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We exist, and we are living creatures. It follows that the universe we live in must be compatible with the existence of life. However, as scientists have studied the fundamental principles that govern our universe, they have discovered that the odds of a universe like ours being compatible with life are astronomically low. We can model what the universe would have looked like if its constants—the strength of gravity, the mass of an electron, the cosmological constant—had been slightly different. What has become clear is that, across a huge range of these constants, they had to have pretty much exactly the values they had in order for life to be possible. The physicist Lee Smolin has calculated that the odds of life-compatible numbers coming up by chance is 1 in 10229.

Physicists refer to this discovery as the “fine-tuning” of physics for life. What should we make of it? Some take this to be evidence of nothing other than our good fortune. But many prominent scientists—Martin Rees, Alan Guth, Max Tegmark—have taken it to be evidence that we live in a multiverse: that our universe is just one of a huge, perhaps infinite, ensemble of worlds. The hope is that this allows us to give a “monkeys on typewriters” explanation of the fine-tuning. If you have enough monkeys randomly jabbing away on typewriters, it becomes not so improbable that one will happen to write a bit of English. By analogy, if there are enough universes, with enough variation in the numbers in their physics, then it becomes statistically likely that one will happen to have the right numbers for life.

This explanation makes intuitive sense. However, experts in the mathematics of probability have identified the inference from the fine-tuning to the multiverse as an instance of fallacious reasoning. Specifically, multiverse theorists commit the inverse gambler’s fallacy, which is a slight twist on the regular gambler’s fallacy. In the regular gambler’s fallacy, the gambler has been at the casino all night and has had a terrible run of bad luck. She thinks to herself, “My next roll of the dice is bound to be a good one, as it’s unlikely I’d roll badly all night!” This is a fallacy, because for any particular roll, the odds of, say, getting a double six are the same: 1/36. How many times the gambler has rolled that night has no bearing on whether the next roll will be a double six.

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