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Science fiction has a pretty standard setup: advanced aliens detect our radio broadcasts, plot a course to Earth, and show up. It makes for great storytelling. It also bears almost no resemblance to how the universe actually works.
This Sidequest is a follow-up to the Roswell episode, and it tackles the inevitable question from that story: even setting aside what crashed in New Mexico in 1947, how likely is it that any alien civilization could actually find us?
The numbers are the starting point, and they’re a little hard to process. The Milky Way contains somewhere between 100 and 400 billion stars. Most of them probably have planets. Earth is one world orbiting one ordinary, unremarkable star among all of those. To find us, an alien civilization would need to pick our sun out of billions of candidates and then detect a rocky planet whose reflected light is roughly equivalent to a firefly circling a stadium spotlight. Most detection methods only work if the observer is positioned in a very specific geometric relationship to Earth’s orbital plane — from most vantage points in the galaxy, we’d never be visible at all.
And then there’s the radio signal problem. Our broadcasts have been leaking into space for about a century, which sounds like a lot until you realize that puts us inside a bubble roughly 200 light-years in diameter, in a galaxy 100,000 light-years across. That’s about 0.2% of the galaxy’s diameter. And those signals don’t stay strong: by the time they’ve traveled a few dozen light-years, they’re nearly indistinguishable from background cosmic noise. A civilization trying to intercept our television broadcasts from 30 light-years away would need enormous receiving arrays, very long observation times, and crucially, they’d need to already know where to point them.
There’s also the time dimension, which science fiction almost never addresses. Because of light travel time, no one in the galaxy sees Earth as it is right now. Observers 65 light-years away are watching Earth from the early 1960s. Observers 500 light-years away see no humans at all, just a planet with some biological life. Observers 2,000 light-years away are watching before Rome existed. Every civilization in the galaxy sees a completely different version of our history. Piecing together that we’re here and technologically active requires detecting signals from a moving target across time.
None of this means we’re alone. The universe is vast enough that intelligent life almost certainly exists elsewhere. But the distance problem might explain the Fermi Paradox better than any other answer: civilizations could evolve, flourish, and disappear without any other civilization ever knowing they existed. Not because anyone is hiding. Just because the universe is, in the famous phrasing, vastly, hugely, mind-bogglingly big.
