They’re out there! They’re just really, really, really far away.
Think about it this way; we’re one planet around a relatively average star at no special location in our galaxy, at no special location in the universe. What makes us believe we’re the only form of intelligent life in this vast universe? The trouble lies in the vastness.
Astronomers traditionally tackle this question by means of the Drake Equation, which takes all the pieces we think need to align for life to evolve, and multiplies together all the probabilities that they’ll occur in the same place at the same time within our galaxy. In a simple form, it asks the following series of questions: From the number of stars in our galaxy, what fraction of those stars will have planets? On average, how many planets are at just the right distance from their star, when liquid water can exist at the surface? Of the planets with liquid water hanging around, how many of those should we expect to have any form of life, no matter how simple? What fraction of those planets with life will also have intelligent life? And what fraction of those intelligent life forms are still around now and potentially able to communicate with us?
There are at least 100 billion stars in our galaxy, and hundreds of billions of galaxies in the universe. Pretty much every star we’ve looked at in our galaxy has at least one planet, which means we’re dealing with an overwhelmingly large number of planets in the universe. Our Earth went from ‘liquid surface water’ to 'life’ in a cosmic blink of an eye - that doesn’t seem to be the hard part.The tricky part is getting from bacteria to a species that contemplates the skies above. Even if you’re extremely pessimistic about the fraction of planets that have any form of life, and pessimistic about the fraction of planets with intelligent life, the sheer number of planets out there dominates these calculations. There simply has to be other life out there, and if not in our galaxy, in another.
Here’s the sticking point: the galaxy is huge. It takes light 50,000 years to travel from the centre to the edge of the galaxy: 100,000 years to make it all the way across. And the distances between galaxies are even more extreme. Light from the Andromeda galaxy takes two and a half million years to reach us, and that’s our nearest neighbor. Two and a half million years ago, humans were only at homo habilis. We don’t have any record of modern humans from earlier than 200,000 years ago. Our first radio telescope was built all of 80 years ago.
We can do a few more rough calculations to figure out how bad the problem is. Let’s assume that you do your calculation and work out that by optimistic numbers, there should be 1,000 intelligent, communicable civilizations just in our galaxy. If those civilizations are randomly scattered around the galaxy, on average they’ll be separated by 2,800 light years. But maybe 1,000 is a little too optimistic. Say there are only 15 civilizations in our galaxy; now they’re separated by about 23,000 light years. If there’s only one civilization per galaxy, you’re back to separations of millions of light years. Communication between civilizations (even in the most optimistic of cases) would be effectively impossible.
No matter what, whatever forms of life exist out there will be as uniquely suited to their home planet and home star as we are to our own, and will assuredly look nothing like Hollywood’s favorite “little green men”.
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