Let's crunch the numbers for a rough estimate of the numbers of alien civilizations in our galaxy by combining the "Rare Earth" equation with the older Drake equation.
First the Rare Earth Equation,
https://en.wikipedia.org/wiki/Rare_Earth_hypothesis
N* 500,000,000,000 number of stars in the galaxy
ne 1.000 planets in habitable zone
fg 0.100 stars in the galactic habitable zone
fp 0.500 stars with planets
fpm 0.200 planets that are rocky/metallic
fi 0.100 planets with microbial life
fc 0.100 planets with complex life
fl 0.100 planet's lifespan with complex life
fm 0.001 planets with large stabilizing moon
fj 1.000 stars with a protecting jovian planet
fme 0.010 planets with few extinction events
N 50 total planets with complex life
So only 50 stars in the galaxy have complex life. That does not mean civilization (a trilobite is a complex life form). To calculate the numbers of civilizations, you need the Drake equation:
R* rate of star formation
fp stars with planets
ne planets that could support life
f1 planets that actually develop life
fi planets with life that develop intelligent life
fc planets with intelligent life that create civilizations
L planets with civilizations that could send signals into space
N number of alien civilizations in the galaxy
The rare Earth Equation replaces the first four factors in the Drake Equation with a starting number of 50 planets with complex life.
From this number of only 50, using the remining factors in the Drake Equation, we have to guess how many planets with complex life develop intelligent life, and how many of those create civilizations (an ocean world, for example, may have highly sophisticated cephalopods but no civilization since it can't make fire), and how many of those civilizations survive long enough to send signals to the stars.
If the Rare Earth Equation is correct, there is only one – and we are all alone in the galaxy.
At best, with more optimistic assumptions in the Rare Earth Equation and Drake Equation, there may be 1,000 planets with complex life and maybe a dozen civilizations scattered across the galaxy.
To quote the "Hitchhikers Guide to the Galaxy", "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.”
The Milky Way has a radius of 52,850 light years. Its orbital plane covers a roughly circular area of 8.8 billion square light years. With an average thickness of 1,000 light years it has a volume of 8.8 trillion cubic light years. At 500 billion stars, its average stellar density (varying widely between the galactic fringe and the galactic core) is only 1 star every 17,550 cubic light years, the equivalent of a sphere with a radius of little more than 16 light years. Earth's neighborhood is somewhat denser with Earth being only 4.25 light years from Proxima Centauri.
So lets say the Rare Earth Hypothesis is a tad pessimistic and there are actually a dozen potentially space faring alien civilizations in the galaxy. If spaced more or less evenly around the galactic plane, each civ would have 731 million square light years on the galactic plane to itself, a circular area with a radius of 15,256 light years. Each would have to itself an average volume of 731 billion cubic light years containing 42 million stars.
Unless we violate the laws of physics by inventing warp drive, space folding, wormholes or hyperspace it is going to be a very, very, very long time before we see each other, bump into each other or can hear each other's radio signals.
Assuming I haven't made a bone headed math mistake we really don't get within shouting distance (say, 100 light years apart) of each other unless there are more than 1,000,000 alien civs in the galaxy (each with a galactic plane area of almost 9 thousand square light years and a radius of 53 light years, a volume of 9 million cubic light years and 500 star neighbors).
Not even the most enthusiastic Star Trek fan envisions 1,000,000 alien civs.
So why we may not be perfectly alone and all by ourselves in the galaxy, the basic principles of the Rare Earth Hypothesis (even if we can only conjecture the exact numbers that go into the calculation) ensure that for all practical intents and purposes, we are alone.
Maybe we aren't completely alone, but we are very, very lonely.
Fermi Paradox – Maybe We Aren't Alone, But We Are Very Lonely
byu/Celtiberian2023 inspace
4 Comments
They are here though. They have been for a while studying us and possibly in communication with our governments.
Thought experiments like this one are fine and good as long as we acknowledge that this is all they are: Thought experiments.
With just one data point, all we can do is make up numbers to fill in the variables in the equation.
We just do not know.
There’s another dimension you haven’t considered, too: Time. The universe is 14+ billion years old.
Whatever complex life might have evolved (or will evolve) out there, it could come and go (or not be at a stage where we could detect even if close enough) outside our very narrow window of time.
People be setting up chatGPT bots to ask stupid questions and give their own armchair astrophysicist theory’s in here lately I swear.