I used to read Lubos Motl’s blog (maybe between 2005-2010 or something?), first because I had had him as a QFT professor and liked him personally, and later because, I dunno, I found his physics posts informative and his non-physics ultra-right-wing posts weirdly entertaining and interesting in an insane way. Anyway he used to frequently post rants against the Many Worlds Interpretation, and in favor of the Copenhagen interpretation. (Maybe he still does, I dunno.) After reading those rants and sporadically pushing back in the comments, I maybe came to understand his perspective, though I could be wrong.
So, here’s my attempt to describe Lubos’s perspective (which he calls the Copenhagen interpretation) from your (and my) perspective:
Every now and then, you learn something about what Everett branch you happen to be in. For example, you peer at the spin-o-meter and it says “This electron is spin up”. Before you looked, you had written in your lab notebook that the (partial trace) density matrix for the electron was [[0.5, 0], [0, 0.5]]. But after you see the spin-o-meter, you pull out your eraser and write a new (partial trace) density matrix for the electron in your lab notebook, namely [[1, 0], [0, 0]]. That thing you just did there, with the eraser and pencil? That’s called “collapsing the wavefunction”.
So this version of “Copenhagen” is essentially the same as Everett, but with a different definition of words like “real” and “exists”. Since we don’t care about the other Everett branches, we say that whatever is happening in them is not “real” / doesn’t “exist”, and during experiments (and life) we continually track the “real / existing” part of the wavefunction, and “collapse” is the event where we throw out part of the wavefuntion when discovering that it is not part of the Everett branch that we find ourselves in. (Maybe a compromise position would be saying “the other Everett branches are not real to us” or something.) I still think this position is wrong, but it does help me understand how the non-many-worlds story hangs together.
Yeah… to paraphrase Deutsch, that just sounds like multiple worlds in a state of chronic denial. Also, it is possible for other Everett branches to influence yours, the probability just gets so infinitesimally tiny as they decohere that it’s negligible in practice.
the probability just gets so infinitesimally tiny as they decohere that it’s negligible in practice.
(Is this true even when we apply pressure to it (as in, can we design machines or systems that leverage this systematically)? And are there are actually no macroscopic phenomena that are downstream of branches interacting? Like, I feel like one could have said such a sentence about relativity a few decades back, but it would have been pretty obviously wrong, and you end up with weird stuff like black holes if you take relativity seriously. I feel like I would be quite surprised if we ended up with no macroscopic phenomena that doesn’t require explicitly modeling the interference by distant branches.)
Like I mention in the paper, the largest object for which we’ve done this so far (at least that I’m aware of) is Carbon 60 atoms which, while impressive, are far from “macroscopic.” Preventing a superposition from decohering is really, really difficult—it’s what makes building a quantum computer so hard. That being said, there are some wacky macroscopic objects that do sometimes need to be treated as quantum systems, like neutron stars (as I mention in the paper) or black holes (though we still don’t fully understand black holes from a quantum perspective).
There is some reason to think we will never see effects that depend on the other Everett branches, because we could say that a branching event has occurred precisely when the differences between the two components are no longer effectively reversible.
Yeah… to paraphrase Deutsch, that just sounds like multiple worlds in a state of chronic denial
Motl’s point was the opposite..that MWI is Copenhagen in denial because you keep having to get out your eraser and discard what you did not observe. (Which is relevant to the claim that MWI is simple: in terms of the minimal amount of calculation you need to do to get results, it is not simpler).
I used to read Lubos Motl’s blog (maybe between 2005-2010 or something?), first because I had had him as a QFT professor and liked him personally, and later because, I dunno, I found his physics posts informative and his non-physics ultra-right-wing posts weirdly entertaining and interesting in an insane way. Anyway he used to frequently post rants against the Many Worlds Interpretation, and in favor of the Copenhagen interpretation. (Maybe he still does, I dunno.) After reading those rants and sporadically pushing back in the comments, I maybe came to understand his perspective, though I could be wrong.
So, here’s my attempt to describe Lubos’s perspective (which he calls the Copenhagen interpretation) from your (and my) perspective:
Every now and then, you learn something about what Everett branch you happen to be in. For example, you peer at the spin-o-meter and it says “This electron is spin up”. Before you looked, you had written in your lab notebook that the (partial trace) density matrix for the electron was [[0.5, 0], [0, 0.5]]. But after you see the spin-o-meter, you pull out your eraser and write a new (partial trace) density matrix for the electron in your lab notebook, namely [[1, 0], [0, 0]]. That thing you just did there, with the eraser and pencil? That’s called “collapsing the wavefunction”.
So this version of “Copenhagen” is essentially the same as Everett, but with a different definition of words like “real” and “exists”. Since we don’t care about the other Everett branches, we say that whatever is happening in them is not “real” / doesn’t “exist”, and during experiments (and life) we continually track the “real / existing” part of the wavefunction, and “collapse” is the event where we throw out part of the wavefuntion when discovering that it is not part of the Everett branch that we find ourselves in. (Maybe a compromise position would be saying “the other Everett branches are not real to us” or something.) I still think this position is wrong, but it does help me understand how the non-many-worlds story hangs together.
Yeah… to paraphrase Deutsch, that just sounds like multiple worlds in a state of chronic denial. Also, it is possible for other Everett branches to influence yours, the probability just gets so infinitesimally tiny as they decohere that it’s negligible in practice.
(Is this true even when we apply pressure to it (as in, can we design machines or systems that leverage this systematically)? And are there are actually no macroscopic phenomena that are downstream of branches interacting? Like, I feel like one could have said such a sentence about relativity a few decades back, but it would have been pretty obviously wrong, and you end up with weird stuff like black holes if you take relativity seriously. I feel like I would be quite surprised if we ended up with no macroscopic phenomena that doesn’t require explicitly modeling the interference by distant branches.)
Like I mention in the paper, the largest object for which we’ve done this so far (at least that I’m aware of) is Carbon 60 atoms which, while impressive, are far from “macroscopic.” Preventing a superposition from decohering is really, really difficult—it’s what makes building a quantum computer so hard. That being said, there are some wacky macroscopic objects that do sometimes need to be treated as quantum systems, like neutron stars (as I mention in the paper) or black holes (though we still don’t fully understand black holes from a quantum perspective).
Ah, yeah, neutron stars do feel like a good example. And I do just recall you mentioning them.
There is some reason to think we will never see effects that depend on the other Everett branches, because we could say that a branching event has occurred precisely when the differences between the two components are no longer effectively reversible.
Motl’s point was the opposite..that MWI is Copenhagen in denial because you keep having to get out your eraser and discard what you did not observe. (Which is relevant to the claim that MWI is simple: in terms of the minimal amount of calculation you need to do to get results, it is not simpler).