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Nornagest comments on Boltzmann Brains and Anthropic Reference Classes (Updated) - Less Wrong
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Comments (112)
This seems confused. Boltzmann's model only has any interesting consequences if you at least consider external-world skepticism; if you use a causal history to specify any particular agent and throw out anything where that doesn't line up with experiential history, then of course we can conclude that Boltzmann brains (which generally have a causal history unrelated to their experiential history, although I suppose you could imagine a Boltzmann brain with a correct experiential history as a toy example) aren't in the right reference class. But using that as an axiom in an argument intended to prove that Boltzmann brains don't pose a problem to current cosmological models amounts to defining the problem away.
Here's the structure of the purported problem in cosmology:
(1) Model X predicts that most observers with subjective experience identical to mine are Boltzmann brains.
(2) I am not a Boltzmann brain.
(3) The right way to reason anthropically is the SSA.
(4) The appropriate reference class used in the SSA must include all observers with subjective experience identical to mine.
CONCLUSION: Model X wrongly predicts that I'm a Boltzmann brain.
I am not attacking any of the first 3 premises. I am attacking the fourth. Attacking the fourth premise does not require me to establish that I'm not a Boltzmann brain. That's a separate premise in the original argument. It has already been granted by my opponent. So I don't see how assuming it, in an objection to the argument given above, amounts to writing my bottom line first.
Your objection assumes that we can distinguish observers by their causal history rather than their subjective experience, and that we can discard agents for whom the two don't approximately correspond. This is quite a bit more potent than simply assuming you're not a Boltzmann brain personally: if extrapolated to all observers, then no (or very few) Boltzmann brains need be considered. The problematic agents effectively don't exist within the parts of the model you've chosen to look at. Merely assuming you're not a Boltzmann brain, on the other hand, does lead to the apparent contradiction in the parent -- but I don't think it's defensible as an axiom in this context.
Truthfully, though, I wouldn't describe the cosmological problem in the terms you've used. It's more that most observers with your subjective experience are Boltzmann brains under this cosmological model, and Boltzmann brains' observations do not reliably reflect causal relationships, so under the SSA this cosmology implies that any observations within it are most likely invalid and the cosmology is therefore unverifiable. This does have some self-reference in it, but it's not personal in the same sense, and including "I am not a Boltzmann brain" in the problem statement is incoherent.
I'm not sure what you mean by this. I'm claiming we need not consider the possibility that we are Boltzmann brains when we are reasoning anthropically. I'm not claiming that Boltzmann brains are not observers (although they may not be), nor am I claiming that they do not exist. I also think that if a Boltzmann brain were reasoning anthropically (if it could), then it should include Boltzmann brains in its reference class. So I don't think the claims I'm making can be extrapolated to all observers. They can be extrapolated to other observers sufficiently similar to me.
I hope this is not the case, since I don't believe this. I think it's pretty likely that our universe will contain many Boltzmann brain type observers whose subjective experience is not a reliable record of their causal history (or any sort of record at all, really). Could you clarify where my objection relies on this assumption?
The problem is often presented (including by Bostrom) as a straight Bayesian disconfirmation of models like Boltzmann's. That seems like a different argument from the one you present.
Why? The other three premises do not imply that I am a Boltzmann brain. They only imply that model X predicts I'm a Boltzmann brain. That doesn't conflict with the second premise.
That was poorly worded. I'd already updated the grandparent before you posted this; hopefully the revised version will be clearer.
I was talking about my formulation of the problem, not yours. Assuming you're not a Boltzmann brain does lead to a contradiction with one of my premises, specifically the one about invalid observations.