MEPL

FOUR hundred years ago our understanding of the universe changed for ever. On August 25th 1609 an Italian mathematician called Galileo Galilei demonstrated his newly constructed telescope to the merchants of Venice. Shortly afterwards he turned it on the skies. He saw mountains casting shadows on the moon and realised this body was a world, like the Earth, endowed with complicated terrain. He saw the moons of Jupiter—objects that circled another heavenly body in direct disobedience of the church's teaching. He saw the moonlike phases of Venus, indicating that this planet circled the sun, not the Earth, in even greater disobedience of the priests. He saw sunspots, demonstrating that the sun itself was not the perfect orb demanded by the Greek cosmology that had been adopted by the church. But he also saw something else, a thing that is often now forgotten. He saw that the Milky Way, that cloudy streak across the sky, is made of stars.

That observation was the first hint that, not only is the Earth not the centre of things, but those things are vastly, almost incomprehensibly, bigger than people up until that date had dreamed. And they have been getting bigger, and also older, ever since. Astronomers' latest estimates put the age of the universe at about 13.7 billion years. That is three times as long as the Earth has existed and about 100,000 times the lifespan of modern humanity as a species. The true size of the universe is still unknown. Its age, and the finite speed of light, means no astronomer can look beyond a distance of 13.7 billion light-years. But it is probably bigger than that.

Nor does reality necessarily end with this universe. Physics, astronomy's dutiful daughter, suggests that the object that people call the universe, vast though it is, may be just one of an indefinite number of similar structures, governed by slightly different rules from each other, that inhabit what is referred to, for want of a better term, as the multiverse.

Star trek

The shattering of the crystal spheres which Galileo's contemporaries thought held the planets and the stars, with the sphere containing the stars representing the edge of the universe, is (along with Darwin's discovery of evolution by natural selection) the biggest revolution in self-knowledge that mankind has undergone. The world that Galileo was born into was of comprehensible compass. The Greeks had a fair idea of the size of the Earth and the distance to the moon, and so did the medievals who read their work. But these were distances that the imagination might, at a stretch, embrace. And it was easier to believe that a human-sized universe was one that might have been brought into being with humanity in mind. It is harder, though, to argue that the modern version of cosmology, let alone any hypothetical one which is multiversal rather than universal, has come about for mankind's convenience.

Four centuries on, it is difficult to think of Galileo's intellectual heirs, meeting this week in Rio de Janeiro under the auspices of the International Astronomical Union (see article), as firebrand revolutionaries. Yet their discoveries—from planets around other stars that may support alien life, to dark matter and energy of unknown nature that are the dominant stuff of reality—are no less world-changing than his. Moderns may be more comfortable than medievals with the idea that man's notion of his place within the universe can suddenly change. That should not blind them to the wonder of it.