Skeptigator

I know this is a bit of an old article, Feb. 10, 2009 but I just stumbled across it. Alien Census: Can We Estimate How Much Life Is Out There?: Scientific American.

A paper published recently estimates based on computer modeling that “there should be 361 advanced, stable civilizations in the Milky Way” on the low end and upwards of 38,000 depending on some your assumptions (like that panspermia is possible).

The article doesn’t detail what assumptions were made. I’ll have to go pull the paper if I can find it but I’d be curious as to the assumption the author of the paper made regarding the length of time an advanced civilization would last and how much of that time period it would be capable of generating radio signals that could be detected (or other detectable signs of life).

I’ve always found the Time component of the Drake Equation ,N=R* fs fp ne fl fi, to be the most fascinating. Its one thing to figure out how many star systems in a given galaxy could produce intelligent life like we would expect but its another thing entirely to consider that this civilization will only “inhabit” a certain extraordinarily small slice of time, relative to the Universe itself.

Our entire civilization could come and go long before any other civilization could detect us. If memory serves our omni-directional FM radio and TV signals attenuate to the point of being unrecognizable as being generated by an advanced civilization before they even leave our solar system and certainly are so low-powered by the time they reach our nearest stellar neighbor as to be unrecognizable. I think the power of our radio/tv signals are governed by the Inverse Square Law and that if intelligent life on Alpha Centauri had been broadcasting tv signals for the last 20+ years it’s not likely we’d be able to detect even with the Arecibo array. This means we’d literally have to have an alien civilization drive right by us cosmically-speaking. And the only way we’d know someone from another civilization existed is if they shot high-powered, narrowband transmissions right at us. I’m also going off of memory here but I think I remember reading that our earliest transmissions would have only made it a little over 100 Light Years (LY) away by now. Nevermind that they would probably be indistinguishable from background noise.

Alright, enough of that, the point of the preceding was to put into perspective the distances and the narrow slices of time our nearest intelligent neighbors would have to inhabit for us to detect them. So the next question is: 361 sounds like a lot of intelligent life, what’s the average distance in Light Years between us and our potential galactic cousins?

I did some quick lookups and found the volume of the Milky Way is 23 trillion light years but you probably need to take out the galactic center owing to the likely inhospitable supermassive blackhole there and you take out very edges of the Milky Way which are very cool and don’t support very hospitable stars. Yes I’m rigging the game here but let’s just assume that all 361 civilizations roughly inhabit a narrow ring in the Milky Way’s spiral arms and you end up with 250 million light years of volume.*

250,000,000 / 361 = 690,000 cubic light years per civilizations.
Taking the cube root of 690K and you get 630 Light Years between us and our nearest neighbor.

Now look back at those numbers from our own radio and tv signals. Our nearest neighbors would have to have begun broadcasting radio/tv signals during the 1400’s CE in order for them to even begin to get to us and it wouldn’t really matter because their signals would have attenuated so badly we’d have no shot (at least currently) of distinguishing their radio broadcasts from noise.

We really only have a reasonable shot at detecting life at this stage if a) advanced life is waaaay more abundant than we could imagine or b) they somehow know we are here and have for some time and they’ve already been shooting signals straight us. Here’s an interesting article at faqs.org regarding,  How far away could we detect radio transmissions?

When I consider the Universe it can be very humbling. Carl Sagan was right, we inhabit a pale blue dot. I can’t help but feel an tremendous feeling of awe and wonder. Those the best words I’ve got but they seem inadequate when describing the 

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* If you only exclude the galactic core you are left with 8 trillion cubic light years which puts roughly 2,800 Light Years between civilizations. According the faq.org calculations even Arecibo can only get 720 light years out if the signal were very powerful, narrow and directly focused at us.