Marko Horvat, a computer scientist at the University of Zagreb, calculated the odds of detecting alien civilizations of different lifespans from their radio signals. If, for example, 10 civilizations, each with a lifespan of 250,000 years, live within radio reach of Earth, the probability that one of them will be detected is about 9 per cent, assuming near-perfect radio telescopes scanning the sky constantly (not realistic).

If there are 10 alien civilisations with a much longer lifespan, the chances of detection drop to almost zero (www.arxiv.org/abs/0707.0011). That’s because they will probably have developed better means of communication.