Friday, November 30, 2007

SETI: find one of nine galactic civilizations

Damn Interesting has a very nice Drake Equation/SETI review today. Allan Bellow's even touches lightly on the Fermi Paradox, though he doesn't get into the various paradox resolutions.

The highlight of the article is an interactive Drake Equation calculator. Users start with various presets, including the 'rare earth' and the 'Drake 2004' options, then add their own biases. Two of the "terms" of the Drake Equation are now relatively accepted, below I show them and 3 variations on the rest: Drake 2004, rare earth, and me.
Common assumptions
New Milky Way stars per year = 6.00
Proportion of stars which have planets = 50.00%

Drake 2004
Average number of life-compatible satellites = 2.00
Percentage of planets where life does appear = 100.00%
Percentage where intelligent life evolves = 20.00%
Percentage of civilizations which send signals into space = 100.00%
Average years that civilizations will send signals = 10000.00
Average civilizations in our galaxy = 10,000

Rare Earth
Average number of life-compatible satellites = 0.000001
Percentage of planets where life does appear = 33.00%
Percentage where intelligent life evolves = 1.00%
Percentage of civilizations which send signals into space = 1.00%
Average years that civilizations will send signals = 10000.00
Average civilizations in our galaxy = 0

Me
Average number of life-compatible satellites = 0.10
Percentage of planets where life does appear = 87.50%
Percentage where intelligent life evolves = 20.00%
Percentage of civilizations which send signals into space = 90.00%
Average years that civilizations will send signals = 200.00
Average civilizations in our galaxy = 9.5
So Rare Earth ends up with zero civilizations, though I think this might be a bug. The first time I ran the calculations they had 1 civilization, presumably us.

Drake has 10,000 current civilizations that send signals for 10,000 years. This definitely runs into Fermi Paradox territory. If there are so many, and they endure for so long, then over galactic time scales at least one ought to have infested the stars.

I end up with 10 civilizations, of which we are 1. I get that by assuming an upper limit of 200 years of radio signals. We started radiating significantly around 1960, and if our civilization endures I don't think we'll be doing much wasteful radiating by 2160. I suspect we (or our inheritors) will be utterly incomprehensible, and perhaps uninterested in the merely physical universe.

This is a very small number of civilizations across a galaxy. Ssuch a low number makes it very unlikely that one of them will choose to aim a high intensity radio beam directly at us -- and that's all we can detect with today's technology.

We'd need to build a receptor the size of the solar system to pick up accidental signaling. I'm sure we could do that by 2160, but of course that takes us into the realm of the unimaginable.

So SETI won't find much.

Damn.

Update 12/31/09: I think we can rule out "rare earth". Also, another take on the Drake Equation.

2 comments:

GZ said...

Huh? A bunch of percentages of likeliehood and then none to guess if one has a beam pointing at us?

Likeliehood that a beam is pointing as: .000001 %

Likeliehood it is a death ray: 100%

Likeliehood that they won't use it because they are not sure if we constitute intelligent life: 50%

(depends on if all those folks who have been abscounded by UFOs are telling the truth).

John Gordon said...

Well, yes, it was a mostly whimsical post. The confidence intervals are a bit ... wide.

I don't buy lottery tickets, but on the other hand I would buy a few "SETI tickets".

Sometimes there are limits to logic.