Proponents of the multiverse argue that the fact our universe is fine-tuned for life points to the existence of a multiverse. More universes, they claim, leads to a higher chance that there would be at least one universe with the right conditions for life. But Philip Goff here argues this argument is the result of faulty reasoning - the result of what is known as the inverse gambler's fallacy.

This piece was written in response to this article.

According to our current best physics, our universe is fine-tuned for life. Let me be totally clear about what this means, as there are a lot of misunderstandings. The claim is just that, for life to be possible, certain numbers in physics had to fall in a very narrow range. For example, if the force that powers the accelerating expansion of the universe had been a little stronger, everything would have shot apart so quickly that no two particles would have ever met. There would have been no stars, planets, or any kind of structural complexity. Whereas if that force had been significantly weaker, it would not have counteracted gravity, and so the entire universe would have collapsed back on itself a split second after the Big Bang. For there to be structural complexity, and therefore life, that strength of this force had to be – a bit like Goldilocks porridge – not too strong, and not too weak: just right. There are many numbers like this, which is what it means to say our universe is fine-tuned for life.

What follows from the fine-tuning? Some think it points to a multiverse. But I’ve been persuaded by philosophers of probability that there’s some dodgy reasoning at play in such an inference, that it commits what’s known as the inverse gambler’s fallacy. Suppose you and I walk into a casino and the first person we see is someone winning big. I say, ‘Wow, there must be tens of thousands of people playing in the casino tonight!’ You say, ‘What makes you think that?’ I reply, ‘Well, if there are tens of thousands of people playing, it’s not so surprising that at least one person would win big, and that’s what we’ve just observed.’

___

The truism that we couldn’t have observed a universe that lacked the right numbers for life is known as the ‘anthropic principle.

___

Everybody agrees this is dodgy reasoning. Our observational evidence is that this one particular individual won big, and postulating lots of people playing elsewhere in the casino does not explain why this particular individual won big. But this mirrors the attempt to explain fine-tuning in terms of a multiverse. Our observational evidence is that this one particular universe we observe has the right numbers for life, and the existence of lots of other universes outside of our own does not explain why this particular universe is fine-tuned for life.

SUGGESTED VIEWING The mystery of the multiverse With Michio Kaku, Sabine Hossenfelder, Roger Penrose

Multiverse theorists often argue that there is a crucial difference between the above casino scenario and the real-world case of fine-tuning: we couldn’t have observed a universe that didn’t have the right numbers for life, but we could have observed someone failing to win big in a casino. The truism that we couldn’t have observed a universe that lacked the right numbers for life is known as the ‘anthropic principle.’ Is there a way of altering the casino story to capture the anthropic principle? Multiverse theorists have proposed something like the following:

The Winner Selection Story: You’re waiting outside of the casino. If in the first five minutes someone is about to win big, you’ll be rushed into the room where it’s about to happen to observe the big win. If nobody wins big, you’ll stay outside twiddling your thumbs.

It is indeed true that in this new scenario, if you were rushed in to see someone winning big, this would give you evidence that there are many people playing in the casino. This is because it is much more likely that someone’s going to win big if there are many people playing, and in this scenario whoever wins big, you’re going to observe them. And superficially, this set up seems to capture the anthropic principle. You wouldn’t have observed anything in the casino unless someone had won big, just as you wouldn’t be alive to observe any universe if the right numbers for life hadn’t come up.

However, there is another way of adjusting the case to model the anthropic principle:

The Sniper Story: A sniper is waiting in the first room ready to blow your brains before you see anything unless the first person you will observe is about to win big.

In this case also, you couldn’t have observed anything other than someone winning big. And yet in this scenario, observing someone winning big wouldn’t give you grounds for thinking there are many people playing in the casino. This is because whether or not someone wins big elsewhere in the casino has no bearing on your evidence, which solely concerns a specific individual.

To summarise, in the Winner Selection Story, seeing somebody winning big would give you evidence that there are many players in the casino, whereas in the Sniper Story, seeing someone winning big would not give you evidence that there are many players in the casino. The million-dollar question is: Which of the above two modifications of the casino story better captures the real-world situation with fine-tuning?

SUGGESTED VIEWING How to Find a Multiverse With Laura Mersini-Houghton

Following the excellent work of Roger White on this, I contend that it is the Sniper Story that accurately captures real-world fine-tuning. In the Sniper Story, if we’re going to observe anything, it’ll be someone winning big. This accurately captures the real-world situation that if we’re going to observe anything, it’ll be a universe with the right numbers for life. But the Sniper Story doesn’t guarantee that if someone wins big, we’ll be rushed in to observe it. And this lack of a guarantee also mirrors real-world fine-tuning. Just because some universe turned up to have the right numbers for life, it doesn’t follow that we would have been around to observe it. It could have been that the next universe along got the right numbers for life, and some aliens observed it rather than us.

___

We should conclude that our physics having the right numbers for life does not give us evidence that we live in a multiverse.

___

In contrast, in guaranteeing that we will observe whoever in the casino wins big, the Winner Selection Story fails to mirror real-world fine-tuning. As White puts it, the Winner Selection Story would mirror real-world fine-tuning only if we were once disembodied souls floating around outside the multiverse waiting for some universe to get the right numbers for life so that we could be rushed into it.

Let’s bring everything together:

• The Sniper Story mirrors our real-world situation with respect to fine-tuning, whereas the Winner Selection Story corresponds to some strange fantasy world of disembodied spirits.

• The Sniper Story is not one in which we have evidence that there are many people playing in the casino.

We should conclude that our physics having the right numbers for life does not give us evidence that we live in a multiverse.

___

Future historians will find it bizarre that we ignored for so long what is staring us in the face.

___

Moreover, we don’t need to rely on analogies. The flawed reasoning in the multiverse inference can be laid out in terms of well-accepted principles of probabilistic reasoning, as I explain in my book Why? The Purpose of the Universe. I also explain why appeal to scientific evidence for a multiverse doesn’t help, as there is no empirical evidence for a multiverse with different ‘local physics’ in different universes, as opposed to a multiverse where all the universes have the same physics.

SUGGESTED READING Fine-tuning points towards a cosmic purpose By Philip Goff

What then does our universe having the right numbers for life point towards? If we simply apply our standard way of understanding how evidence works, given by Bayes’ theorem, the fine-tuning of physics for life presents us with evidence for some form of goal-directedness towards life in the early universe. I suspect that it is deep-seated bias, a sense that this kind of hypothesis is not ‘proper science’, that is stopping most in the scientific community from following the evidence where it leads. Future historians will find it bizarre that we ignored for so long what is staring us in the face.