intelligent life on them, and there's a good chance that we're looking at the wrong time in the history of those planets -- either intelligent life hasn't developed yet or it has but for one reason or another it's gone dark.

Finding out that there are potentially habitable worlds in our own solar system is exciting, but probably doesn't change the picture that much.  There could well be a technological civilizations in the oceans of Enceladus, but proving that based on what molecules we see puffing out of vents on the surface many kilometers above said ocean seems like a longshot.

With that in mind, let's put some concrete numbers behind a less optimistic scenario.  If there's a 10% chance of detectable intelligent life (as opposed to intelligent life we don't currently know how to detect), and there's a 5% chance we'd have detected it based on what we've done so far and a 1% chance of a false positive (that is, of the scientific community agreeing that life is out there when in fact it's not), then it's 98.6% likely we wouldn't have seen clear signs of life by now.   That seems fine.

While I'm conjecturing intermittently here, my own wild guess is that it's quite likely that some kind of detectable life is out there, something that, while we couldn't unequivocally say it was intelligent, would make enough of an impact on its home world that we could hope to say "that particular set of signatures is almost certainly due to something we would call life".   I'd also guess that it's pretty likely that in the next, say, 20 or 50 or 100 years we would have searched enough places with enough instrumentation to be pretty confident of finding something if it's there.  And it's reasonably likely that we'd get a false positive in the form of something that people would be convinced it was a sign of life when there in fact wasn't -- maybe we'd figure out our mistake in another 20 or 50 or 100 years.

Let's say life of some sort is 90% likely, there's a 95% chance of finding it in the next 100 years if it's there and a 50% chance of mistakenly finding life when it's not there, that is, a 50% chance that at some point over those 100 years we mistakenly convince ourselves we've found life and later turn out to be wrong.  Who knows?  False positives are based on the idea that there's no detectable life out there, which is another question mark.  But let's go with it.

I actually just ran those numbers a few paragraphs ago and came up with a 9.5% chance of not finding anything, even with those fairly favorable odds.

All in all, I'd say we're quite a ways from any sort of paradoxical result.

One final thought occurs to me:  The phrase "Fermi paradox" has been in the lexicon for quite a while, long enough to have taken on a meaning of its own.  Fermi himself, being one of the great physicists, was quite comfortable with uncertainty and approximation, so much so that the kind of "How many piano tuners are there in Chicago?" questions given to interview candidates are meant to be solved by "Fermi estimation".

I should go back and get Fermi's own take on the "Fermi paradox".  My guess was he wasn't too bothered by it and probably put it down to some combination of "we haven't really looked" and "maybe they're not out there".

If I find out I'll let you know.

[As noted above, I did in fact come across something --D.H Oct 2018]

Are we alone in the face of uncertainty?

I keep seeing articles on the Drake equation and the Fermi Paradox on my news feed, and since I tend to click through and read them, I keep getting more of them.  And since I find at least some of the ideas interesting, I keep blogging about them.  So there will probably be a few more posts on this topic.  Here's one.

One of the key features of the Drake equation is how little we know, even now, about most of the factors.  Along these lines, a recent (preprint) paper by Anders Sandberg, Eric Drexler and Toby Ord claims to "dissolve" the Fermi Paradox (with so many other stars out there why haven't we heard from them?), claiming to find "a substantial ex ante probability of there being no other intelligent life in our observable universe".

As far as I can make out, "ex ante" (from before) means something like "before we gather any further evidence by trying to look for life".  In other words, there's no particular reason to believe there should be other intelligent life in the universe, so we shouldn't be surprised that we haven't found any.

I'm not completely confident that I understand the analysis correctly, but to the extent I do, I believe it goes like this (you can probably skip the bullet points if math makes your head hurt -- honestly, some of this makes my head hurt):

• We have very little knowledge of the some of the factors in the Drake equation, particularly fl (probability of life on a planet that might support life) fi (probability of a planet with life developing intelligent life) and L (the length of time a civilization produces a detectable signal)
• Estimates of those range over orders of magnitude.
• Estimates for L range from 50 years to a billion or even 10 billion years.
• The authors do some modeling and come up with a range of uncertainty of 50 orders of magnitude for fl.  That is, it might be close to 1 (that is, close to 100% certain), or it might be more like 1 in 100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.  Likewise they take fi to range over three orders of magnitude, from near 1 to 1 in 1,000.
• Rather than assigning a single number to every term, as most authors do, it makes more sense to assign a probability distribution.  That is, instead of saying "the probability of life arising on a suitable planet is 90%", or 0.01% or whatever, assign probability for each possible value (the actual math is a bit more subtle, but that should do for our purposes).  Maybe the most likely probability of life developing intelligence is 1 in 20, but there's a possibility, though not as likely, that it's actually 1 in 10 or 1 in 100, so take that into account with a probability distribution..
• (bear in mind that the numbers were looking at are themselves probabilities, so we're assigning a probability that the probability is a given number -- this is the part that makes my head hurt a bit)
• Since we're looking very wide ranges of values, a reasonable distribution is the "log normal" distribution -- basically "the number of digits fits a bell curve".
• These distributions have very long tails, meaning that if, say, 1 in a thousand is a likely value for the chance of life evolving into intelligent life, then (depending on the exact parameters) 1 in a million may be reasonably likely, 1 in a billion not too unlikely and 1 in trillion is not out of the question.
• The factors in the Drake equation multiply, following the rules of probability, so it's quite possible that the aggregate result is very small.
• For example if it's reasonably likely that fl is 1 in a trillion and fi is 1 in a million, then we can't ignore the chance that the product of the two is 1 in a quintillion.
• Numbers like that would make it unlikely that there is any life in our galaxy's few hundred billion stars and that ours just happened to get lucky.
• Putting it all together, they estimate that there's a significant chance that we're alone in the observable universe.

I'm not sure how much of this I buy.

There are two levels of probability here.  The terms in the Drake equation represent what has actually happened in the universe.  An omniscient observer that knew the entire history of every planet in the universe (and exactly what was meant by "life" and "intelligent") could count the number of planets, the number that had developed life and so forth and calculate the exact values of each factor in the equation.

The probability distributions in the paper, as I understand it, represent our ignorance of these numbers.  For all we know, the portion of "habitable" planets with intelligent life is near 100%, or near 1 in a quintillion or even lower.  If that's the case, then the paper is exploring to what extent our current knowledge is compatible with there being no other life in the universe.  The conclusion is that the two are fairly compatible -- if you start with what (very little) we know about the likelihood of life and so forth, there's a decent chance that the low estimates are right, or even too optimistic, and there's no one but us.

Why?  Because low probabilities are more plausible than we think, and multiplying probabilities increases that effect.  Again, the math is a bit subtle, but if you have a long chain of contingencies, any one of them failing breaks the whole chain.  If you have several unlikely links in the chain, the chances of the chain breaking are even better.

The conclusion -- that for all we know life might be extremely rare -- seems fine.  It's the methodology that makes me a bit queasy.

I've always found the Drake equation a bit long-winded.  Yes, the probability of intelligent life evolving on a planet is the probability of life evolving at all multiplied by the probability of life evolving into intelligent life, but does that really help?

On the one hand, it seems reasonable to separate the two.  As far as we know it took billions of years to go from one to the other, so clearly they're two different things.

But we don't really know the extent of our uncertainty about these things.  If you ask for an estimate of any quantity like this, or do your own estimate based on various factors, you'll likely* end up with something in the wide range of values people consider plausible enough to publish (I'm hoping to say more on this theme in a future post).  No one is going to say "zero ... absolutely no chance" in a published paper, so it's a matter of deriving a plausible really small number consistent given our near-complete ignorance of the real number -- no matter what that particular number represents or how many other numbers it's going to be combined with.

You could almost certainly fit the results of surveying several good-faith attempts into a log-normal distribution.  Log-normal distributions are everywhere, particularly where the normal normal distribution doesn't fit because the quantity being measured has something exponential about it -- say, you're multiplying probabilities or talking about orders of magnitude.

If the question is "what is the probability of intelligent life evolving on a habitable planet?" without any hints as to how to calculate it, that is, one not-very-well-determined number rather than two, then the published estimates, using various methodologies, should range from a small fraction to fairly close to certainty depending on the assumptions used by the particular authors.  You could then plug these into a log normal distribution and get some representation of our uncertainty about the overall question, regardless of how it's broken down.

You could just as well ask "What is the probability of any self-replicating system arising on a habitable planet?", "What is the probability of a self-replicating system evolving into cellular life?"  "What is the probability of cellular life evolving into multicellular life?" and so forth, that is, breaking the problem down into several not-very-well-determined numbers.  My strong suspicion is that the distribution for any one of those sub-parts will look a lot like the distribution for the one-question version, or the parts of the two-question version, because they're basically the same kind of guess as any answer to the overall question.  The difference is just in how many guesses your methodology requires you to make.

In particular, I seriously doubt that anyone is going to cross-check that pulling together several estimates is going to yield the same distribution, even approximately, as what's implied by a single overall estimate.  Rather, the more pieces you break the problem into, the more likely really small numbers become, as seen in the paper.

I think this is consistent with the view that the paper is quantifying our uncertainty.  If the methodology for estimating the number of civilizations requires you to break your estimate into pieces, each itself with high uncertainty, you'll get an overall estimate with very high uncertainty.  The conclusion "we're likely to be alone" will lie within that extremely broad range, and may even take up a sizable chunk of it.  But again, I think this says much more about our uncertainty than about the actual answer.

I suspect that if you surveyed estimates of how likely intelligent life is using any and all methodologies*, the distribution would imply that we're not likely to be alone, even if intelligent life is very rare.  If you could find estimates of fine-grained questions like "what is the probability of multicellular life given cellular life?" you might well get a distribution that implied we're an incredibly unlikely fluke and really shouldn't be here at all.  In other words, I don't think the approach taken in the paper is likely to be robust in the face of differing methodologies.  If it's not, it's hard to draw any conclusions from it about the actual likelihood of life.

I'm not even sure, though, how feasible it would be to survey a broad sample of methodologies.  The Drake formulation dominates discussion, and that itself says something.  What estimates are available to survey depends on what methods people tend to use, and that in turn depends on what's likely to get published.  It's not like anyone somehow compiled a set of possible ways to estimate the likelihood of intelligent life and prospective authors each picked one at random.

The more I ponder this, the more I'm convinced that the paper is a statement about the Drake equation and our uncertainty in calculating the left hand side from the right.  It doesn't "dissolve" the Fermi paradox so much as demonstrate that we don't really know if there's a paradox or not.  The gist of the paradox is "If intelligent life is so likely, why haven't we heard from anyone?", but we really have no clear idea how likely intelligent life is.

* So I'm talking about probabilities of probabilities about probabilities?

Did clickbait kill the aliens?

Disclaimer: This post is on a darker topic than most.  I've tried to adjust the tone accordingly, but if anything leads you to ask "How can he possibly say that so casually?", rest assured that I don't think any of this is a casual matter.  It's just that if we're talking at the scale of civilizations and stars we have to zoom out considerably from the everyday human scale, to the point where a truly horrible cataclysm becomes just another data point.


As I've noted elsewhere, the Fermi paradox is basically "It looks likely that there's life lots of other places in the universe, so why haven't we been able to detect it -- or why haven't they made it easy by contacting us?"  Or, as Fermi put it, "Where is everybody?"

One easy answer, though something of a downer, is "They're all dead."^*

This is the idea that once a species gets to a certain level of technological ability, it's likely to destroy itself.  This notion has been floated before, in the context of the Cold War: Once it became technically possible, it took shockingly little time for humanity to develop enough nuclear weapons to pose a serious threat to itself.  One disturbingly ready conclusion from that was that other civilizations hadn't contacted us because they'd already blown themselves up.

While this might conjure up images of a galaxy full of  the charred, smoking cinders of once vibrant, now completely sterile planets, that's not exactly what the hypothesis requires.  Before going into that in detail, it's probably worth reiterating here that most planets in the galaxy are much too far away to detect directly against the background noise, or to be able to carry on a conversation with (assuming that the speed of light is the cosmic speed limit we think it is).  In order to explain why we haven't heard from anyone, we're really trying to explain why we haven't heard from anyone within, say, a hundred light years.  I've argued elsewhere that that narrows the problem considerably (though maybe not).


A full-scale nuclear exchange by everyone with nuclear weapons would not literally kill all life on Earth.  There are a lot of fungi and bacteria, and a lot of faraway corners like hydrothermal vents for all kinds of life to hide.  It probably wouldn't even kill all of humanity directly, but -- on top of the indescribable death and suffering from the bombing itself -- it would seriously damage the world economy and make life extremely difficult even in areas that weren't directly affected by the initial exchange.  Behind the abstraction of the "world economy" is the flow of food, medicine, energy and other essentials.

There is an extensive literature concerning just how bad things would get under various assumptions, but at some point we're just quibbling over levels of abject misery.  In no realistic case is bombing each other better for anyone involved than not bombing each other.

For our purposes here, the larger point is clear: a species that engages in a full-scale nuclear war is very unlikely to be sending out interstellar probes or operating radio beacons aimed at other stars.  It may not even be sending out much in the way of stray radio signals at all.  It might well be possible for a species in another star system to detect life in such a case without detecting signs of a technological civilization, much less communicating with it.

So how likely is a full-scale nuclear war?  We simply don't know.  So far we've managed to survive several decades of the nuclear age without one, but, as I've previously discussed, that's no time at all when it comes to estimating the likelihood of finding other civilizations.  To totally make up some numbers, suppose that, once nuclear weapons are developed, a world will go an average of a thousand years without seriously using them and then, after the catastrophe, take a couple of centuries to get back to the level of being able to communicate with the rest of the universe.

Again, who knows?  We (fortunately) have very little data to go on here.  In the big picture, though, this would mean that a planet with nuclear weaponry or something similarly dangerous would be 10-20% less likely to be detected than one without.  We also have to guess what portion of alien civilizations would be subject to this, but how likely is it, really, that someone would develop the ability to communicate with the stars without also figuring out how to do anything destructive with its technology?

My guess is  that "able to communicate across interstellar distances" is basically the same as "apt to destroy that ability sooner or later".  This applies particularly strongly to anyone who could actually send an effective interstellar probe.  The  kinetic energy of any macroscopic object traveling close to light speed is huge.  It's hard to imagine being able to harness that level of energy for propulsion without also learning how to direct it toward destruction.

For purposes of calculation, it's probably best to assume a range of scenarios.  In the worst case, a species figures out how to genuinely destroy itself, and perhaps even life on its planet, and is never heard from.  In a nearly-as-bad case, a species spends most of its time recovering from the last major disaster and never really gets to the point of being able to communicate effectively across interstellar distances, and is never heard from.  The upshot is a reduction in the amount of time a civilization might produce a detectable signal (or, in a somewhat different formulation, the average expected signal strength over time).

Our own case is, so far, not so bad, and let's hope it continues that way.  However, along with any other reasons we might not detect life like us on other planets, we can add the possibility that they're too busy killing each other to say hello.


With all that as context, let's consider a recent paper modeling the possibility that a technological civilization ends up disrupting its environment with (from our point of view here, at least) pretty much the same result as a nuclear war.   The authors build a few models, crunch through the math and present some fairly sobering conclusions: Depending on the exact assumptions and parameters, it's possible for a (simulated) civilization to reach a stable equilibrium with its (simulated) environment, but several other outcomes are also entirely plausible: There could be a boom-and-bust that reduces the population to, say, 10% of its peak.  The population could go through a repeating boom/bust cycle.  It could even completely collapse and leave the environment essentially unlivable.

So what does this add to the picture?  Not much, I think.

The paper reads like as a proof-of-concept of the idea of modeling an alien civilization and its environment using the same mathematical tools (dynamical system theory) used to model anything from weather to blood chemistry to crowd behavior and cognitive development.  Fair enough.  There is plenty of well-developed math to apply here, but the math is only as good as the assumptions behind it.

The authors realize this and take care only to make the most general, uncontroversial assumptions possible.  They don't assume anything about what kind of life is on the planet in question, or what kind of resources it uses, or what exact effect using those resources has on the planet.  Their assumptions are on the order of "there is a planet", "there is life on it", "life consumes resources" and so forth.

Relying on few assumptions means that any conclusions you do reach are very general.  On the other hand, if the assumptions support a range of conclusions, how do you pick from amongst them?  Maybe once you run through all the details, any realistic set of assumption leads to a particular outcome -- whether stability or calamity.  Maybe most of the plausible scenarios are in a chaotic region where the slightest change in inputs can make an arbitrarily large difference in outputs.  And so forth.

As far as I can make out, the main result of the paper is that planets, civilizations and their resources can be modeled as dynamical systems.  It doesn't say what particular model is appropriate, much less make any claims about what scenarios are most likely for real civilizations on real exoplanets.  How could it?   Only recently has there been convincing evidence that exoplanets even exist.  The case that there is life on at least some of them is (in my opinion) reasonably persuasive, but circumstantial.  It's way, way too early to make any specific claims about what might or might not happen to civilizations, or even life in general, on other planets.

To be clear, the authors don't seem to be making any such claims, just to be laying some groundwork for eventually making such claims.  That doesn't make a great headline, of course.  The article I used to find the paper gives a more typical take: Climate change killed the aliens, and it will probably kill us too, new simulation suggests.

Well, no.  We're still in the process of figuring out exactly what effect global warming and the resulting climate change will have on our own planet, where we can take direct measurements and build much more accurate models than the authors of the paper put forth.  All we can do for an alien planet is lay out the general range of possibilities, as the authors have done.  Trying to draw conclusions about our own fate from our failure (so far) to detect others like us seems quite premature, whether the hypothetical cause of extinction is war or a ruined environment.



There's a familiar ring to all this.   When nuclear destruction was on everyone's mind, people saw an obvious, if depressing, answer to Fermi's question.  As I recall, papers were published and headlines written.  Now that climate-related destruction is on everyone's mind, people see an obvious, if depressing, answer to Fermi's question, with headlines to match.  It's entirely possible that fifty years from now, if civilization as we know it is still around (as I expect it will be) and we haven't heard directly from an alien civilization (as I suspect we won't), people will see a different obvious, if depressing, answer to Fermi's question.  Papers will be written about it, headlines will do what headlines do, and it will all speak more to our concerns at the time than to the objective state of any alien worlds out there.


I want to be clear here, though.  Just because headlines are overblown doesn't mean there's nothing to worry about.  Overall, nuclear weapons take up a lot less cultural real estate than they did during the height of the cold war, but they're very much still around and just as capable of wreaking widespread devastation.  Climate change was well underway during that period as well, and already recognized as a hazard, but not nearly as prominent in the public consciousness as it is today.

It's tempting to believe in an inverse relationship between the volume of headlines and the actual threat: If they're making a big deal out of it, it's probably nothing to worry about.  But that's an empirical question to be answered by measurement.  It's not a given.  Without actually taking measurements, the safest assumption is the two are unrelated, not inversely related.  That is, how breathless the headlines are is no indication one way or another as to how seriously to take the threat.

My own guess, again without actually measuring, is that there's some correlation between alarming headlines and actual danger.  People study threats and publish their findings.  By and large, and over time, there is significant signal in the noise.  If a range of people working in various disciplines say that something is cause for concern, then it most likely is -- nuclear war and climate change are real risks.  Some part of this discussion finds its way into the popular consciousness, with various shorthands and outright distortions, but if you take the time to read past the headlines and go back to original sources you can get a reasonable picture, and one that will bear at least some resemblance to the headlines.

Going back to original sources and getting the unruly details may not be as satisfying as a nice, punchy one-sentence summary, but I'd say it's worth the effort nonetheless.



(*) A similar but distinct notion is the "Dark forest" hypothesis: They're out there, but they're staying quiet so no one else kills them -- and we had best follow suit.  That's fodder for another post, though I think at least some of this post applies.

Detectability and the Drake Equation

I've argued in posts on the Drake Equation (really more a framework for trying to work out the odds of finding extraterrestrial life), that the L factor, representing the amount of time for which an intelligent civilization is detectable on a planet, is both underappreciated and overestimated.  That is, it's not just important whether a planet can develop life -- which is were a lot of attention is -- but just how long a planet with intelligent life is detectable as such.  If that time span is not very long, then there might well be intelligent civilizations out there that we don't know about, or have much hope of ever knowing about.

Radio transmissions are often used as a proxy for intelligent life.  Clearly, if we detect a radio signal coming from planet X with a structure we can't explain by natural means, we have to seriously consider the possibility that some intelligent life form sent the signal.  Artificial-looking radio signals strongly imply intelligent life, but lack of them doesn't imply lack of intelligent life.

Radio isn't the only way to go looking for intelligent life.  We're already able to get some idea of the atmospheres of exoplanets based on their effect on light from the parent star as they transit between us and that star, provided everything is in a favorable alignment.  That ability is liable to improve over time, to the point where we'll be able to detect whether a planet has a chemical composition that's likely to be produced by something like life as we know it.  That's pretty impressive, if you think about what it entails, and we're just getting started.  Astronomy has gotten really good at gleaning ridiculously faint signals from vast fields of noise, and while there are some fundamental limits to what we can gather, there's clearly a lot more we can do within those limits.

Likewise, if we can detect some signal related to a planet's surface, and we can observe the same planet from different angles (which is often possible since planets rotate) we can get some idea of any changes in the surface as seen from different angles.  Similar techniques were used to get a very rough map of Pluto's surface prior to the New Horizons mission.  It may also be possible to detect the polarization of light coming from a planet, and there are probably other sources of data.  Put together enough such hints and we may be able to measure whether a planet has anomalously dull or shiny or hot or cold regions or similar that might indicate ... something, maybe enough to say that there's probably a civilization something like ours on a given planet, and not just an odd configuration of protoplanetary dust.


So suppose that some twin Earth in the general vicinity -- say a few dozen or a few hundred light-years -- develops along similar lines to ours.  Suppose that some time after a species like ours arises it discovers radio, but not long after that it finds more efficient ways of communicating than blasting radio waves in all directions (including a tiny fraction headed towards us).  In my previous posts I argued that that's probably about it.  We won't be able to detect any signs of intelligent life (life, yes, civilization no) except for a tiny portion of the planet's existence, and so the odds are very low that we happen to be listening at the same time they're broadcasting.

But surely twin Earth will have cities and other large artifacts for longer than it has detectable radio emissions, and have them both before and after their radio era.  Our ability to detect such artifacts will only improve over time.  Suppose our techniques get to the point where we could detect the analog of a city of, say, 100,000 people by way of its structures and overall impact on the surrounding environment.  There are thousands of those on Earth now and, more to the point, there have been for quite a while.  Thousands of years, versus decades for radio transmission.  It's at least possible that there will continue to be cities for thousands of years more.  This is a couple of orders of magnitude longer than our detectable radio era might end up being, not something easy to write off.

Nonetheless, I don't know that it changes the picture much.  On the one hand, even this larger time window is still pretty small on a planetary time scale.  On the other hand, it's not at all clear to me that detecting a signature consistent with cities means that there are cities there.  I'd want to see a lot of work to rule out natural formations that we haven't thought of, and even then city-like collections of life don't necessarily mean intelligent civilization.  We are not the only life forms on Earth that can gather in numbers or have a significant environmental effect.

It's also entirely possible that we'd see the same effect with cities on Earth as with radio, just on a slower scale.  That is, we or our counterparts might have cities for a long time, but not have detectable cities for very long.  I'm not going to predict that humanity will necessarily lessen its overall impact on the environment over time, but it's possible.  If we become cleaner and (much the same thing) more efficient, we become harder to spot, and likewise for a hypothetical alien civilization.

Nonetheless, it seems dangerous to assume that whatever impact we do have, or an alien civilization has, will be undetectable from interstellar distances.  It will probably be detectable as an overall signature.  The question is what could we make of such a signature.  We'd probably be able to associate it with life, but what kind of life?

[ Re-reading an earlier post I see I already took this point into account, although in a more abstract way -- D.H.]

Reworking the Drake equation

In speculating about life on other worlds (here and here for example) the Drake Equation provides a useful framework.  This equation multiplies a number of factors to arrive at the number of civilizations in the Milky Way that would be technologically capable of communicating with us.

When it was first formulated, most if not all of the factors had such wide error bars that it's hard to argue that any meaningful number could come out of it.  An answer of the form "2.5 million, but maybe zero and maybe several billion or anything in between", while honest, is not a particularly useful result.  For much of the time the Drake Equation has been around, it's been useful more as a  framework for reasoning about the possibility of alien civilizations (and, in my opinion, a reasonable one) than as a way of producing a meaningful number.

Recently, though, a couple of the error ranges have tightened considerably.  Let's look at the factors in question:

• the average rate of star formation in our galaxy.  This is currently estimated at 1.5 - 3 stars per year
• the fraction of formed stars that have planets. This is quite likely near 100%
• the average number of planets per star that can potentially support life.  There is some dispute over this.  You can find numbers from 0.5 to 4 or 5, and even outside that range.  My personal guess is toward the high end. 
• the fraction of those planets that actually develop life.  At this point we can only extrapolate from life on Earth, a minimal and biased sample.  It's noteworthy that life now seems to have begun shortly (in geological terms) after suitable conditions arose.
• the fraction of planets bearing life on which intelligent, civilized life has developed.  Developing intelligent life as we understand it took considerably longer: billions of years.  Again extrapolating from our one known example, this implies that a large fraction of life-bearing planets haven't been around long enough to develop intelligent life.
• the fraction of these civilizations that have developed technologies that release detectable signals into space.  Still extrapolating, this fraction may be pretty high.   On geological scales, humanity developed radio pretty much instantaneously, suggesting it was nearly inevitable.
• the length of time, L, over which such civilizations release detectable signals.  I've argued that this is probably quite short (see the links above and the discussion below for a bit more detail).

Looking at the units in those factors, we have

• civilizations = (stars/time) * (a bunch of fractions that amount to civilizations/star) * time

which is perfectly valid.  However, I'm not sure it's the best match for the problem that we're trying to solve.  I've argued previously that timing is important.  The last factor (length of time a civilization produces detectable signals) takes that into account, but the other time factor, in the rate of star formation, seems less relevant.  There are billions of stars in the galaxy.  At a rate of a couple of stars per year that's not going to change meaningfully over human timescales.

So let's try the same general idea but with different units:

• expected signal = planets * (expected signal / planet)

First, shift the focus from stars to planets.  For our purposes here that includes objects like planet-sized moons of gas giants.  This cuts out the estimation of star formation and planets per star, since we can now observe planets (in some cases even directly) and get a pretty good count of them.  Or at least we're now guessing about planets directly, instead of guessing about stars and planets.

Then, let's pull back a bit from the details of how a planet would produce a signal of intelligent life, and focus on the signal itself, by estimating how strong a signal we can expect from a given planet.   This consolidates the estimates of life evolving, civilization evolving, civilization developing technology and the duration of any signal into a single factor.

The "expected" means we're looking at weighted probabilities.  To take a familiar example, if you roll a six-sided die and I pay you $10 per pip that comes up, you should expect to get $35 on average and you shouldn't pay more than that to play the game.  This really only holds up if you expect to play the game a number of times.  If you only roll the dice once, you could always just get a bad roll (or a good one).

Likewise, if we say that a planet is producing a signal of a given expected strength, we're saying that's the average strength over all the possibilities for that planet -- maybe it's young with only one-celled life, maybe it's harboring a civilization that's producing radio signals, etc.  We're not claiming that it's actually producing a signal of that strength.  We can get away with this, more or less, because we'll be adding up expectations over a reasonably large number of planets.

Looking at expected signal accounts for a couple of factors.  What a planet emits in the radio spectrum will vary over time.  The raw strength will vary.  Earth has gone from watts to at least gigawatts in the past century or so.  The signal to noise ratio will also vary.  As we make better use of encryption, compression and such, our signal looks more like noise.  Signal strength also accounts for distance.  A radio signal falls off as the square of the distance. 

A given planet will have a particular profile of signal strength over time.  Ours is zero for most of our history, rises significantly as humans develop radio and (I've argued), will drop off significantly as we come to use radio more efficiently and use broadcast less and less.

There are two sources of uncertainty in what strength of signal we would expect to detect, knowing how far away a planet is and how much background noise there is:  We don't know what the signal strength profile for a given planet is, and we don't know where we are in that profile, that is, just how old the planet is at the moment.

For the first uncertainty, the best we can currently do is compare to our experience on earth.  My best guess is that we should expect a very brief blip (brief on planetary scales).  If we expect a blip on the order of hundred years and a planetary age on the order of billions of years, this reduces the expected signal -- again, "expected" in the probabilistic sense -- at any given time to a very low level.  This would be true even if planets occasionally send out strong, targeted transmissions, as ours does.

In the absence of anything better, we can account for the second uncertainty by averaging the signal strength over the expected age of the planet.  That is, we assume the planet could be at any point in its history with equal probability.  In real life, we may be able to do better by looking at factors like the age of the star and the amount of dust around it.

Strictly speaking we should be talking about intervals rather than instants, since listening for a million years is more likely to turn something up than listening for a hundred, but human timescales are tiny enough that this doesn't really affect our calculations of what we should expect with current or near-future technology over our lifetimes.  Either way, we can still define expected signal.

We also need to account for the distribution of planets in space.  If stars were uniformly distributed in space and background noise didn't matter, this would cancel out the effect of decreasing signal strength, since the number of stars at a given distance would increase as the square of the distance.

But they're not.  If they were then the nighttime sky would also be uniformly bright in all directions.  The Milky way is only about a thousand light years thick.  After about half that distance the number of stars increases much more slowly than the square of the distance.  This means we're really looking at a weighted sum of expectations rather than just multiplying planets by expectation per planet, but that doesn't greatly change the overall analysis.

Finally, we should take background noise into account.  As the strength of a signal (actual, not expected strength) drops toward zero, our ability to detect it doesn't drop in tandem.  Once the signal becomes weaker than the general background noise in that part of the sky, our chances of detecting it are already very near zero.  This correction should be applied to the signal profile before averaging over time.

My engineering intuition tells me that the upshot is that we can neglect planets more than a relatively short distance away, say tens of light-years.  At some point background noise will wash everything out.  That's more or less the limit for having a meaningful conversation anyway, since it takes a year for a radio signal to travel a light-year.

So where does that leave us?

Estimating the probability of a detectable signal from a planet requires knowing

• The distribution of planets as a function of distance.  Our knowledge of this has sharpened dramatically over the past couple of decades.
• The effect of distance on the strength of a signal we detect.  This is fairly well understood.
• The background noise for any particular location in the sky.  This is directly observable.
• The expected strength of the signal emitted by a planet, averaged over its lifetime.  This is where the uncertainty is concentrated.

Essentially we've consolidated all the various fractions of the Drake equation into a single factor and characterized it in terms of signal strength over time (which we then average over time unless we can think of something better).

When searching for life, "signal" doesn't necessarily mean "radio signal".  Soon we will be able to search for signatures such as high levels of oxygen in the atmosphere, which suggest that there is life of a similar form to ours, though not necessarily intelligent, technological or whatever.  This signal would have a much different profile from radio.  In our case it would rapidly jump from zero to full strength relatively early in our history and stay there for billions of years.  It may also be a stronger signal than radio leakage in the sense that we can feasibly detect it from further away.

If we take our experience on earth as a basis, this implies it's quite likely that we'll detect life on other planets, but unlikely that we'll detect radio signals (and probably other smoking-gun signs of civilization as we know it).  Looking for signatures of life in general is probably going to be more informative in any case.  If we don't find any radio signals from other planets, which seems more and more likely, it could just be because even planets with intelligent life don't tend to emit high signal-to-noise radio signals for long.  If we find chemical signatures indicating life on X% of planets with detectable atmospheres, that gives a strong estimate on the probability of life arising in general.  This is true whether X is 0, 100 or something in between.

[Technical note: Somewhat ironically, since I started out talking about unit analysis, the units here are less clear than they might be.  If we're talking about radio, then at any given moment a planet is emitting radio signals at a given power, say X Watts.  Power is energy per unit time.  Probably the most natural way of expressing what we actually detect over time is an amount of energy, say Y Joules -- power times time is energy.  We'd like that to stay the same whether we're talking about an actual measurement or a probabilistic estimate.  So the quantity we're trying to estimate for a given planet is power.

If we assume a particular profile of power over time, and we average it, we're summing up power over time to get total energy, then dividing by the total time span over which we think we might be looking -- the age of the planet -- to get power again.  Accounting for distance still gives power, that is energy we expect to receive per unit time.  Using units of power also accounts for the amount of time we spend looking.  If we look for 100 years we expect to detect 10 times as much signal (energy) as if we look for 10 years.  I tried to gloss over that in the main article on the grounds that the numbers are all likely to be too small to matter.  But it's better to think of a minuscule amount of power over a shorter or longer time than to try to assume everything's an instant.

I've made a few edits to the main article, mainly changing "signal strength" to "signal" in several places to try to reflect this.]

[And having gone through all that, and thought it over a bit more ... the really natural units to use here are bits and bits per second.  At the end of the day, we're trying to glean information from listening to the skies, and information is measured in bits.  This accounts for several troublesome factors:

• We're trying to estimate detectable information from other planets.  This starts by estimating what information they transmit over time, as measured by an observer in the near vicinity (say, in low Earth orbit or on the Moon in our case)
• I've argued that as we use compression and encryption more, our signal looks more like noise.  This is quantifiable in terms of bits and bit rates.
• If a planet is far away or in a noisy area of the sky, we're less likely to detect a signal from it.  There are well-established formulas relating signal power, bandwidth and signal/noise ratios that can be used to translate an estimate of what radio signals a planet emits to an estimate of bits/second we could detect.
• As above, integrating bits/time over time spent listening gives us the total information we would expect to detect, which is arguably the quantity of interest in the whole exercise.
• So
• bits detected = sum over time of the sum over planets of bits per second we expect to detect from each planet
• leaving out the sums, which don't change the units: bits = (bits/second)/planet * planets * seconds

]

Doctors Fermi, Drake and Strangelove

By now it's well-accepted that there are large numbers of planets in the universe that could plausibly support life more or less as we know it.  From this, it follows that unless civilizations like ours are exceedingly rare on such planets, there must be a great number of them in the universe, if not now then at least over history.  A recent paper argues that "... as long as the probability that a habitable zone planet develops a technological species is larger than ∼10⁻²⁴ [that is, about one in a trillion trillion], humanity is not the only time technological intelligence has evolved".

I've argued elsewhere that numbers like that are beyond our ability to understand directly.  For practical purposes, we can call one in a trillion trillion "zero".  The paper is essentially concluding that, based on what we know now, there (practically) certainly have been other intelligent civilizations in the universe.

In evaluating a statement like that it's important to keep in mind the scales involved.  We're talking about the whole universe here, of which our galaxy is only a tiny part, and we're talking about the entire history of the universe, of which human history is only a tiny part.  The authors make a point of not addressing the question of how many such civilizations there might ever have been in our galaxy, much less close enough for communication with Earth to be practical.

They also make a point of not addressing how many such civilizations there might be right now (regardless of where they might be).  I want to get into the significance of that.


Questions of how many intelligent civilizations there might be generally center around the Drake Equation, which is probably best thought of as a framework for breaking down the problem.  The breakdown is that the number of civilizations we could communicate with must be the product of

• Three factors representing the rate at which planets form that might support life appear (we're assuming here, for better or worse, that life lives on planets)
• Three factors representing what portion of those actually produce life that would put out a detectable signal
• How long those civilizations actually put out a detectable signal (the 'L' factor, for 'lifetime').

We now have a pretty good handle on the first bullet point above.  On the other hand, we don't really know how likely it is that a planet that could support life actually develops life or how likely it is that such life actually puts out a detectable signal.  I've previously argued that, because of the distances involved there's a big difference between "detectable" and "detectable by us" and that the last factor, how long there would be a detectable signal, could be very, very short on a cosmic scale.

The paper I referenced sidesteps these questions by considering everything everywhere and over all time, regardless of whether we could hope to make contact or would even be around to try.  That's fine, but in doing so it shifts from the practical question of "Are we alone?", or Fermi's "Where is everyone?", to the more philosophical question of "Are we unique?".  That's an interesting question, but it somehow lacks the emotional resonance of the other two.

I grew up during the Cold War.  I remember the electricity of the Berlin Wall opening, and the profound feeling of disorientation that came with it.  All my life the East and West had been locked in a permanent stalemate with no sign of an end.  And then it ended.  Now what?

For the most part, life went on.  That's not to say that the transition was smooth, particularly if you had lived in the Soviet Union or its satellites.  My point is more that the "western" developed world, at least, went on more or less as it was.  McDonalds is still McDonalds, Hollywood still makes films, football (or soccer, if you prefer) is still the world's sport, the US still doesn't care greatly that it is, and so forth.  MTV is still the place to go for music videos ... oh, wait ...

Except for nukes.

The amount of nuclear weaponry developed during the Cold War is staggering.  The only two nuclear weapons that have actually been used militarily, the ones dropped on Hiroshima and Nagasaki, yielded under 150TJ (or if you prefer, around 35 kilotons) .  We saw what that did.

Modern nuclear warheads are generally in the thousands of TJ, and tens of thousands of those have been made.  While you can't just multiply numbers and say "Ten thousand times as many bombs each yielding ten times as much means a hundred thousand times as many people killed," it was really no exaggeration, at all, to say that humanity now had the means to cause much, much more destruction than had ever been possible before.

This was a fact of life growing up in the cold war.  My high school newspaper once had a debate in the editorial columns about whether a nuclear war could be survived, at all, and if so whether you should even try.  The bidding started at "The US government would no longer exist" and from there it wasn't far to "Industrial civilization would collapse, bringing about a new Dark Age lasting centuries" or "All humans would die as nuclear winter wiped out agriculture and plunged temperatures by 20 degrees Celsius for decades".  It wasn't completely outlandish to speculate that multicellular life would be wiped out.

This colored our outlook on the world.

Today, not so much, which is interesting since there are still thousands of extremely powerful nuclear weapons in the world and it's not clear that they're as tightly controlled now as they had been.  Just why attitudes might have changed is for another discussion.  For now, let's just take it as a given that "nukes could kill us all" is not nearly as prominent a thought in the early 21st century as it was in the mid to late 20th.

That L factor of the Drake equation represents the amount of time during which an intelligent civilization puts out a detectable signal.  This could be a very short time, on cosmic scales, if only because unless you're actually trying to be detected, putting out radio or other signals that could be detected dozens or hundreds of light years away is a large waste of energy.

If you're streaming video over the internet, for example, no one has to broadcast a signal from a tower.  Even if radio signals are involved they are more likely beamed from one microwave station to another or otherwise narrowly focused.  An intelligent species could quite likely get along just fine for almost all of its existence without producing a detectable signal, if it so chose.

When the Drake equation was first developed, however, this wasn't the interpretation that people tended to use.

At the time, we had no idea whether there were many habitable planets out there, but we had made a few efforts to contact other stars and to listen for signs of life on them (including Drake's own Project Ozma), without any clear success.  That suggested that the factors of the Drake equation must multiply out to a small number.

Since we knew even less then than we know now, most of the factors of the equation were little more than wild guesses.  But we did have at least one data point for an intelligent species (at least by our own definition of "intelligent"), and there was one ready explanation that fit with our understanding of that species and the lack of signs of other species like it: Intelligent species didn't last long.

There was ample reason to believe that.  Perhaps it was inevitable that, at least on the cosmic scale, it would not be long between a species developing technology that could have a major impact on its planet and that species destroying itself.  In 1961, when Frank Drake put forth his equation, it had been less than 20 years since the end of World War II and nuclear weapons testing was in full swing.  It was the most natural thing in the world to wonder if we would make it another 20 years.

Now that we've made it over fifty years since then, it may be more natural to assume that we'll still be here in another fifty, or thousand, or whatever, and either to assume that the L factor could be small for any number of non-lethal reasons or to neglect it altogether on the assumption that we'll be around and detectable forever.  What strikes me here is how much room, within the broad limit that our theories need to be consistent with the facts as we know them, there is for them to reflect who we are at the moment. Then as well as now.

More on the invisible oceans, and their roundness

Previously, in speculating about what it would take for an inhabitant of one of the several subsurface oceans thought to exist in our solar system to discover that their world was round, I said

Figuring out that the world is round would be a significant accomplishment.  The major cues the Greeks used -- ships sinking below the horizon, lunar eclipses, the position of the noontime sun at different latitudes -- would not be available.  The most obvious route left is to actually circumnavigate the world.  And figure out that you did it.

Fortunately, I was smart enough to leave myself some wiggle room: "I'm very reluctant to say 'such and such would be impossible because ...'"  After all, humanity is in a similar situation in trying to figure out the shape of our universe, though in our case circumnavigating doesn't seem to be even remotely close to an option.  Even so, we've had some apparent success.

On further reflection, there are at least two ways an intelligent species living in a world like Ganymede's subsurface ocean could figure out that the world is round.

First, and perhaps most likely, it's not necessary for a Ganymedean to circumnavigate the world, if something can.  In this case that something would be sound.  Under the right conditions, sound can travel thousands of kilometers underwater.  The circumference of Ganymede is around 10,000km, which may be too far.  Europa's is around 6,000km, which as I understand it is on the edge of what experiments in Earth's oceans have been able to detect (pretty impressive, that).

We've already postulated that sound would be one of the more important senses in such a world.  It doesn't seem impossible that someone would notice that especially loud noises tended to be followed a couple of hours later by similar noises from all directions.  This assumes that the the oceans are unobstructed, but if they're not, you have landmarks to measure by, which would make the original "circumnavigate and tell that you did it" less of a challenge.

Second, if some sort of light-production and light-detection evolve, then one of the cues the Greeks used is indeed available, at least in theory: one can see things disappear over the horizon.  To actually make use of this one would need to be able to see far enough to tell that the object was disappearing due to curvature and not behind some obstacle, or simply because it was too far away to see.  The exact details depend on how smooth the inner surface is.

Humans noticed the effect with ships because a calm sea is quite flat, that is to say, quite close to perfectly round.  If the inner surface of the ocean is rough, one might have to float at a considerable distance from it, and thus wait for the object to recede a considerable distance, to be sure of the effect.  On the other hand, floating a considerable distance from the inner surface would be much easier than floating the same distance from the surface of the earth.  For that matter, it would also be possible to note what's visible and what's not at different distances from the inner surface.

A little back-of-the-envelope estimation suggests that one would have to be able to see objects kilometers or tens of kilometers away.  By way of comparison, Earth's oceans are quite dark at a depth of one kilometer, so this seems like a longshot.  Nor does it help that it's possible to hear long distances, since sound doesn't necessarily propagate in a straight line (neither does light, but that's a different can of worms).


As I originally disclaimed, it's not a good idea to rule something out as impossible just because you can't think of a way to do it.  The inhabitants of a subsurface ocean would have thousands, if not millions, of years to figure things out, even if they wouldn't have the advantage of already knowing approximately what their world looks like.

The invisible oceans

Life as we know it needs water.  Two conclusions that don't follow from that premise: Life needs water, and where there is water, there must be life.  Nonetheless, in our present ignorance, the best bet for finding other life is to find water.  Liquid water, that is.

Over the past few years there has been a steady stream of discoveries of likely liquid water in our solar system.  Jupiter's moons Ganymede and Europa probably each have more than Earth does, even though both are considerably smaller than Earth.  Enceladus (a small moon of Saturn) probably contains significant amounts.  The planet asteroid dwarf planet Ceres also shows possible signs of an ocean.  Pluto and Charon might also.  We should know more about them in a few months as I write this.  In the case of Pluto and the moons, tidal flexing generates the heat that keeps the water liquid.  The case of Ceres is less clear, so from here on I'll restrict discussion to the moons.

What these masses of water all have in common, of course, is that they lie deep beneath the surface.  Tens of kilometers, at least.  Some, at least are also salty enough to conduct electricity well, as evidenced by their interaction with magnetic fields.  It's not clear what kind of pressure they are under, or what their range of temperatures is, except that they are likely sandwiched between layers of ice, or maybe between ice and rock, or maybe in alternating layers of various forms of ice (yes, there is more than one kind).  They will thus be literally ice cold in at least some regions, though they are probably considerably warmer in other places.  They certainly receive no sunlight.

Could life evolve under such conditions?  Who knows?  Personally I'd be reluctant to rule it out.  We've found life in all kinds of unlikely habitats on Earth, and in any case we just don't know that much about how life develops.  So suppose it did develop on one of these moons.  What would it be like, and what would be our chances of encountering it?


The first question is whether there is enough in these oceans to get microbial life going.  The short answer: no idea.  We don't know very much at all about the chemical composition of these oceans, though we can make some informed guesses, and again we don't know very much at all about how microbial life on Earth developed, though we can make some informed guesses about that, too.

It's pretty clear that there will be various impurities in the ocean water, so there might well be potential for some sort of self-replicating, information-carrying polymer, similar to RNA, to develop.  There is an outside source of heat from tidal flexing, and there will be temperature gradients as a result [perhaps as much as 40K over a few hundred kilometers], so the laws of thermodynamics don't rule anything out.  Let's assume that microbial life of some sort can develop.

The path from single-cell organisms to colonies of single-cell organisms to colonies of single-cell organisms with different forms (but the same genetics) to something we may as well call a multicellular organism is reasonably clear, although at least in the case of life on Earth it seems to have taken a good long time to develop.

Since we're totally speculating, let's assume there are multicellular organisms analogous to our own sea creatures, but possibly very different in form.  Again, in reality there might be nothing at all, or only single-celled organisms, or some sort of microbe without clearly defined cells at all, or who knows what.

What kinds of features might these critters have?

• Some sort of chemical sense analogous to smell or taste seems like a good bet.  It's almost a defining property of life that it will respond to chemical stimuli in some way or another.  Microbes do.  Animals of all sizes do.  Even plants and fungi do.  I wouldn't necessarily call a slime mold constituent detecting another's chemical signal "taste" or "smell" but ... some sort of chemical sense.
• For similar reasons, a temperature sense seems likely.
• Food webs, predator/prey relationships, mutualism, parasitism, commensalism, amensalism and any number of other relationships among organisms are pretty much inevitable when there is more than one kind of organism.
• Some way of shuffling genetic material as with sexual reproduction.  A source of variability beyond random mutation can be useful in surviving long-term in harsh, ever-changing environments.  Even microbes do this to at least some extent.
• Various ways of physically manipulating objects ... tentacles, pincers, pseudopods, maybe even something resembling hands
• Some sort of nervous system for communicating signals, including sensations from the senses, from one part of the body to another
• Sociality in at least some species.  We're assuming there are multicellular organisms, which is not so different from sociality at the cell level.  This is the next level: sociality among multicellular organisms.  Sociality requires some means of communication between organisms.  Chemical signals and sound seem like plausible candidates.

So far we have a world comprising organisms of various sizes and forms, some herding/schooling together, some hunting others, with the ability to grab things and move them around ... much like life as we know it, except quite likely totally different.

But then, we assumed many important premises based on experience with life as we know it, particularly cells, and we assumed that evolution would follow essentially the same rules as here.  The second assumption, at least, seems pretty reasonable, but who knows?

It's natural to think such an ocean would be dark and all its inhabitants blind, but maybe not.  Plenty of deep-sea creatures find it useful to have eyes and even to produce light.  Light-producing molecules are not that complex.  There are ones that require only carbon, hydrogen and oxygen, so we don't need to assume phosphorous or sulfur, just some source of carbon in the water, which is probably inevitable.

The evolutionary pathway to light and sight is not so clear, though.  On Earth there is a source of light independent of life and it's not surprising that eyes would evolve in the sunlit portion of the ocean, at least.  In naturally pitch-dark oceans, there would have to be some source of light, as a side-effect of something else, before light-detecting cells and organs become useful.

So let's assume it's dark.  Being visual animals, we might assume that that's a big deal, but maybe not.  There are plenty of other ways to get a good picture of the world without seeing it.  In particular, hearing will be important.

Sound carries well in water, and given that the whole world is constantly flexing, there ought to be at least some natural sources of sound.  Unlike the case of vision, it's almost inevitable that organisms that are reasonably large and able to move and to move things will end up making some noise doing so.  This all suggests it would useful to be able to hear.  If it's useful to hear, it becomes useful to be able to make sounds, both for communication and possibly for active sonar.

So we have a dark, noisy and probably fairly cold place teeming with organisms of various shapes and sizes, and schools of this and that all trying to eat and not be eaten.  Will we ever see it?  All we'd have to do to see it is fly a probe a billion kilometers or so and have it dig through kilometers of ice.  Keeping in mind that at the surface temperatures we're looking at, ice acts more like any other mineral than the softish stuff you can chew (to the horror of your dentist).

Since we're speculating, let's say that something we would recognize as an intelligent species develops.  Are they likely to come visit?

Humanity went into space driven by (among other things) the curiosity inspired by the sun, moon, stars and planets.  Even if you don't buy that, you have to admit that we could at least see the sun, moon, stars and planets without any technological help.  Our hypothetical ocean-dwellers could live indefinitely with no clue that there was a solar system out there, or anything else at all.  They would have no direct way of knowing that the parent planet, or even the surface of their own moon, existed.

Even the existence of gravity might be the subject of intense debate.  These moons are relatively small.  The surface gravity of Ganymede, for example, is about 0.1g.  If you're considerably beneath the surface the influence of gravity decreases since you also have matter above you, but that's probably not a major factor at the scales we're dealing with.  If our own oceans are any clue, most things will be near neutral buoyancy.

Put that together and you have a general tendency for most living things to float around in an ocean layer dozens of kilometers deep (thick?) and thousands of kilometers around.  Some inanimate things would tend to drift, very slowly, toward the inner surface (that is, sink).  Others would tend to drift, very slowly, toward the outer surface (that is, float).  This would not be nearly so easy to sort out as the general tendency of things to fall quickly to the ground on Earth.

Figuring out that the world is round would be a significant accomplishment.  The major cues the Greeks used -- ships sinking below the horizon, lunar eclipses, the position of the noontime sun at different latitudes -- would not be available.  The most obvious route left is to actually circumnavigate the world.  And figure out that you did it.

I'm very reluctant to say "such and such would be impossible because ...", but I think it's safe to say that a number of things we take for granted living on the solid surface of a planet with an atmosphere transparent in many wavelengths would be a lot harder in these worlds.  On the other hand, if an inhabitant ever did make it to space, they'd probably have a much better intuitive feel for it than we do.

There's one major factor I haven't mentioned here, that's been cited as a reason that no underwater species could ever develop technology.  Fire doesn't work.  Without fire, a host of things become much harder, if not impossible, including metal extraction and heat engines (steam, internal combustion, etc.).  Underwater rocketry also seems like a stretch, though jet propulsion is not a problem (the difference is that jet propulsion uses the medium one is traveling through, while a rocket creates its own exhaust).

From an earthbound perspective, knowing about our technologies, it's easy to say what familiar technologies probably wouldn't work in such a world, at least not the way they work here.  It's harder, though, to say what unfamiliar technologies could work.  This makes it tempting to say things like "There's no way that life in Ganymede's oceans could contact us.  They wouldn't even have fire."

But we've had, depending on how you count, at least thousands of years to figure technology out.  Depending on how things develop, particularly the transition from single-celled to multi-celled organisms, our hypothetical counterparts might have had tens of thousands, or millions of years.  Or no time at all since they haven't gotten far enough yet.

So what's at least possible in such a world?  Here are some wild, sketchily-informed guesses:

Mathematics: "Can mathematics develop?" seems very much the same as "Can intelligent life develop?", if only as a matter of definition.  If it can't handle at least some form of mathematical thought, can we really call it intelligent? Let's assume that these sea creatures have learned not only to count, but to "think abstractly", whatever that means.  While we're at it, let's assume something we would recognize as a language.

Writing: It's unlikely that anyone is going to whip out a ballpoint pen and write on the back of a napkin, so we need to define "writing" a bit more abstractly.  What we're really after is a permanent means of recording language in via discrete symbols, that is, in digital form.  This doesn't require much, for example, some sort of solid material that can be manipulated and will hold its form.  For example, it doesn't seem unlikely that there could be something resembling rope, which would enable something like the quipu.

Physics: If our critters have mathematical ability and curiosity, they will begin to notice and codify basic facts about their physical world.  Fluid dynamics would be an obvious subject of study, along with some aspects of thermodynamics, particularly at water/ice boundaries.  Electromagnetism, or at least magnetism, is not out of the question.

On the other hand, Newtonian mechanics might take quite a while.  We can ignore wind resistance and get reasonable answers for many problems.  Water resistance is a whole different matter, so it might take quite a while to develop a notion of inertia.  Modern physics -- particle physics, quantum mechanics, relativity, plasma physics, low-temperature physics, etc., requires something like modern technology, of which more in a bit.

Chemistry: This will certainly be tricky.  You can't just pour something in a beaker or dump a solid chemical into a liquid solvent -- at least not without producing the solid chemical in the first place.  But who knows?  If there is life, there are chemical reactions going on all the time naturally.  Perhaps someone learns that a particular gland-y thing from one creature does funny things when you stick a bone-y thing from another creature in it, or that you can use thus-and-such material to isolate a kind of water that acts unusually, and eventually this becomes a systematic body of knowledge.  Not out of the question, and not as much of a leap as astronomy would be, but still doesn't seem like it would be a strong suit.

Biology: Large parts of biology -- cell theory, for example -- require some sort of microscope, but other aspects just require careful observations of other living things.  Evolution is an interesting example.  It's not clear what kind of fossil record there might be, though non-living things would tend to float or sink, but evolution fundamentally requires figuring out that there's a family relationship among seemingly different living things, and realizing that the world is old.  That's certainly not out of the question.

Materials science: Metallurgy requires a supply of metal, which would likely be hard to come by, but one could learn a lot about the properties of the materials around -- tensile strength, hardness, elasticity and even, with more careful observation than we need in our environment, density, heat capacity and such.

Cities: There doesn't seem to be any fundamental reason there couldn't be something we might call  agriculture, and floating cities, at least, forming around it.  Suppose again that there's some sort of rope-like material, and suppose something edible likes to grow on it.  It might then be natural to build largish structures and garden them.  This in turn would provide a reason to stay close instead of wandering off, and along come civilization and its discontents.

Fire: At some point a technological species needs to figure out how to do things that don't happen easily in the natural world.  For us, you could argue it was extracting metals sometime in prehistory, or maybe harnessing steam, or maybe something in between.  For an undersea world, creating an environment where things could burn would be not only a significant achievement, but a gateway to what we might consider industrial technology.  If they can make fire, then it's hard to think what human technology would be out of reach, because to make fire, you need to recreate an environment not too different from ours.  At the very least you need a bubble of some sort of oxidizing atmosphere.  Once that happens, pretty much everything that seems implausible on the list above becomes possible.


So who knows?  There doesn't seem to be anything in principle keeping life on some extraterrestrial ocean from being able to get to us.  It just requires a long chain of unlikely events.  But then, life is full of those.  If you roll the dice billions of times and you get to keep lucky changes around for the next roll, the extremely unlikely can become quite plausible.  In this case, our chain of events looks something like

• A self-replicating molecule develops
• Microorganisms develop
• Multicellular organisms develop
• Some of them develop what we might call intelligence
• These develop a body of mathematical and scientific knowledge
• From that, they develop what we would recognize as technological tools, including
• Ways of storing energy and converting it to work
• Probably some way of creating sizable bubbles of gas, and ways of working inside them
• One way or another they find their way to the surface (which is much farther away than humanity has ever managed to dig into the Earth's crust)
• And then they get in contact with us, somehow.

On the other hand, we know we're coming their way (if they're there).  It's going to be quite some time before we can send a probe to their habitat, but there's one other way we could meet up: One of the reasons we believe that Ganymede and other worlds have water: cryovulcanism, that is, liquids coming to the surface through cracks in the crust (the "cryo" part is there because these liquids are a lot colder than the magma we're familiar with).

If water is coming up from the liquid interior of one of these moons, it might well carry something along with it.  With luck, it might be something like a tardigrade that can survive being dried out and frozen.  Or there might just be a fossil bed of life forms that might not have even made it to the surface alive.

Or, maybe, just maybe (and by "just maybe" I mean "almost certainly not, given all the things that would have to go right"), it might be a hardy explorer, wearing some sort of protective suit, struggling against the surface gravity that we would consider negligible, and wondering at the bizarre new world, and ... what's this thing coming at me from the ... what do you even call it without knowing "stars" and "empty space"?