Science & Fun - Jonathan Graehl

How many alien planets have intelligent life now? How many have ever had it? How long does an intelligent species usually survive? How cheap can interstellar travel become, and how hard is it to make the necessary discoveries?

These are all interesting questions that are somewhat constrained by our observations: in the last 50 or so years we’ve been watching, we’ve seen no evidence of lightspeed messages, and, crackpot stories aside, no alien or alien-device visitors. It can’t be the case that intelligent species arise from nearly every one of billions of stars in our galaxy, that last a long time, and discover effective interstellar travel, or we’d have been colonized or visited already (surely not all of them would be cautiously isolationist). The argument that we haven’t recognized any signals isn’t too convincing to me, because efficient use of limited spectrum means that the signal should look like random noise (excessive regularities indicate a wasteful encoding).

I mention this to introduce a cool synthetic (computer model only) experiment that suggests that our planet is extremely fortunately stable in its climate, which of course encourages (like in rainforests) diverse and interdependent species. It’s not a stretch to think that after a radical environmental change, humans would be extinct, but some simpler forms would survive. Of course, we shouldn’t be surprised to find that we live in precisely the type of environment that makes intelligent life more likely than usual. In fact, any seemingly-odd coincidence (e.g. that we have a tide-locked moon orbiting us that shows the same hemisphere, give or take, at all times), may be well explained by such an effect (of course, truly meaningless coincidences are possible too).

Three separate tests are investigated: (1) Earth-Moon properties and their effect on obliquity; (2) Individual planet locations and their effect on eccentricity variation; (3) The overall structure of the Solar System and its effect on eccentricity variation. In all three cases, the actual Earth/Solar System has unusually low [large-scale astronomically-caused climate change] frequencies compared to similar alternative systems. All three results are statistically significant at the 5% or better level, and the probability of all three occurring by chance is less than 10^-5

- Testing Anthropic Selection: A Climate Change Example

1 in 10000 isn’t THAT amazing, but it is cool. This doesn’t mean the odds of life in an earth-like planet are 1 in 10000; far from it. It means that we should expect to find similarly stable long-term climate conditions 1 in 10000 times. Also, don’t forget that this is *given* an earth-like planet (we don’t know how exactly how frequent those are in our galaxy etc. - yet).

found via Robin Hanson - the comments so far there don’t really discuss the post; people are just using it as a forum to share their pet views on anthropics

On learning that our solar system is unusually well-suited to developing intelligent life, we should feel less worried about the likelihood of intelligent civilizations failing before they can broadcast for long, or develop interstellar travel+colonization, as suggested by the low apparent frequency of alien visits to us. The less likely intelligent life is to arise, the happier we should be. Advanced aliens are scary. Unlike the movies, we won’t win. I’d hope they are merciful, and able to trust us or contain us without destroying us.

Of course, in some sense “less worried” is meaningless. If there’s not much you can do about a worry, hopefully you weren’t dwelling on it. Most of us probably don’t have the right estimate (and estimate of uncertainty of that estimate) of the likelihood of our civilization imploding, or aliens invading, before we learned of this evidence; if your estimates were vague enough to begin with, then you still won’t have a sharp belief even after many such pieces of suggestive anthropic evidence.