Estimating Galactic Population – Any Nearby?
[Note this is part of a continuing series, the introduction is here.]
The factor that is missing from the Drake equation is the frequency of advanced civilizations that are close enough to be detectable. This is a very important factor in resolving the Fermi paradox.
This last factor can be expressed very simply:
Nd = Nc * Fd
Nc is the number of technological civilizations with advanced communications capability, as we calculated from previous posts.
Fd is the frequency that a given Nc will be near enough to us for our SETI research to detect them.
And Nd is the number detectable by us.
My estimate of Fd is fairly straightforward, I assume that we can detect communications efforts within 5,000 light years. The Milky Way is about 100,000 light years in diameter. Allow for the fact that they will not be in certain parts of the galaxy, like the bar structure. About 1.2% chance of being within a 5000 ly radius of us. Even within this close distance a significant portion may not be detectable due to attenuation through clouds, radio interference, and so forth. I end up with 0.9% frequency of being detectable by us. Plugging in the numbers:
Nd = Nc x Fd = 146 x 0.9% = 1.3 worlds with advanced technological civilizations detectable by us. And one of those is ourselves.
On the bright side, there are probably around 20 trillion worlds in the universe that have given birth to advanced technological civilizations. But, unless there is a drastic change in our understanding of physics that allows travel and communications at faster than the speed of light, we will never meet any of them.
We are not alone, but we might just as well be.