“Being a scientist, Fermi attempted to add a degree of numerical rigour to the discussion with some back-of-the-envelope calculations. By combining educated guesses about the frequency of Earth-like planets and the possibility of life arising, he came to the conclusion that alien civilisations should be numerous – and yet we see no credible signs of their existence. This is the heart of what has become known as the Fermi Paradox; a conflict between an overwhelming probability and a total lack of evidence.”