Something I’ve often wondered about Earth – would our civillisation be detectable from another star, by the light of our cities? It may sound silly at first, but hear me out.
Most of the street lights in the world are sodium lamps. They work by passing an electric current through sodium vapour, which excites the sodium atoms and causes them to emit that familiar orange glow, characteristic of cities all across the world. That orange glow, however, has a very distinctive spectrum. There’s no natural mechanism through which a planet like Earth would be able to emit light like it. Thus our city lights are quite an obvious indicator that this planet is inhabited, and that the inhabitants have some level of technology at their disposal. Whatever the form of artificial lighting, it would be very obviously artificial in nature. Interestingly, a couple of scientists named Abraham Loeb and Edwin Turner have recently proposed a similar concept to try and look for civillisations on other worlds.
Science fiction writers have hypothesised the concept of the ecumenopolis – a planetwide city. The most well known example is Coruscant from the Star Wars universe. It’s quite uncertain whether or not such a place might actually ever exist, but it’s not unreasonable to suppose that a sufficiently advanced civillisation might be able to construct a planetwide city. If population growth on Earth continues at the rate it currently has, the inevitable consequence would be increasing amounts of conurbation. Ultimately, a society without any sense of environmental conservation might decide to build up their entire world.
Just supposing a place like Coruscant might exist, whatever form of artificial illumination they use would stand out like a handful of sore thumbs against the spectrum of the planet. Perhaps the idea of detecting an extraterrestrial civillisation this way isn’t so unreasonable. It’s certainly no less unusual than the idea of detecting alien vegetation. In fact conceptually it’s very similar. Both Earth’s cities and forests are likely to be easily detectable from light years away. Provided, of course, that anyone was looking in the right direction and using a sufficiently powerful telescope.
That is, of course, assuming any extraterrestrial civillisation needs artificial lighting in the same way that we do. Assuming they even build cities the way we would recognise them. Assuming that they do, in fact, even have eyes.