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Nisan comments on Stupid Questions Open Thread - Less Wrong Discussion
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That line of reasoning puzzles me, because the anthropic-principle explanation of fine tuning works just fine without MWI: Out of all the conceivable worlds, of course we find ourselves in one that is habitable.
This only works if all worlds that follow the same fundamental theory exist in the same way our local neighborhood exists. If all of space has just one set of constants even though other values would fit the same theory of everything equally well, the anthropic principle does not apply, and so the fact that the universe is habitable is ordinary Bayesian evidence for something unknown going on.
The word "exist" doesn't do any useful work here. There are conceivable worlds that are different from this one, and whether they exist depends on the definition of "exist". But they're still relevant to an anthropic argument.
The habitability of the universe is not evidence of anything because the probability of observing a habitable universe is practically unity.
Can you clarify why a conceivable world that doesn't exist in the conventional sense of existing is relevant to an anthropic argument?
I mean, if I start out as part of a group of 2^10 people, and that group is subjected to an iterative process whereby we split the group randomly into equal subgroups A and B and kill group B, then at every point along the way I ought to expect to have a history of being sorted into group A if I'm alive, but I ought not expect to be alive very long. This doesn't seem to depend in any useful way on the definition of "alive."
Is it different for universes? Why?
I agree with all that. I don't quite see where that thought experiment fits into the discussion here. I see that the situation where we have survived that iterative process is analogous to fine-tuning with MWI, and I agree that fine-tuning is unsurprising given MWI. I further claim that fine-tuning is unsurprising even in a non-quantum universe. Let me describe the though experiment I have in mind:
Imagine a universe with very different physics. (1) Suppose the universe, by nature, splits into many worlds shortly after the beginning of time, each with different physical constants, only one of which allows for life. The inhabitants of that one world ought not to be surprised at the fine-tuning they observe. This is analogous to fine-tuning with MWI.
(2) Now suppose the universe consists of many worlds at its inception, and these other worlds can be observed only with great difficulty. Then the inhabitants still ought not to be surprised by fine-tuning.
(3) Now suppose the universe consists of many worlds from its inception, but they are completely inaccessible, and their existence can only be inferred from the simplest scientific model of the universe. The inhabitants still ought not to be surprised by fine-tuning.
(4) Now suppose the simplest scientific model describes only one world, but the physical constants are free parameters. You can easily construct a parameterless model that says "a separate world exists for every choice of parameters somehow", but whether this means that those other worlds "exist" is a fruitless debate. The inhabitants still ought not to be surprised by fine-tuning. This is what I mean when I say that fine-tuning is not surprising even without MWI.
In cases (1)-(4), the inhabitants can make an anthropic argument: "If the physical constants were different, we wouldn't be here to wonder about them. We shouldn't be surprised that they allow us to exist." Does that makes sense?
Ah, I see.
Yes, I agree: as long as there's some mechanism for the relevant physical constants to vary over time, anthropic arguments for the "fined-tuned" nature of those constants can apply; anthropic arguments don't let us select among such mechanisms.
Thanks for clarifying.
Hm, only the first of the four scenarios in the grandparent involves physical constants varying over time. But yes, anthropic arguments don't distinguish between the scenarios.
Huh. Then I guess I didn't understand you after all.
You're saying that in scenario 4, the relevant constants don't change once set for the first time?
In that case this doesn't fly. If setting the constants is a one-time event in scenario 4, and most possible values don't allow for life, then while I ought not be surprised by the fine-tuning given that I observe something (agreed), I ought to be surprised to observe anything at all.
That's why I brought up the small-scale example. In that example, I ought not be surprised by the history of A's given that I observe something, but I ought to be surprised to observing anything in the first place. If you'd asked me ahead of time whether I would survive I'd estimate a .0001 chance... a low-probability event.
If my current observed environment can be explained by positing scenarios 1-4, and scenario 4 requires assuming a low-probability event that the others don't, that seems like a reason to choose 1-3 instead.
I'm saying that in all four scenarios, the physical constants don't change once set for the first time. And in scenarios (2)-(4), they are set at the very beginning of time.
I was confused as to why you started talking about changing constants, but it occurs to me that we may have different ideas about how the MWI explanation of fine-tuning is supposed to run. I admit I'm not familiar with cosmology. I imagine the Big Bang occurs, the universal wavefunction splits locally into branches, the branches cool down and their physical constants are fixed, and over the next 14 billion years they branch further but their constants do not change, and then life evolves in some of them. Were you imagining our world constantly branching into other worlds with slightly different constants?
No, I wasn't; I don't think that's our issue here.
Let me try it this way. If you say "I'm going to roll a 4 on this six-sided die", and then you roll a 4 on a six-sided die, and my observations of you are equally consistent with both of the following theories:
Theory T1: You rolled the die exactly once, and it came up a 4
Theory T2: You rolled the die several times, and stopped rolling once it came up 4
...I should choose T2, because the observed result is less surprising given T2 than T1.
Would you agree? (If you don't agree, the rest of this comment is irrelevant: that's an interesting point of disagreement I'd like to explore further. Stop reading here.)
OK, good. Just to have something to call it, let's call that the Principle of Least Surprise.
Now, suppose that in all scenarios constants are set shortly after the creation of a world, and do not subsequently change, but that the value of a constant is indeterminate prior to being set. Suppose further that life-supporting values of constants are extremely unlikely. (I think that's what we both have been supposing all along, I just want to say it explicitly.)
In scenario 1-3, we have multiple worlds with different constants. Constants that support life are unlikely, but because there are multiple worlds, it is not surprising that at least one world exists with constants that support life. We'd expect that, just like we'd expect a six-sided die to come up '4' at least once if tossed ten times. We should not be surprised that there's an observer in some world, and that world has constants that support life, in any of these cases.
In scenario 4, we have one world with one set of constants. It is surprising that that world has life-supporting constants. We ought not expect that, just like we ought not expect a six-sided die to come up '4' if tossed only once. We should be surprised that there's an observer in some world.
So. If I look around, and what I observe is equally consistent with scenarios 1-4, the Principle of Least Surprise tells me I should reject scenario 4 as an explanation.
Would you agree?
By 'explain observed fine-tuning', I mean 'answer the question why does there exist a universe (which we inhabit) which is fine-tuned to be life-friendly.' The anthropic principle, while tautologically true, does not answer this question, in my view.
In other words, the existence of life does not cause our universe to be life-friendly (of course it implies that the universe is life friendly); rather, the life-friendliness of our universe is a prerequisite for the existence of life.
We may have different ideas of what sort of answers a "why does this phenomenon occur?" question deserves. You seem to be looking for a real phenomenon that causes fine-tuning, or which operates at a more fundamental level of nature. I would be satisfied with a simple, plausible fact that predicts the phenomenon. In practice, the scientific hypotheses with the greatest parsimony and predictive power tend to be causal ones, or hypotheses that explain observed phenomena as arising from more fundamental laws. But the question of where the fundamental constants of nature come from will be an exception if they are truly fundamental and uncaused.
You're right that observing that we're in a habitable universe doesn't tell us anything. However, there are a lot more observations about the universe that we use in discussions about quantum mechanics. And some observations suit the idea that we're know what's going on better than others. "Know what's going on" here means that a theory that is sufficient to explain all of reality in our local neighborhood is also followed more globally.