[I]n the 1960s, Princeton astrophysicist Robert Dicke offered a very different explanation for the big number coincidence. The starting point for his theory was the connection between biological evolution and the evolution of the universe.

The current age of the universe now, he reasoned, is not some random moment; it's the moment when life has evolved enough to produce beings able to measure it. Dicke wondered what the pre-requisites were for intelligent life in the universe, and how that might relate to the mystery of 1040.

All known life is based on the element carbon. But carbon did not exist in the early universe; it was formed later by nuclear processes inside stars. When large stars grow old and die, they explode, and their life-encouraging carbon can end up in the next generation of stars and their planets. That meant, Dicke reasoned, that life in general, and intelligent beings in particular, could not exist until at least one generation of stars had lived and died.

Stars are made mainly of hydrogen - a form of nuclear fuel - held together by gravity. They burn steadily through this fuel until it is spent and the energy released has radiated away.

If gravity were stronger, the stars would consume their nuclear fuel faster, because they would be squeezed harder and burn brighter. But a star only dies once all the heat produced from burning its fuel has radiated into space, and its escape depends on electromagnetism: photons formed deep inside the stars have to plough through a thick soup of electrons and protons, scattering this way and that.

Dicke showed that the lifetime of a typical star hinges precisely on the ratio of electric to gravitational forces in the hydrogen atom.

The upshot of Dicke's calculation is that the current age of the universe is about the same as the ratio of electric to gravitational forces. If gravity were weaker, stars would live longer and carbon-based life would have emerged later. And if gravity were stronger, life would have arisen sooner.