This is a linkpost for https://frvser.com/posts/multiverse-finetuning.html
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.
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:
far more likely, you just get a few barren rocks circling a few billion times before their sun chars or consumes them.
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.
If the observer is distinct from Alice, absolutely. If the observer is Alice, nothing needs explaining in either case.
To put a bit of a crude metaphor on it, if you were to pick a random number uniformly between 0 and 1,000,000, and pre-commit to having a child on iff it's equal to some value X - from the point of view of the child, the probability that the number was equal to X is 100%.
Apologies if there's something more subtle with your answer that I've missed.
To put a bit of a crude metaphor on it, if you were to pick a random number uniformly between 0 and 1,000,000, and pre-commit to having a child on iff it's equal to some value X - from the point of view of the child, the probability that the number was equal to X is 100%.
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 observing that they exist. They can notice it's very low, and therefore be rightfully surprised that they exist at all and update in favor of some hypothesis that would make their unconditional existence more likely.
I find the anthropic reasoning to be structurally incoherent when evaluated with rigorous probabilistic reasoning. Conditioning on the fact of existence does not dissolve the explanatory asymmetries introduced by fine-tuning nor does it render hypotheses about design/multiverse/underlying law epistemically inert in any way. That's an illusion that arises from conflating necessity of explanation with inevitability of observation (yet these are not interchangeable categories).
I don't think it quite hits the target because the real question isn't how probable is it that we'd observe a universe compatible with our existence - as that is trivially true and obvious - of course we only observe such universes. That, however, was never the engine of the inference at issue.
The real inferential fulcrum is comparative, namely: how much more probable is a life-permitting universe under a design/multiverse hypothesis, versus under a null hypothesis of chance without structure?
Sober’s infamous formulation:
P(R | D, OSE) > P(R | ¬D, OSE)
is rejected by some on the grounds that both sides allegedly reduce to 1 due to anthropic inevitability. But this is a mischaracterization of what’s actually being conditioned on. It stems from a confusion between indexical necessity and probabilistic expectation. The fact that we observe fine-tuning is not the same as asserting that it is guaranteed under all hypotheses. Observation selection constraints are not symmetry erasers; they are filters applied over distributions. You still need a distribution to filter over.
When the only information you account for is “we observe that we exist,” you obliterate the probabilistic structure you’re trying to reason about. This is what I call "probability laundering" - i.e., you sneak structure into a tautology and pretend nothing remains to be explained.
The implicit claim that the existence of observers guarantees their post hoc astonishment is meaningless because wherever observers do arise, they'll naturally observe favorable conditions - but this collapses as it treats observer existence as a universal selection function rather than a hypothesis-sensitive outcome. Consider, arguendo, two hypotheses:
H1: A single-roll universe with no fine-tuning mechanisms.
H2: A structured universe or multiverse that probabilistically amplifies life-permitting conditions.
Under H1, the likelihood of observers conditional on the base parameters is vanishingly small. Under H2, it is orders of magnitude higher. When you discover you’re in a universe fine-tuned for life, Bayesian updating must favor the hypothesis under which such a discovery is less surprising. That’s not nullified by the fact that you could only have made the observation under favorable conditions. It’s precisely because such conditions are rare under H1 that the observation carries inferential weight.
It’s the same logic behind any conditional probability inference. Suppose I survive a rare disease after an experimental treatment: it’s no rebuttal to say “Well, you couldn’t have observed otherwise - you’re only alive because the treatment worked.” Sure - but that survival is still stronger evidence for treatment efficacy than against it.
And, finally, perhaps the deepest flaw here (and in much of the anthropic literature post-Carter) is the collapse of epistemic granularity. That is, if we treat observer selection effects as categorical absolutes (rather than filters over structured distributions) - then no observation could ever update our beliefs on the nature of physical reality - which is epistemically fatal. Worse - if anthropic reasoning is always sufficient, then ANY observed structure becomes vacuously necessary, which immediately erodes the boundary between explanation and observation. We might as well assert that "Of course the laws of physics look elegant - we wouldn't be here if they didn't" - but that is the ultimate Copernican anti-shift - it turns us back to epistemic geocentrists who imagine our observations are self-justifying.
If a theory predicts nothing but observers in observer-compatible worlds, it’s not making a prediction - it’s making a category statement, and that's unfalsifiable. Science dies under that standard.
If we knew with absolute certainty that there was only a single solar system and required far more specific circumstances, would you be equally unsurprised to be alive as compared to living in a vast multiverse where the requirements were very low?
Outaide view: if there's no difference in your level of surprise it seems like something is going wrong in your reasoning.
There should be some amount of suspicion, and that amount should change based on how likely it was from the beginning. You update a certain amount based on the result, but you shouldn't end up in the same probability distribution.
As I see it, there are two possible explanations for a fine-tuned universe:
The argument for multiverse is precisely that it does not require the insane amount of luck.
Your argument is that with the insane amount of luck, the multiverse is not necessary.
That is correct, but then we are back to the original question: why did we get so extremely lucky?
If the only observable outcome is the one in which we got that extremely lucky, then it doesn't need explaining. You only can observe outcomes compatible with the fact of you making an observation.
Suppose, someone comes to you and shows you a red ball. Then they explain that they had two bags one with all red balls and one with balls of ten different colors. They have randomly picked the bag from which to pick the ball and then randomly picked the ball from this bag. Additionally they've precommited to come to you and show you the ball iff it happened to be red. What are the odds that the ball that is being shown to you have been picked from the all-red bag?
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.