Alien Civilizations
There have probably been trillions of alien civilizations, and yet we may still never see one
Reuters/NASA/Handout
Earlier in more than five decades of scanning the heavens, the search for extraterrestrial intelligence (SETI) has found no sign of alien life. Yet now two American astronomers in the scientific equivalent of a back-of-the-envelope calculation are estimating that over the course of its history the universe has seen at least half a trillion technologically advanced species.
From the paper in Astrobiology by Adam Frank and Woodruff Sullivan notes that in just the last few years, we’ve gained a much clearer sense of how hospitable the universe is to life. NASA’s Kepler space telescope has identified thousands of planets in our neighbourhood of the galaxy along with their sizes and distances from their stars. From there it’s fairly easy to guess how many may hold liquid water, which is probably essential for complex life. In our Milky Way galaxy alone there are, by this estimate some 60 billion such “habitable” planets, written by Frank and Sullivan.
That the big remaining unknown is how many of these planets give rise to the kinds of life forms that build advanced technology (if nuclear weapons and Oculus Rifts can be called “advanced”). Since Earth is the only one we know of the guesses vary wildly, but one such civilization per 10 billion habitable planets is generally considered “highly pessimistic,” written by Frank in the New York Times yesterday (pay wall). In astronomy-speak, this means the figure could be 10, 100 or even 1,000 times too low.
By using that “pessimistic” proportion, and other numbers from Frank and Sullivan’s paper, It is calculated how many alien civilizations should have emerged within various sub regions of the universe during its history:
Total advanced civilizations in the history of the Universe...
No. of Galaxies No. of Civilizations
• Milky Way 1 6
• Local Group 30 180
• Galactic Cluster 300 1800
• Super Cluster 3000 18000
• Observable Universe 70000000000 420000000000
Please remember, 420 billion intelligent civilizations is the “pessimistic” estimate. But sadly or happily, depending on your view of aliens it doesn’t make us any less alone.
Though Frank and Sullivan wisely avoid putting a number on how many alien species are knocking around right now, we can do our own back-of-the-envelope reckoning. A crucial unknown factor is how long a technologically advanced civilization lasts before either going extinct or blasting itself back to the Stone Age. Judging by the past century of human history, even a thousand years might be optimistic. But let’s be really optimistic and call it a million years. That’s the average lifespan of a mammalian species that doesn’t invent the means of its own destruction.
It is also going to be assumed that, though the universe is 13.8 billion years old, advanced species didn’t begin to appear until a couple of billion years ago. It took most of the universe’s history to form the kinds of planets, rich in heavier elements, on which creatures like us could evolve.
Thus if there have been 420 billion civilizations in the past 2 billion years each one lasting a million years, then on average, about 210 million of them have existed simultaneously at any given moment.
That may seem like a lot of aliens to talk to. But not in a cosmos as big as ours. The observable universe is an estimated 93 billion light years in diameter. If you sprinkle 210 million civilizations throughout it like raisins in a cake, they’ll be spaced about 125 million light years apart.*
This galaxy is only about 100,000 light years wide, so that’s a journey of 1250 Milky Way’s laid end to end before you come to the next intergalactic refuelling stop. Even waving hello from a distance is pretty much out of the question, given that the furthest planets we can currently detect are just 25000 light years away. (For what it’s worth, the SETI people have higher hopes.)
Of course this assumes civilizations are evenly distributed throughout space. In reality, the universe is clumpy, so they’ll be more concentrated in parts. And sheer random luck might have planted one within easy reach of us. Then again, that might be very bad luck indeed.
*Calculation: In a sphere of radius 46.5 billion light years, volume 4.21 x 1032 cubic l.y., with 210 million civilizations, there will be one civilization per 2 x 1024 cubic l.y., i.e., in a cube 125 million l.y. across.
There have probably been trillions of alien civilizations, and yet we may still never see one
Reuters/NASA/Handout
Earlier in more than five decades of scanning the heavens, the search for extraterrestrial intelligence (SETI) has found no sign of alien life. Yet now two American astronomers in the scientific equivalent of a back-of-the-envelope calculation are estimating that over the course of its history the universe has seen at least half a trillion technologically advanced species.
From the paper in Astrobiology by Adam Frank and Woodruff Sullivan notes that in just the last few years, we’ve gained a much clearer sense of how hospitable the universe is to life. NASA’s Kepler space telescope has identified thousands of planets in our neighbourhood of the galaxy along with their sizes and distances from their stars. From there it’s fairly easy to guess how many may hold liquid water, which is probably essential for complex life. In our Milky Way galaxy alone there are, by this estimate some 60 billion such “habitable” planets, written by Frank and Sullivan.
That the big remaining unknown is how many of these planets give rise to the kinds of life forms that build advanced technology (if nuclear weapons and Oculus Rifts can be called “advanced”). Since Earth is the only one we know of the guesses vary wildly, but one such civilization per 10 billion habitable planets is generally considered “highly pessimistic,” written by Frank in the New York Times yesterday (pay wall). In astronomy-speak, this means the figure could be 10, 100 or even 1,000 times too low.
By using that “pessimistic” proportion, and other numbers from Frank and Sullivan’s paper, It is calculated how many alien civilizations should have emerged within various sub regions of the universe during its history:
Total advanced civilizations in the history of the Universe...
No. of Galaxies No. of Civilizations
• Milky Way 1 6
• Local Group 30 180
• Galactic Cluster 300 1800
• Super Cluster 3000 18000
• Observable Universe 70000000000 420000000000
Please remember, 420 billion intelligent civilizations is the “pessimistic” estimate. But sadly or happily, depending on your view of aliens it doesn’t make us any less alone.
Though Frank and Sullivan wisely avoid putting a number on how many alien species are knocking around right now, we can do our own back-of-the-envelope reckoning. A crucial unknown factor is how long a technologically advanced civilization lasts before either going extinct or blasting itself back to the Stone Age. Judging by the past century of human history, even a thousand years might be optimistic. But let’s be really optimistic and call it a million years. That’s the average lifespan of a mammalian species that doesn’t invent the means of its own destruction.
It is also going to be assumed that, though the universe is 13.8 billion years old, advanced species didn’t begin to appear until a couple of billion years ago. It took most of the universe’s history to form the kinds of planets, rich in heavier elements, on which creatures like us could evolve.
Thus if there have been 420 billion civilizations in the past 2 billion years each one lasting a million years, then on average, about 210 million of them have existed simultaneously at any given moment.
That may seem like a lot of aliens to talk to. But not in a cosmos as big as ours. The observable universe is an estimated 93 billion light years in diameter. If you sprinkle 210 million civilizations throughout it like raisins in a cake, they’ll be spaced about 125 million light years apart.*
This galaxy is only about 100,000 light years wide, so that’s a journey of 1250 Milky Way’s laid end to end before you come to the next intergalactic refuelling stop. Even waving hello from a distance is pretty much out of the question, given that the furthest planets we can currently detect are just 25000 light years away. (For what it’s worth, the SETI people have higher hopes.)
Of course this assumes civilizations are evenly distributed throughout space. In reality, the universe is clumpy, so they’ll be more concentrated in parts. And sheer random luck might have planted one within easy reach of us. Then again, that might be very bad luck indeed.
*Calculation: In a sphere of radius 46.5 billion light years, volume 4.21 x 1032 cubic l.y., with 210 million civilizations, there will be one civilization per 2 x 1024 cubic l.y., i.e., in a cube 125 million l.y. across.
Connect With Us