Imagine an alien civilisation is looking up at the night sky, scanning the heavens, trying to detect signs of life. We as a species have been sending out radio signals for about a hundred years and we have followed that up with television signals. Those signals have gone into space and travelled at the speed of light and by now have reached hundreds of stars and hundreds of planets around them.
So if there was an alien civilisation in that distance and they were pointing their detectors roughly in our direction, they would have heard the first BBC Broadcasts and by now they would have seen many many years of Coronation Street and have a rough idea of how our humanity works. Some very prominent scientists think it’s quite silly that we’re sharing quite so much about ourselves to the galaxy. Aliens may not be very kind and maybe we shouldn’t be giving away our position and our entertainment choices. Maybe we should all just collectively stop advertising ourselves to the galaxy. So let’s say we do go quiet. Could aliens still find us?
So imagine that same alien civilisation looking at the night sky, but now there are no radio signals, no television signals, just three hundred billion beautiful white lights quietly shining away. But it is that white light, seemingly so fundamental and pure, that holds the key to unlocking the secrets of all those planets around all those stars. White light is only pure until you crack it open. With a prism or a drop of water, white light shows what really is within it – the rainbow. And the rainbow isn’t just made up of seven colours. With a powerful enough prism, you can get all of the reds and all of the violets, all of the infinite colours between them and beyond them in the infra-red and the ultra-violet. So if that alien civilisation pointed a very expensive prism at our sun, they’d be able to see our sun’s rainbow. And if they were very patient and looked carefully, they would notice that every year a few new shadows would appear and disappear. And they would appear and disappear. And those shadows are caused by the molecules in our atmosphere. And they’ll be able to look at those shadows in the rainbow and know that we have an extremely lively planet, that we have vast inhabited oceans, that we are oxygen reliant life forms, that we have at least one technological species and also that we pollute a lot.
The earth has a living atmosphere. We call that a biosphere. And that biosphere is a planet-sized unintentional message to any aliens out there who care to listen. It tells the story of who lives on this planet and what we do with it. So we could stop sending messages into space, but even if we do go quiet, they can still tell that we’re here. It’s the galactic equivalent of thinking I’m invisible now.
And can we do the same to them? Could we detect aliens even if they have no radio or television? Well we can already look at some alien atmospheres, although we can’t quite look at a full terrestrial atmosphere yet. But we’re very close, we are developing tools to try to decipher these potential alien biospheres. Those tools are how we will be listening into alien messages. And of course, we would love to receive the alien version of The Simpsons coming done from the heavens. Until them, we’ll be looking for the signs of life that the aliens didn’t even mean to create.