I’m old enough to remember when we had nine planets in the Solar System, and zero outside. The news since then has been mixed. Here in our neighborhood we’re down to only eight planets; but in the wider galaxy, we’ve obtained direct evidence for about a thousand, with another several thousand candidates. [Thanks to Peter Edmonds for a correction there.] Now that we have real data, what used to be guesswork gives way to best-fit statistical inference. How many potentially habitable planets are there in the Milky Way, given some supposition about what counts as “habitable”? Well, there are about 200 billion stars in the galaxy. And about one in five are roughly Sun-like. And now our best estimate is that about one in five of them has a somewhat Earth-like planet. So you do the math: about eight billion Earth-like planets. (Here’s the PNAS paper, by Petigura, Howard, and Marcy.)
“Earth-like” doesn’t mean “littered with human-esque living organisms,” of course. The number of potentially habitable planets is a big number, but to get the number of intelligent civilizations we need to multiply by the fraction of such planets that are home to such civilizations. And we don’t know that.
It’s surprising how many people resist this conclusion. To drive it home, consider a very simplified model of the Drake equation.
x equals a times b. Now I give you a, and ask you to estimate x. Well, you can’t. You don’t know b. In the abstract this seems obvious, but there’s a temptation to think that if a (the number of Earth-like planets) is really big, then x (the number of intelligent civilizations) must be pretty big too. As if it’s just not possible that b (the fraction of Earth-like planets with intelligent life) could be that small. But it could be! It could be 10-100, in which case there could be billions of Earth-like planets for every particle in the observable universe and still it would be unlikely that any of the others contained intelligent life. Our knowledge of how easy it is for life to start, and what happens once it does, is pretty pitifully bad right now.
On the other hand — maybe b isn’t that small, and there really are (or perhaps “have been”) many other intelligent civilizations in the Milky Way. No matter what UFO enthusiasts might think, we haven’t actually found any yet. The galaxy is big, but its spatial extent (about a hundred thousand light-years) is not all that forbidding when you compare to its age (billions of years). It wouldn’t have been that hard for a plucky civilization from way back when to colonize the galaxy, whether in person or using self-replicating robots. It’s not the slightest bit surprising (to me) that we haven’t heard anything by pointing radio telescopes at the sky — beaming out electromagnetic radiation in all directions seems like an extraordinarily wasteful way to go about communicating. Much better to send spacecraft to lurk around likely star systems, à la the monolith from 2001. But we haven’t found any such thing, and 2001 was over a decade ago. That’s the Fermi paradox — where is everyone?
It isn’t hard to come up with solutions to the Fermi paradox. Maybe life is just rare, or maybe intelligence generally leads to self-destruction. I don’t have strong feelings one way or another, but I suspect that more credence should be given to a somewhat disturbing possibility: the Enlightentment/Boredom Hypothesis (EBH).
The EBH is basically the idea that life is kind of like tic-tac-toe. It’s fun for a while, but eventually you figure it out, and after that it gets kind of boring. Or, in slightly more exalted terms, intelligent beings learn to overcome the petty drives of the material world, and come to an understanding that all that strife and striving was to no particular purpose. We are imbued by evolution with a desire to survive and continue the species, but perhaps a sufficiently advanced civilization overcomes all that. Maybe they perfect life, figure out everything worth figuring out, and simply stop.
I’m not saying the EBH is likely, but I think it’s on the table as a respectable possibility. The Solar System is over four billion years old, but humans reached behavioral modernity only a few tens of thousands of years ago, and figured out how to do science only a few hundred years ago. Realistically, there’s no way we can possibly predict what humanity will evolve into over the next few hundreds of thousands or millions of years. Maybe the swashbuckling, galaxy-conquering impulse is something that intelligent species rapidly outgrow or grow tired of. It’s an empirical question — we should keep looking, not be discouraged by speculative musings for which there’s little evidence. While we’re still in swashbuckling mode, there’s no reason we shouldn’t enjoy it a little.
I’m late to the party here, but the EBH is an old idea that goes back at least 65 years. The great Arthur C Clarke even wrote a novella about it in 1949 “The Lion of Comarre”
See here: http://en.wikipedia.org/wiki/The_Lion_of_Comarre_and_Against_the_Fall_of_Night
One must also necessarily ask what this means for our over 10,000 religions. With 100 to 200 billion stars in our galaxy and upwards of 10 billion Earth-like planets plus another 100 billion or so galaxies in the known universe (that’s 100 billion x 10 billion…), is it not fair to ask why God chose this lonely outpost, i.e., Earth, to manifest in human form? Why here and not elsewhere? And if elsewhere, does this mean that there are trillions of “Bibles”, each with their own Gospels, scattered throughout the universe?
A literal reading and interpretation of the Bible would conclude that we are alone, would it not? As far as I know, there is nothing in the “word of God” about life, human or otherwise, elsewhere in the universe – a universe He created. And if this is just an “oversight” and life exists elsewhere, wouldn’t it have to be human since we are created in His likeness? Just asking…
There’s a lot we don’t know.
Read “The World-Thinker” by Jack Vance.
I plan on this being the last I have to say on this, but I thought it worthwhile to actually do some calculations.
http://thefurloff.com/2013/11/17/towards-realistic-probabilities-how-many-licks-does-it-take-to-get-to-the-center-of-a-tootsie-pop-or-for-et-to-build-a-radio/
Total side note: I just started reading Asimov’s “Nemesis”, and I was struck by a brief mention early on where one of the characters is remarking that (roughly paraphrasing) “we know of not a single earth-like planet, and very few planets in general”. Well, that has changed mightily since that book was written! 🙂
If we have the greatest of troubles surviving even moderately harsh weather conditions and environments here on earth, and cannot communicate meaningfully with *any* of the other life-forms that share this world with us, then any attempts to meet and greet aliens on their home worlds are pretty much doomed to abysmal failure. Imagine a planet where typhoon Haiyan was a mild day and the brilliant local creatures flop around like axolotls, going “gloop, gloop,” bored with their enlightenment.
Thank you so much for giving me such good information for free. I read SciAm for the play-by-lay and you for the color commentary.
So the universe accelerating has nothing to do with non-zero…great. That solves all my problems…*hugegulp*
I dislike any solution to the Fermi Paradox — which is in short “if there’s intelligent life out there, why haven’t we seen it yet?” — by focusing entirely on the first part and figuring out what limitations there might be on the first part. Such as — there is lots of life, but they self-destruct or get bored of life or whatever. Things we really have zero idea about.
Rather than focusing on the scenario where we know there are serious limitations: Our ability to see.
How do we know that alien colonists didn’t observe our solar system billions of years ago, and of the potential worlds to inhabit they found one undergoing a runaway greenhouse effect, another that already had life established that was pumping out poisonous oxygen, and another that had a suitable climate and atmosphere but was too small and inactive to maintain that atmosphere in the long term? So they moved on.
How do we know that there isn’t an alien probe — many times larger than all human observatories put together — studying our system from the vantage of the Neptune-Sun L5 — and beaming information back to its homeworld to satisfy their curosity?
We don’t. That’s my point — limitations on the development of advanced civilizations are unknown, but limitations on our ability to *directly see evidence* for such are multitudinous, well known, and not speculative at all.
Not that speculating about the unknowns like Sean is doing isn’t fun and interesting! But I’d warn anyone else that any conclusion from Fermi’s Paradox that isn’t basically “we have no way of knowing if it’s really a ‘paradox’ at all yet” is misguided.