The standard argument usually goes something like this:
Alice: there's a lot about the Earth that seems suspiciously fine-tuned for the evolution of complex life.
- Our orbit happens to lie in this fairly narrow band that allows for liquid water, and it seems other solvents don't work nearly as well in supporting biochemistry.
- We've lucked into an ozone layer and a strong magnetic field, shielding us from cosmic rays and solar radiation.
- Our moon is far larger than we'd expect for a planet our size - to the extent that it might be more correct to classify the Earth-Moon system as a double planet. It's plausible that the lunar-powered periodic cycle of tidal conditions could have been crucial as a sort of temperature input in the simulated annealing process of evolution - very simple local optima are less likely be stable and dominant in a dynamic environment.
- We've been dealt a perfect hand of accessible elements - H, C, N, O, P, and S - and we're unique among the terrestrial planets in our active plate tectonics to recirculate and buffer stuff that would otherwise build up or deplete from the biosphere.
- Plus, I've heard some people mention the position of Jupiter might on-net prevent a lot of asteroids from bombarding Earth.
The odds of any one of these factors happening by chance is low, but putting them all together...
Alice isn't that bright.
Bob: Even granting that complex life needs an Earth-like planet - that some utterly alien tidally-heated briny pocket or subsurface ocean, or a floating ecosystem in gas giant clouds, or weirder things still wouldn't suffice - the universe is immense. There are a lot of planets. Cast the dice enough times and you'll get lucky.
Imagine these factors didn't work out for Earth, and it was yet another uninhabitable rock. We'd be standing on some distant shore, having the same conversation, wondering why Florpglorp-iii was so perfectly fine-tuned.
You've forgotten to condition on the fact that you're here to have this conversation in the first place. The fact of your observation means it's 100% likely that you're standing on a planet able to produce observers.
Boltzmann brains and simulations aside, Bob is more right than wrong. Alice is unconvinced.
Alice: But it's not just the earth that's fine-tuned. Between GR and the standard model, our current best theories of reality have 19 arbitrary constants. For many of them, even a slight change would have made a universe unable to support the kinds of complex chemistry needed for life.
Again, astronomically low odds that you just happen to land on a combination of all 19 that allows for life, so...
The usual way I see this play out online: Bob points at one of many plausible ways physics could allow for a multiverse with varying physical constants across universes (something using eternal inflation seems to be in vogue among the types arguing about this stuff). He completes the case:
Bob: Assuming there's some vast multiverse, then anthropics ensures that we find ourselves within one of the sub-universes with constants that allow for observers.
But let's take a step back. Imagine the universe consisted only of a single tiny galaxy, with a very small number of planets. If one of them happens to be Earth-like, then there are observers to marvel at the incredibly unlikely fine-tuning of it all. If no planets are able to support life, then there are no observers to consider how very predictable the lack of their own existence is.
Scale it back further. A universe containing only a single solitary star, with a proto-planetary disk that will coalesce into a few planets. With extraordinarily low odds, one of them could be earth-like, and the lonely humans that crawl from its mud will gaze at the empty sky with complete confidence that their creation was the result of intentional intervention. Or, far more likely, you just get a few barren rocks circling a few billion times before their sun chars or consumes them.
Of course, the same is true of our universe. If the constants of reality didn't permit observers, we would not observe those constants. No multiverse necessary.
Whether the mere fact of our existence counts as evidence toward a multiverse - as such a scenario would predict more total observers - is a much subtler question. My point is simpler: in principle, anthropic reasoning is sufficient to explain fine-tuning. A physical multiverse is not strictly required.
This is valid. Alice is doing the usual mistake of confusing P(One Particular Thing) with P(Any Thing from a Huge Set of Things) and Bob is right to correct her.
However, when we decrease the size of our Set of Things the difference between the two probabilities decreases. In the universe with billions of galaxies:
P(Life on at Least One Planet) is much much much higher than P(Life on One Particular Planet).
In the universe with only one galaxy the difference is not as dramatic. In the universe with only one solar system, with a few planets these probabilities become similar.
And so if Alice finds herself in such a universe it's indeed rightfully surprising. As you've said yourself:
On the other hand, if vast multiverse with many more planets exist then the difference between our two probabilities once again becomes high. So we would need to invoke multiverse to account for this kind of fine-tuning.
Conditional P(X|X) = 1.
However, unconditional P(X) = 1/1,000,000.
Just like you can still reason about unconditional probability of a fair coin even after observing an outcome of the toss, the child can still reason about unconditional probability of their existence even after o... (read more)