“Suppose aliens existed, and that some had been watching our planet for its entire forty-five million centuries, what would they have seen? Over most of that vast time-span, Earth’s appearance altered very gradually. Continents drifted; ice-cover waxed and waned; successive species emerged, evolved and became extinct. But just in a tiny sliver of Earth’s history – the last hundred centuries – the patterns of vegetation altered much faster than before. This signaled the start of agriculture – and then urbanization.” We are at the very end of that “sliver”. So begins a new book by Martin Rees, Britain’s Astronomer Royal, “On the Future: Prospects for Humanity”. There is much in this short, very readable, book – the disturbing, sometimes positive, sometimes negative, future of biotech, artificial intelligence, global warming, medicine, ageing, communications, nuclear energy, weapons development, sustainability, agricultural research, poverty and employment, but its centerpiece is astronomy. Over the...