I think in many-worlds you have to say things like “A photon passed through the measurement apparatus, in the branch of the wavefunction where we’re speaking.”
The more general idea is: you have a fact with a “location tag” of where that fact is true.
In a more everyday example, I can say “The temperature is 29° here”. The word “here” tags a time and place.
Or in the other direction, consider “3 quarks can bind together into a proton”. This seems to be a universal fact that therefore doesn’t need any “location tag”.… But is it really? No! In some plausible theories of physics, there are many universes (and this is not related to quantum many-worlds), and they all have different fundamental constants, and in some of these universes, 3 quarks can not bind together into a proton. So you should really say “3 quarks can bind together into a proton (in the universe where I’m speaking)”.
So I would just go with the paradigm of “many facts need to come with a ‘location tag’ specifying where that fact is meant to be valid”, and if you’re OK with that, quantum many-worlds is fine.
Sorry if I’m misunderstanding your point. I am an expert on QM but not a mathematical and philosophical expert ;-)
Many worlds plus a location tag is the Bohm interpretation. You need theory for how locations evolve into other locations (in order to talk about multiple events happening in observed time), hence the nontriviality of the Bohm interpretation.
Many worlds plus a location tag is the Bohm interpretation.
Really? I don’t think I agree with that. In many-worlds, you can say “The photon passed through the apparatus in the branch of the wavefunction I find myself in”, and you can also say “The photon did not pass through the apparatus in other branches of the wavefunction that I do not find myself in”. The Bohm interpretation would reject the latter.
And if the measurement just happened on Earth, but you’re 4 lightyears away near Alpha Centauri, space-like-separated from the measurement, you can say “The photon passed through the apparatus in some branches of the wavefunction but not others. Right now, it is not yet determined which kind of branch I will eventually find myself in. But ~4 years from now (at the soonest), there will be a fact of the matter about whether I am in a photon-passed-through-the-apparatus branch of the wavefunction or not, even if nobody tells me.”
The Bohm interpretation would reject that quote, and say there is a fact of the matter about measurements from which you are space-like-separated.
You need theory for how locations evolve into other locations
I understand you as saying “you’re in some branch of the wavefunction now, and you’ll be in some branch of the wavefunction tomorrow, and you need a theory relating those”. I would say: That theory is the Schrodinger equation (also keeping in mind quantum decoherence theory, which is a consequence of that). Plus the postulate that you will find yourself in any given branch of the wavefunction with a probability proportional to its squared absolute amplitude. (And see also “consistent histories”.) Is something missing from that?
Consistent histories may actually solve the problem I’m talking about, because it discusses evolving configurations, not just an evolving wave function.
I think in many-worlds you have to say things like “A photon passed through the measurement apparatus, in the branch of the wavefunction where we’re speaking.”
The more general idea is: you have a fact with a “location tag” of where that fact is true.
In a more everyday example, I can say “The temperature is 29° here”. The word “here” tags a time and place.
Or in the other direction, consider “3 quarks can bind together into a proton”. This seems to be a universal fact that therefore doesn’t need any “location tag”.… But is it really? No! In some plausible theories of physics, there are many universes (and this is not related to quantum many-worlds), and they all have different fundamental constants, and in some of these universes, 3 quarks can not bind together into a proton. So you should really say “3 quarks can bind together into a proton (in the universe where I’m speaking)”.
So I would just go with the paradigm of “many facts need to come with a ‘location tag’ specifying where that fact is meant to be valid”, and if you’re OK with that, quantum many-worlds is fine.
Sorry if I’m misunderstanding your point. I am an expert on QM but not a mathematical and philosophical expert ;-)
Many worlds plus a location tag is the Bohm interpretation. You need theory for how locations evolve into other locations (in order to talk about multiple events happening in observed time), hence the nontriviality of the Bohm interpretation.
Really? I don’t think I agree with that. In many-worlds, you can say “The photon passed through the apparatus in the branch of the wavefunction I find myself in”, and you can also say “The photon did not pass through the apparatus in other branches of the wavefunction that I do not find myself in”. The Bohm interpretation would reject the latter.
And if the measurement just happened on Earth, but you’re 4 lightyears away near Alpha Centauri, space-like-separated from the measurement, you can say “The photon passed through the apparatus in some branches of the wavefunction but not others. Right now, it is not yet determined which kind of branch I will eventually find myself in. But ~4 years from now (at the soonest), there will be a fact of the matter about whether I am in a photon-passed-through-the-apparatus branch of the wavefunction or not, even if nobody tells me.”
The Bohm interpretation would reject that quote, and say there is a fact of the matter about measurements from which you are space-like-separated.
I understand you as saying “you’re in some branch of the wavefunction now, and you’ll be in some branch of the wavefunction tomorrow, and you need a theory relating those”. I would say: That theory is the Schrodinger equation (also keeping in mind quantum decoherence theory, which is a consequence of that). Plus the postulate that you will find yourself in any given branch of the wavefunction with a probability proportional to its squared absolute amplitude. (And see also “consistent histories”.) Is something missing from that?
See my reply here.
Consistent histories may actually solve the problem I’m talking about, because it discusses evolving configurations, not just an evolving wave function.