In Drake equation terms I guess I’m looking at n_e (planets/star which can support life) and f_i (fraction developing intelligent life, which the emphatic caveat that even if unhampered evolution wandered toward intelligence reliably, which we don’t know, there’s a big question of how often evolution gets to run without giant whacks happening. I suspect we’re still fundamentally ignorant here.

He says something in “The Data Point” section referring to a of hyper-expansion of our descendants going beyond the scale of a single disaster, so it’s there peripherally.

Your filter components for nonevents is interesting; now I wonder why it’s not there too. Robin tends to formulate his analysis of problems using Bayesian methods, so it seems like these would be natural probabilities (1-p1), (1-p2), … to add to his other filter components.

In these last 10 years since he wrote the essay (I have a draft version from 1996 which is not very different), astronomers have made large discoveries. The extrasolar planets program was just beginning, the Mars rock (which he does mention) with the proposed evidence for life was under intense analysis, the snowball earth idea was new, automated searches for NEOs was in the beginning stages, for example. You or I could suggest making a re-evaluation or update of that essay (and he could respond reasonably that he doesn’t have time…)

Well…I’d say that doesn’t make it any less of a coincidence, just a coincidence that we may have reason to believe happened anyway. But that’s probably just semantics.

I just re-read Robin’s Great Filter essay. One thing he doesn’t mention is mass extinctions, the probability of life getting set back, or even completely wiped out, somewhere along the way.

Hansen may not mention that, but Ward and Brownlee do go into some length in Rare Earth about most or all of the factors you list. Then again, as Amara mentioned, their book is considered rather controversial. In fact, though I didn’t know this until I did some websearching yesterday, one of their biggest detractors is one David Darling, who’s written a book of his own, Life Everywhere: The Maverick Science of Astrobiology, in which he counters each of Ward and Brownlee’s arguments, point by point. I haven’t read Darling’s book (yet), so I don’t know what he has to say about those particular arguments.

Anyway, though, again, I guess at this point it’s just a matter of opinion; even the experts who’ve done substantial research on this subject are strongly divided on their conclusions, so we’re not going to settle the matter here. Doesn’t mean it’s not interesting to talk about, though.

The Sun has been getting warmer; Earth has been getting cooler for tens of millions of years, perhaps because the Himalayas have been scrubbing CO2 out of the atmosphere. What if we had no Himalayas? No ice ages, maybe even greenhouse.

Someone has to be first. And it needn’t be a big coincidence: the mediocrity principle can be opposed by the anthropic principle. If spreading is easy then the galaxy is winner-take-all, and there’s only ever one naturally occurring intelligent species per galaxy. The fact that we exist at all tells us we’re it.

If life is really easy it should have arisen multiple times on Earth, right? Except that the first cell swept the oceans and ruined it for anyone else. (Maybe; not like we know what really happened.)

That’s the Fermi paradox in a nutshell, basically.

*Why* have there got to be some issues? Maybe complex (or more to the point, technological) life *is* unlikely. Or maybe we’re first in our light-cone.

But then why is it unlikely? See the Great Filter essay–according to our current understanding of the scientific principles involved, it shouldn’t be unlikely. Therefore, if it is unlikely, there are some issues we don’t understand, q.e.d. Similarly, why would we be first in our light-cone? If life is as common as our current understanding of the scientific principles would seem to imply, we shouldn’t be, unless by a really big coincidence.

I’ve gotten curious enough I’ve been looking through the scientific literature for a few hours on various takes on the Fermi paradox. Yes, the “zoo” idea is one that has seen print, but it’s not one that strikes me as too probable. There have been closer examinations of the metallic-elements issue, but they don’t all seem to agree. There are other ideas I’ve run across that I hadn’t heard of before. For instance, the abstract to one paper (LK Scheffer, Machine Intelligence, The Cost of Interstellar Travel, and Fermi Paradox, Quarterly Journal of the Royal Astronomical Society 35 (2): 157-175 JUN 1994) asserted the following:

If machine intelligence is possible, and the computers on which it is based resemble today’s computers in some very fundamental aspects, then interstellar travel can be accomplished by data exchange as opposed to the physical movement of matter. Assuming data exchange by radio, such travel is many orders of magnitude cheaper than physical travel. This low cost provides a huge incentive for an emerging society to join an existing galactic civilization as opposed to physically colonizing the galaxy. It is likely, therefore, that there is at most one advanced civilization per galaxy. This civilization may well have unified goals and objectives, thus removing the strongest underpinning of Fermi’s paradox.

An interesting idea. Could be. I wouldn’t bet on it, but I can see it as a possibility.

To me, personally, the possibility that there was something about the conditions in the early universe that prevented complex life from forming long before us strikes me as the most plausible explanation. But it’s certainly not the only one. Maybe you’re right and we are in some kind of “zoo”. Maybe Robin Hanson’s right and there’s some stage in the development of complex life that’s much less probable than we think. We just don’t know enough to know why we haven’t encountered any complex extraterrestrial life; all we can do is state our opinions. And given the amount of debate and disagreement that seems to exist in the published literature on the subject, I doubt we’re going to be able to settle much here.

*Why* have there got to be some issues? Maybe complex (or more to the point, technological) life *is* unlikely. Or maybe we’re first in our light-cone.

Way I see it, either tech life can spread beyond a solar system or it can’t. If it can, then either we’re first in the local area, or we’re in some well-maintained zoo. This is because I assume spacefaring life would make a visible difference to the universe, if not actively gobble most things up.

If it can’t, then we get a common SETI vision of lots of old civs locked into their systems, or even their planets. We’ve got company, but don’t expect to meet them, and expect humanity (and descendants) to die with the Sun.

Basically I ignore the astronomical details and look at my expectations for self-replicating entities.

I’ve never seen a resolution of the Fermi paradox that I really found satisfying. Even the Great Filter essay you linked to above focuses on the idea that there must be something about the development of complex life we don’t understand that makes it extremely unlikely. I don’t see it as any more implausible to think that maybe rather than there being something we don’t understand that makes complex life rare and unlikely as a whole, there’s something we don’t understand that just made the development of complex life impossible in our universe (or prohibitively unlikely) until very recently. Either way, there have got to be some issues we don’t understand to explain the Fermi paradox; I just don’t think the development of complex life being extremely unlikely is the only possible conclusion.

Ah, well. Perhaps there are more things in heaven and Earth than are dreamt of in our philosophy…

Yes, I agree with most of what it says in the review you linked to. (Although I didn’t think the book was as thoroughly sloppy as that review makes it out to be–then again, it was many years ago that I read it; maybe I’d think less of it if I reread it now.) In fact, I find it kind of gratifying to find out that there are so many others who disagree with Ward’s and Brownlee’s conclusions; I’d kind of feared maybe I was just being stubborn in finding fault with their thesis.

However, that’s not really the issue at hand here. I’m not arguing for Ward’s and Brownlee’s conclusions–which, as I said, I don’t agree with myself. All I said is that I got that one point, about the supernovas and the build-up of radioactive elements, from their book, and that, whatever I thought of their central thesis, that one point struck me as an interesting possibility, if not as the sure thing they made it out to be. Unfortunately, the review you linked to doesn’t address that particular point at all.

Now, as I said, it’s been a long time since I read the book, and I may be remembering the argument incorrectly. I’ll reread that part when I get home and see if there’s anything I missed.

But anyway, once more, I want to emphasize I’m not trying to argue (as Ward and Brownlee were) that complex life must be an extremely rare phenomenon. In fact, I’m trying to argue just the opposite. I was bringing up what I thought was a possible explanation for why complex life might not arise until relatively late in the universe–which could be one way of resolving the Fermi paradox. Maybe the reason we haven’t been contacted by any much older races of intelligent life is because the conditions in the universe weren’t right for intelligent life until relatively recently. Again, I’m not convinced that’s the case, but I’m more comfortable with that as an explanation of the Fermi paradox than assuming we must be nearly alone in the universe as Ward and Brownlee do–and as the author does of the Great Filter essay you linked to in a previous post, approaching the issue from a different direction. Obviously, the fact that I’m more comfortable with the explanation doesn’t make it right, and for all I know we are alone in the universe–but I’d like to examine other possibilities.