Don’t believe in coincidences but stuck for an explanation? Time to call up the anthropic principle and the multiverse

PHYSICISTS dislike coincidences such as those set out on these pages, suspecting them of covering up some new principle they don’t yet grasp. But when they run out of theories, there’s a one-size-fits-all explanation that can answer everything without really answering much at all: the universe is as it is because we’re here to see it.

This piece of circular logic is the anthropic principle. A universe inhospitable to life would have no human beings around to observe it, so the one we see must, by definition, possess features essential to accommodating intelligent life. But that doesn’t tell us whether a slightly different universe might still host life, why our particular universe exists and not some other, or why we see finely balanced features with no bearing on the emergence of life.

And yet there is an idea that sweeps all these objections away: all conceivable universes exist side by side in a patchwork multiverse. We merely inhabit one out of the infinite selection.

Our implausible universe Our cosmos’s five most startling coincidences – and what lies behind them

Why believe in the multiverse? Because a process such as inflation (see “The universe is flat as a pancake. Coincidence?“), if left unchecked, could produce a multitude of causally disconnected universes. String theory, still most physicists’ favoured route to an overarching theory of everything, conjures up an entire landscape of maybe 10100 or more universes, all with slightly varying properties.

But some still see the multiverse as an abdication of scientific responsibility: a fancier way of simply saying “coincidences happen”. And, if true, it means some astronomers out there are forced to justify a universe even more replete with coincidences than ours, while others could be bored stiff in a completely random cosmos.

This article appeared in print under the headline “Best of all possible worlds?”