Why does it get dark at night? It's a simple question, but the answer has taken astronomers half a millennium of groping in the black skies to find. The question is not about the rotation of the Earth but about starlight, and as telescopes expanded, along with theories of the universe, so too did what became known as Olbers' paradox.
The problem is this. Imagine looking at a postage-stamp-size area of the sky. Within that space, the naked eye may see a couple of bright stars, those closest to us. But behind them are thousands of others receding into space, and the further into space you go, the greater the number of stars. While individual stars may not be bright enough to see, the number of them packed into one's sight-line means that the whole postage stamp should be glowing. Why isn't it?
Professor Al-Khalili's new book Paradox looks at some of science's famous conundrums from some interesting modern angles. He begins with the logical paradoxes of Zeno, including the old favourite of Achilles and the Tortoise. But ancient Greek paradoxes that can be solved with Newtonian mathematics are not the real meat of his project. They are useful as the templates for later problems. The best example is Zeno's arrow paradox, in which the philosopher posited that an arrow observed during a moment of flight could also be perceived as static – a paradox which was picked up again by some quantum physicists in the Seventies, trying to explain why a watched radioactive atom would never decay.
To the sci-fi fan, al-Khalili's meditations on the theory of relativity and time travel will be enlightening, and Schrodinger's cat is let out of his box again. Al-Khalili, a master of making the complex simple, concludes his book with the paradox that could potentially emerge if CERN reports this summer that particles travelling faster than the speed of light do indeed exist, challenging Einstein's laws of physics.
With anecdotal explanations and good graphics, it's a well-written and lucid book. And worthy of repetition at dinner parties is the brainteaser which a Scottish physicist put to Al-Khalili: "Every Scotsman who travels south to England raised the average IQ of both countries." As for Olbers' paradox, the simple answer is that one day, a long time hence, the night sky could well be bright. It's just that starlight from galaxies more than 14 billion light years away has yet to reach us.