Scientists have discovered a surprising fact about our universe in the past 40 years: against incredible odds, the numbers in basic physics are exactly as they need to be to accommodate the possibility of life. If gravity had been slightly weaker, stars would not have exploded into supernovae, a crucial source of many of the heavier elements involved in life. Conversely, if gravity had been slightly stronger, stars would have lived for thousands rather than billions of years, not leaving enough time for biological evolution to take place. This is just one example – there are many others – of the “fine-tuning” of the laws of physics for life.
Some philosophers think the fine-tuning is powerful evidence for the existence of God. However, in his 2010 book The Grand Design (co-authored with Leonard Mlodinow), Stephen Hawking defended a naturalistic explanation of fine-tuning in terms of the multiverse hypothesis. According to the multiverse hypothesis, the universe we live in is just one of an enormous, perhaps infinite, number of universes. If there are enough universes then it becomes not so improbable that at least one will chance upon the right laws for life.
In Hawking’s older version of the multiverse hypothesis, there is great variety among the laws in different universes. In some gravity is stronger, in some weaker, and so on. However, physicists have come to see problems with such a heterogenous multiverse, especially if the number of universes is infinite. We work out the predictions of a given multiverse hypothesis by asking how probable our universe is according to that hypothesis. But if there is an infinite number of universes, that question becomes meaningless. And hence in his final paper, A Smooth Exit from Eternal Inflation?, Hawking and his co-writer, Thomas Hertog, formulate strict limits to the kind of universes that populate the multiverse.
The problem is that the less variety there is among the universes, the less capable the multiverse hypothesis is of explaining fine-tuning. If there is a huge amount of variation in the laws across the multiverse, it is not so surprising that one of the universes would happen to have fine-tuned laws. But if all of the universes have exactly the same laws – as in Hawking and Hertog’s proposal – the problem returns, as we now need an explanation of why the single set of laws that govern the entire multiverse is fine-tuned.
Hertog seems not to agree, arguing that the paper does make progress on fine-tuning: “This paper takes one step towards explaining that mysterious fine-tuning … It reduces the multiverse down to a more manageable set of universes which all look alike.” However, this merely puts off the explanation of fine-tuning, for the result is that the laws underlying the generation of the multiverse are fine-tuned. We now need to explain not only why our universe is fine-tuned but why every universe is fine-tuned! In terms of explaining the fine-tuning, this is not a step forward but a step back.
All is not lost. Hawking was exploring models of the multiverse based on inflationary cosmology, and his paper casts doubt on the potential of this kind of multiverse to explain fine-tuning. But there is another source of scientific support for a multiverse theory: the “many worlds” interpretation of quantum mechanics. While physicists have been exploring inflationary explanations of fine-tuning, philosophers of physics have been exploring quantum mechanical explanations of fine-tuning. If, in the earliest period of our universe, our laws were shaped by the right kind of probabilistic process, the many worlds theory could furnish us with enough variety of laws across the many worlds so as to make it likely that one would be fine-tuned. We don’t yet have evidence that our laws were shaped by such a process. But if the alternative is the postulation of a supernatural creator, then this seems like the more plausible proposal.
There is still hope for a scientific account of fine-tuning. However, by ruling out one of the two scientifically credible options for doing this, Hawking and Hertog have slightly strengthened the alternative explanation in terms of God. It is ironic that the atheist Hawking should, in his final contribution to the science, make God’s existence less improbable.
• Philip Goff is the author of Consciousness and Fundamental Reality