Something wrong with this post, which I didn’t appreciate back in 2008, when it was made, is that it misunderstands how quantum mechanics is interpreted by most practicing physicists.
According to the post, physicists believe in wavefunction collapse, and in doing so they follow the rules of Science, but if they followed the rules of Bayes, they would believe that the wavefunction does not collapse, and thus in many worlds.
Now quite apart from the problems of many worlds, which I have pointed out here and at other posts, it is not even true that physicists, as a rule, believe in wavefunction collapse in the way it is represented here, i.e. as an actually occurring physical process.
The cognitive facts about what all the world’s physicists individually believe regarding quantum mechanics would be rather complicated—there is a diversity of opinion among physicists, and an internal inconsistency of opinion within many individual physicists—but the standard view is not wavefunction realism. The wavefunction (or quantum state vector) is like a probability function; it is a mathematical entity from which probabilities of outcomes can be calculated. There is no wavefunction in space, which evolves smoothly when not observed and which jumps discontinuously when it is observed. What’s actually there are particles (fields, strings, whatever), with quantitative properties (“observables”) which take values with probabilities derived according to the projection postulate (or according to some mathematically equivalent rule).
Theoretical physics lacks an agreed-upon picture which specifies which observables take what values and when, at all times. Quantum mechanics only says “if you care, this is what it might be doing right now”; it offers a dynamics for the wavefunction, i.e. for the probabilities, but it doesn’t offer an underlying objective dynamical framework from which wavefunction dynamics can be derived. There are various proposals (e.g. Bohmian mechanics), but they all have problems, and there are well-known difficulties (e.g. Kochen-Specker theorem, Hardy’s theorem) facing the construction of a fully objective theory which reproduces quantum mechanics.
The prevailing attitudes in physics towards quantum foundations may be confused or even deplorable, but nonetheless, my point is that the argument of this article is wrong. In fact it’s wrong twice over. First of all, most physicists do not believe in wavefunction collapse as a physical process—this is what I have just been saying—and so the starting point of the argument only describes the views of a minority. Second, the assertion that many worlds provides a quantitatively simpler theory than objective wavefunction collapse is a highly dubious one, because there is no good derivation within many worlds of the probabilities which contain all of the actual predictive content of quantum mechanics. It’s as if I were to say, “My theory of physics is blah blah blah, and though I can’t explain why in terms of blahs, my theory happens to give exactly the same predictions as orthodox quantum mechanics. Therefore, it is at least as good as orthodox quantum mechanics.” Which is the criticism I was making back in 2008.
My problem with the collapse version of QM—and this may stem from the fact that Eliezer’s explanation is the only one I’ve read that I’ve actually had a decent understanding of it (such that I am relatively confident I could pass along the basic concepts to someone else without becoming “Goofus” in some of EY’s earlier examples) - is that there is no apparent reason for the collapse.
Take a coin toss. We say the probability of a heads or tails on a fair coin is .5 for each outcome. When heads eventually happens, the truth of the matter is that if we had information like the state of the coin pre-flip, the position of the hand flipping the coin, the force of the arm as it moves up and the exact position and force of the thumb on the coin itself, we could raise our estimation of the probability for that flip to be heads up to probably .9+. Given more precise information, we could conceivably get the probability up to .99. Excluding quantum effects, the actual probability that the coin would come up heads in that particular instance was essentially 1.
This does not seem to be the case with quantum mechanics. There does not seem to be any new information that could give any insight as to why the electron went through the first slit instead of the second, or vice versa. It’s not just that the information is hidden, it doesn’t seem to exist at all. Instead, the probability itself appears to be “baked into” reality, with no reason to prefer one outcome over the other. The CI response seems to be “It just does, Born probabilities blah blah blah accept it” without even attempting to explain what seems to me to be a major problem with the way reality works under this interpretation. CI doesn’t actually explain the Born probabilities any better than MWI, as far as I’ve read, they just seem to have “claimed” them. For this reason, I don’t think CI satisfies your criteria of having a “derivation… of the probabilities which contain all of the actual predictive content of quantum mechanics” criteria either. At least not any better than MWI.
If the wave functions aren’t a real property of the universe, then why the hell does reality seem to follow them? And if they are real, why did A happen when there is no reason B didn’t happen? This seems to imply that luck is a fundamental property of the universe!
It’s these two basic questions that I haven’t seen answered satisfactorily from the CI or more general collapse perspective (if such a thing exists separate from mainstream CI). The fact that most physicists believe some variation MWI bolsters my confidence, even though the idea that decoherance effectively produces zillions of universes continuously simply blows my mind.
Something wrong with this post, which I didn’t appreciate back in 2008, when it was made, is that it misunderstands how quantum mechanics is interpreted by most practicing physicists.
According to the post, physicists believe in wavefunction collapse, and in doing so they follow the rules of Science, but if they followed the rules of Bayes, they would believe that the wavefunction does not collapse, and thus in many worlds.
Now quite apart from the problems of many worlds, which I have pointed out here and at other posts, it is not even true that physicists, as a rule, believe in wavefunction collapse in the way it is represented here, i.e. as an actually occurring physical process.
The cognitive facts about what all the world’s physicists individually believe regarding quantum mechanics would be rather complicated—there is a diversity of opinion among physicists, and an internal inconsistency of opinion within many individual physicists—but the standard view is not wavefunction realism. The wavefunction (or quantum state vector) is like a probability function; it is a mathematical entity from which probabilities of outcomes can be calculated. There is no wavefunction in space, which evolves smoothly when not observed and which jumps discontinuously when it is observed. What’s actually there are particles (fields, strings, whatever), with quantitative properties (“observables”) which take values with probabilities derived according to the projection postulate (or according to some mathematically equivalent rule).
Theoretical physics lacks an agreed-upon picture which specifies which observables take what values and when, at all times. Quantum mechanics only says “if you care, this is what it might be doing right now”; it offers a dynamics for the wavefunction, i.e. for the probabilities, but it doesn’t offer an underlying objective dynamical framework from which wavefunction dynamics can be derived. There are various proposals (e.g. Bohmian mechanics), but they all have problems, and there are well-known difficulties (e.g. Kochen-Specker theorem, Hardy’s theorem) facing the construction of a fully objective theory which reproduces quantum mechanics.
The prevailing attitudes in physics towards quantum foundations may be confused or even deplorable, but nonetheless, my point is that the argument of this article is wrong. In fact it’s wrong twice over. First of all, most physicists do not believe in wavefunction collapse as a physical process—this is what I have just been saying—and so the starting point of the argument only describes the views of a minority. Second, the assertion that many worlds provides a quantitatively simpler theory than objective wavefunction collapse is a highly dubious one, because there is no good derivation within many worlds of the probabilities which contain all of the actual predictive content of quantum mechanics. It’s as if I were to say, “My theory of physics is blah blah blah, and though I can’t explain why in terms of blahs, my theory happens to give exactly the same predictions as orthodox quantum mechanics. Therefore, it is at least as good as orthodox quantum mechanics.” Which is the criticism I was making back in 2008.
My problem with the collapse version of QM—and this may stem from the fact that Eliezer’s explanation is the only one I’ve read that I’ve actually had a decent understanding of it (such that I am relatively confident I could pass along the basic concepts to someone else without becoming “Goofus” in some of EY’s earlier examples) - is that there is no apparent reason for the collapse.
Take a coin toss. We say the probability of a heads or tails on a fair coin is .5 for each outcome. When heads eventually happens, the truth of the matter is that if we had information like the state of the coin pre-flip, the position of the hand flipping the coin, the force of the arm as it moves up and the exact position and force of the thumb on the coin itself, we could raise our estimation of the probability for that flip to be heads up to probably .9+. Given more precise information, we could conceivably get the probability up to .99. Excluding quantum effects, the actual probability that the coin would come up heads in that particular instance was essentially 1.
This does not seem to be the case with quantum mechanics. There does not seem to be any new information that could give any insight as to why the electron went through the first slit instead of the second, or vice versa. It’s not just that the information is hidden, it doesn’t seem to exist at all. Instead, the probability itself appears to be “baked into” reality, with no reason to prefer one outcome over the other. The CI response seems to be “It just does, Born probabilities blah blah blah accept it” without even attempting to explain what seems to me to be a major problem with the way reality works under this interpretation. CI doesn’t actually explain the Born probabilities any better than MWI, as far as I’ve read, they just seem to have “claimed” them. For this reason, I don’t think CI satisfies your criteria of having a “derivation… of the probabilities which contain all of the actual predictive content of quantum mechanics” criteria either. At least not any better than MWI.
If the wave functions aren’t a real property of the universe, then why the hell does reality seem to follow them? And if they are real, why did A happen when there is no reason B didn’t happen? This seems to imply that luck is a fundamental property of the universe!
It’s these two basic questions that I haven’t seen answered satisfactorily from the CI or more general collapse perspective (if such a thing exists separate from mainstream CI). The fact that most physicists believe some variation MWI bolsters my confidence, even though the idea that decoherance effectively produces zillions of universes continuously simply blows my mind.