If the many worlds interpretation of quantum mechanics is true isn’t anthropic reasoning involved in making predictions about the future of quantum systems. There exists some world in which, from the moment this comment is posted onward, all attempts to detect quantum indeterminacy fail, all two-slit experiments yield two distinct lines instead of a wave pattern etc. Without anthropic reasoning we have no reason to find this result at all surprising. So either we need to reject anthropic reasoning or we need to reject the predictive value of quantum mechanics under the many worlds interpretation. Right?
(Apologies if this has been covered, I’m playing catch-up and just trying to hash things out for myself. Also should I expect to be declared a prophet in the world in which quantum indeterminacy disappears from here on out?)
If the many worlds interpretation of quantum mechanics is true isn’t anthropic reasoning involved in making predictions about the future of quantum systems.
Basic QM seems to say that probability is ontologically basic. In a collapse point of view, it’s what we usually think of as probability that shows up in decision theory. In MWI, both events happen. But you could talk about usual probability either way. (“classical probability is a degenerate form of quantum probability” with or without collapse)
Anthropics is about the interaction of probability with the number of observers.
Replacing usual probability with QM doesn’t seem to me to make a difference. Quantum suicide is a kind of anthropics, but it’s not clear to me in what sense it’s really quantum. It’s mainly about rejecting the claim that the Born probabities are ontologically basic, that they measure how real an outcome is.
But in MWI isn’t the observed probability of some quantum state just the fraction of worlds in which an observer would detect that quantum state? As such, doesn’t keeping the probabilities of quantum events as QM predicts require that “one should reason as if one were a random sample from the set of all observers in one’s reference class” (from a Nick Bostrom piece). The reason we think our theory of QM is right is that we think our branch in the multi-verse didn’t get cursed with an unrepresentative set of observed phenomena.
Wouldn’t a branch in the multi-verse that observed quantum events in which values were systematically distorted (by random chance) come up with slightly different equations to describe quantum mechanics? If so, what reason do we have to think that our equations are correct if we don’t consider our observations to be similar to the observations made in other possible worlds?
It’s not just world counting… (Although Robin Hanson’s Mangled World’s idea does suggest a way that it may turn out to amount to world counting after all)
essentially one has to integrate the squared modulus of quantum amplitude over a world. This is proportional to the subjective probability of experiencing that world.
Yes… that it isn’t simple world counting does seem to be a problem. This is something that we, or at least I, am confused about.
As I said, that’s something I’m confused about, and apparently others are as well.
We’ve got the linear rules for how quantum amplitude flows over configuration space, then we’ve got this “oh, by the way, the subjective probability of experiencing any chunk of reality is proportional to the square of the absolute value” rule.
Would you expand and sharpen your point? Woit comes to mind.
At one point you claim, possibly based on MWI, that “there is some world in which …”. As far as I can tell, the specifics of the scenario shouldn’t have anything to do with the correctness of your argument.
This is how I would paraphrase your comment:
According to MWI, there exists some world in which unlikely things happen.
We find this surprising.
Anthropic reasoning is necessary to conclude 2.
Anthropic reasoning is involved in making predictions about quantum systems.
In step 2: Who is the “we”? What is the “this”? Why do we find it surprising?
In step 3: What do you mean by “anthropic reasoning”? In general, it is pretty hard metareasoning to conclude that a reasoning step or maneuver is necessary for a conclusion.
We don’t need anthropic reasoning under MWI in order to be surprised when finding ourselves in worlds in which unlikely things happen so much as we need anthropic reasoning to conclude that an unlikely thing has happened. And our ability to conclude that an unlikely thing has happened is needed to accept quantum mechanics as a successful scientific theory.
“We” is the set of observers in the worlds where events, declared to be unlikely by quantum mechanics actually happen. An observer is any physical system with a particular kind of causal relation to quantum states such that the physical system can record information about quantum states and use the information to come up with methods of predicting the probability of previously unobserved quantum processes (or something, but if we can’t come up with a definition of observer then we shouldn’t be talking about anthropic reasoning anyway).
According to MWI, the (quantum) probability of a quantum state is defined as the fraction of worlds in which that state occurs.
The only way an observer somewhere in the multi-verse can trust the observations used that confirm quantum mechanics probabilistic interpretations is if they reason as if they were a random sample from the set of all observers in the multi-verse (one articulation of anthropic reasoning) because if they can’t do that then they have no reason to think their observations aren’t wrong in a systematic way.
An observer’s reason for believing the standard model of QM to be true the first place is that they can predict atomic and subatomic particles behaving according a probabilistic wave-function.
Observers lose their reason for trusting QM in the first place if they accept the MWI AND are prohibited reason anthropically.
In other words
If MWI is likely, then QM is likely iff AR is acceptable.
I think one could write a different version of this argument by referencing expected surprise at discovering sudden changes in quantum probabilities (which I was conflating with the first argument in my first comment) but the above version is probably easier to follow.
I hadn’t come to that conclusion until you said it… but yes, that is about right. I’m not sure I would say all evidence is anthropic- I would prefer saying that all updating involves a step of anthropic reasoning. I make that hedge just because I don’t know that direct sensory information is anthropic evidence, just that making good updates with that sensory information is going to involve (implicit) anthropic reasoning.
If the many worlds interpretation of quantum mechanics is true isn’t anthropic reasoning involved in making predictions about the future of quantum systems. There exists some world in which, from the moment this comment is posted onward, all attempts to detect quantum indeterminacy fail, all two-slit experiments yield two distinct lines instead of a wave pattern etc. Without anthropic reasoning we have no reason to find this result at all surprising. So either we need to reject anthropic reasoning or we need to reject the predictive value of quantum mechanics under the many worlds interpretation. Right?
(Apologies if this has been covered, I’m playing catch-up and just trying to hash things out for myself. Also should I expect to be declared a prophet in the world in which quantum indeterminacy disappears from here on out?)
Basic QM seems to say that probability is ontologically basic. In a collapse point of view, it’s what we usually think of as probability that shows up in decision theory. In MWI, both events happen. But you could talk about usual probability either way. (“classical probability is a degenerate form of quantum probability” with or without collapse)
Anthropics is about the interaction of probability with the number of observers.
Replacing usual probability with QM doesn’t seem to me to make a difference. Quantum suicide is a kind of anthropics, but it’s not clear to me in what sense it’s really quantum. It’s mainly about rejecting the claim that the Born probabities are ontologically basic, that they measure how real an outcome is.
But in MWI isn’t the observed probability of some quantum state just the fraction of worlds in which an observer would detect that quantum state? As such, doesn’t keeping the probabilities of quantum events as QM predicts require that “one should reason as if one were a random sample from the set of all observers in one’s reference class” (from a Nick Bostrom piece). The reason we think our theory of QM is right is that we think our branch in the multi-verse didn’t get cursed with an unrepresentative set of observed phenomena.
Wouldn’t a branch in the multi-verse that observed quantum events in which values were systematically distorted (by random chance) come up with slightly different equations to describe quantum mechanics? If so, what reason do we have to think that our equations are correct if we don’t consider our observations to be similar to the observations made in other possible worlds?
It’s not just world counting… (Although Robin Hanson’s Mangled World’s idea does suggest a way that it may turn out to amount to world counting after all)
essentially one has to integrate the squared modulus of quantum amplitude over a world. This is proportional to the subjective probability of experiencing that world.
Yes… that it isn’t simple world counting does seem to be a problem. This is something that we, or at least I, am confused about.
Thanks. Good to know. I don’t suppose you can explain why it works that way?
As I said, that’s something I’m confused about, and apparently others are as well.
We’ve got the linear rules for how quantum amplitude flows over configuration space, then we’ve got this “oh, by the way, the subjective probability of experiencing any chunk of reality is proportional to the square of the absolute value” rule.
There’re a few ideas out there, but...
Would you expand and sharpen your point? Woit comes to mind.
At one point you claim, possibly based on MWI, that “there is some world in which …”. As far as I can tell, the specifics of the scenario shouldn’t have anything to do with the correctness of your argument.
This is how I would paraphrase your comment:
According to MWI, there exists some world in which unlikely things happen.
We find this surprising.
Anthropic reasoning is necessary to conclude 2.
Anthropic reasoning is involved in making predictions about quantum systems.
In step 2: Who is the “we”? What is the “this”? Why do we find it surprising? In step 3: What do you mean by “anthropic reasoning”? In general, it is pretty hard metareasoning to conclude that a reasoning step or maneuver is necessary for a conclusion.
We don’t need anthropic reasoning under MWI in order to be surprised when finding ourselves in worlds in which unlikely things happen so much as we need anthropic reasoning to conclude that an unlikely thing has happened. And our ability to conclude that an unlikely thing has happened is needed to accept quantum mechanics as a successful scientific theory.
“We” is the set of observers in the worlds where events, declared to be unlikely by quantum mechanics actually happen. An observer is any physical system with a particular kind of causal relation to quantum states such that the physical system can record information about quantum states and use the information to come up with methods of predicting the probability of previously unobserved quantum processes (or something, but if we can’t come up with a definition of observer then we shouldn’t be talking about anthropic reasoning anyway).
According to MWI, the (quantum) probability of a quantum state is defined as the fraction of worlds in which that state occurs.
The only way an observer somewhere in the multi-verse can trust the observations used that confirm quantum mechanics probabilistic interpretations is if they reason as if they were a random sample from the set of all observers in the multi-verse (one articulation of anthropic reasoning) because if they can’t do that then they have no reason to think their observations aren’t wrong in a systematic way.
An observer’s reason for believing the standard model of QM to be true the first place is that they can predict atomic and subatomic particles behaving according a probabilistic wave-function.
Observers lose their reason for trusting QM in the first place if they accept the MWI AND are prohibited reason anthropically.
In other words If MWI is likely, then QM is likely iff AR is acceptable.
I think one could write a different version of this argument by referencing expected surprise at discovering sudden changes in quantum probabilities (which I was conflating with the first argument in my first comment) but the above version is probably easier to follow.
Can I paraphrase what you just said as:
“If many-worlds is true, then all evidence is anthropic evidence”
I hadn’t come to that conclusion until you said it… but yes, that is about right. I’m not sure I would say all evidence is anthropic- I would prefer saying that all updating involves a step of anthropic reasoning. I make that hedge just because I don’t know that direct sensory information is anthropic evidence, just that making good updates with that sensory information is going to involve (implicit) anthropic reasoning.