To the best of our knowledge we are alone in the universe. But if we’re not, do the aliens know that they aren’t alone? And if they know we exist, how much do they know about us?
It largely depends on who they are and where they live. If they inhabit planets orbiting nearby stars – up to a few dozen light years away – and are of comparable intelligence and technological development to us, they might know we are here, thanks to various messages we have deliberately beamed to an assumed interstellar audience. But that also means their knowledge of us is selective at best, confined to the Beatles, Doritos, theremins and a few other trivialities.
If our neighbours are more advanced – enough to have blanketed their moons in receivers, for instance – they might have been watching the weak TV signals that have beamed from Earth at the speed of light since transmissions really took off after the second world war (see diagram). Assuming they managed the relatively simple task of decoding the signal, they would know a lot about us: what we look like, how we behave and relate to one another, our breeding habits, culture, history – and predilection for watching trash.
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There probably are a few Earth-like exoplanets that are close enough for our broadcasts to have reached them. But the chances of advanced life being on one them at just the right time to listen in is minuscule. And if they are there, they have done a good job of hiding their presence from us.
Beyond such relatively limited distances, the odds of advanced life could be higher, purely on the basis of the numbers. There are several hundred billion stars in our galaxy, a million of them within 1000 light years. But our artificial signals haven’t had time to reach them yet. Could civilisations at these distances know anything about us?
Technically it is feasible to conceive of telescopes capable of gathering information from vast distances, although it would require a truly astronomical aperture to see anything in detail. One way around this is to use a massive object, like a star or a black hole, as a giant gravitational lens, by seeing how it bends light. If we used our sun in this way, which is technically possible although horrendously complicated and expensive, we could make out road networks or detect street lighting on the surfaces of nearby exoplanets, should they exist.
For a much more advanced society, it is conceivable that our ancestors’ burgeoning imprint on Earth could have been observed from hundreds or thousands of light years away – although the farther away they are, the less of our history is accessible to them.
What about societies even more distant? Given our current ability to detect exoplanets, it isn’t a huge leap to imagine that civilisations millions of light years from Earth know that our rocky planet is here, in the habitable zone of a yellow dwarf star in an arm of a spiral galaxy. Our planet’s 2-billion-year-old oxygen signature may have been picked up along with other spectral hints of ancient life. But the existence of intelligent life would be inaccessible to them.
“If I were an alien society just 100 years more advanced than us, my astronomy text books would surely contain huge tables of habitable worlds. Earth would be among them,” says Seth Shostak, director of the SETI Institute in California. Although they would call it something different, perhaps as functional and unglamorous as the names we give to the habitable worlds we know about. Greetings planet KOI-3010.01, we come in peace…
Read more: “The human universe: Exploring our place in space”
This article appeared in print under the headline “Does the cosmos know we’re here?”
