tag:blogger.com,1999:blog-22973357.post2860559121294069901..comments2021-04-15T11:40:51.301-04:00Comments on Sabine Hossenfelder: Backreaction: The Trouble with Many WorldsSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger278125tag:blogger.com,1999:blog-22973357.post-21012263199411857542020-01-08T15:18:07.088-05:002020-01-08T15:18:07.088-05:00Doesn't the branching/splitting of the Univers...Doesn't the branching/splitting of the Universe happen before anyone makes the measurement? For example, in Schrodinger's cat thought experiment the cat/box being macroscopic already caused decoherence and became entangled with the environment before anyone can look inside the box. Thus QM predicts that there will be different patterns we call people that look like the observer looking Nathan Leehttps://www.blogger.com/profile/09539788447914313742noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-24467871788234776902019-12-13T08:00:05.768-05:002019-12-13T08:00:05.768-05:00JimV: "Nobody knows why the universe is such...JimV: <i> "Nobody knows why the universe is such that we can make progress in understanding it with our reasoning capacities."</i><br /><br />What exactly is "progress" in understanding the universe? I think the WAP is just too specific; it has nothing to do with "anthropic", it really only requires the universe behaves in some mostly deterministic fashion. It doesn&Dr. A.M. Castaldohttps://www.blogger.com/profile/17988116835722393503noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-84006988493154259802019-12-12T14:29:37.643-05:002019-12-12T14:29:37.643-05:00Reply to "The mystery is not why, if you know...Reply to "The mystery is not why, if you know that there are regularities in the universe and that we are able to infer these regularities mathematics is a good tool to do that. The mystery is why the universe is this way to begin with."<br /><br />(Done without the reply function because it is easier for me at least to find things if they are in chronological order.)<br /><br />So it JimVhttps://www.blogger.com/profile/10198704789965278981noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-32415545752056318982019-10-22T09:12:40.336-04:002019-10-22T09:12:40.336-04:00"it is perfectly conceivable that the wave fu..."it is perfectly conceivable that the wave function always evolves according to the Schrödinger equation"<br /><br />This makes sense when the wave function describes an ensemble. But that's not Sabine's view. She insists that the Schrödinger equation refers to a single system. If you consider an individual system, you would have to have discontinuities in the A(t) and B(t) Wernerhttps://www.blogger.com/profile/08502954437062856468noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51716166902902729672019-10-22T04:19:09.581-04:002019-10-22T04:19:09.581-04:00"I also find wave function "collapse&quo..."I also find wave function "collapse" repugnant."<br /><br />I'm only interested in whether the collapse is *necessary* on theoretical or empirical grounds. If it were necessary, then deploring its aesthetic demerits seems moot.<br /><br />"But some people are so infatuated with the smooth deterministic evolution dictated by Schrödinger's equation that they wish single_world_interpreterhttps://www.blogger.com/profile/02370138516934087366noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-11486339150464497672019-10-21T04:17:04.261-04:002019-10-21T04:17:04.261-04:00"the absence of some specific predictions abo..."the absence of some specific predictions about the decay does not make it any less real"<br /><br />Rest assured that I consider quantum mechanics a statistical theory and find the mathematical apparatus (though not our understanding!) sufficient. I also find wave function "collapse" repugnant.<br /><br />But some people are so infatuated with the smooth deterministic Wernerhttps://www.blogger.com/profile/08502954437062856468noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-65159035033487350862019-10-20T06:00:58.489-04:002019-10-20T06:00:58.489-04:00Werner: "Do you consider the Born rule part o...Werner: "Do you consider the Born rule part of quantum theory proper, or merely a piece of the interpretational "überbau"?"<br /><br />The point was that the Born rule is not the same as the Collapse. (At least that was the point I was making. Not speaking for Udi Fuchs, of course.) I don't think the Collapse is needed for the derivation of Fermi's Golden Rule, and Isingle_world_interpreterhttps://www.blogger.com/profile/02370138516934087366noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86571986584411420592019-10-20T05:02:41.177-04:002019-10-20T05:02:41.177-04:00Udi Fuchs wrote: "(To) derive Fermi's gol...Udi Fuchs wrote: "(To) derive Fermi's golden rule you need a Hamiltonian and the Born rule."<br /><br />Yes, of course. Do you consider the Born rule part of quantum theory proper, or merely a piece of the interpretational "überbau"?<br /><br />single_world_interpreter wrote: "You do not need a discontinuous change of state for those predictions."<br /><br />Wernerhttps://www.blogger.com/profile/08502954437062856468noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-27680754479618402682019-10-19T05:03:13.307-04:002019-10-19T05:03:13.307-04:00Werner: Schrödinger's equation may not be the ...Werner: Schrödinger's equation may not be the whole story to Fermi's Golden rule, but still I don't see that its derivation would require a nonlinear change of state anywhere. The only thing required besides unitary time evolution seems to be a variant of the Born rule.<br /><br />"The decay time of a single neutron is observable. But it never enters the discussion!"<br /><single_world_interpreterhttps://www.blogger.com/profile/02370138516934087366noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-4012845602412212552019-10-18T17:56:52.747-04:002019-10-18T17:56:52.747-04:00Werner wrote:
“What a sad conclusion!”
Depends o...Werner wrote:<br /><br />“What a sad conclusion!”<br /><br />Depends on your perspective. Some might be happy to know that quantum mechanics does not require any additional complications.<br /><br />“Yet there is an important lesson to be learned here: by taking the extreme view that there is only unitary evolution, MWI reduces this to an absurdity. Fermi's golden rule certainly is one of theUdi Fuchshttps://www.blogger.com/profile/02529460830838964526noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-29318613345722858232019-10-18T04:43:35.659-04:002019-10-18T04:43:35.659-04:00Udi Fuchs wrote: "... studying the foundation...Udi Fuchs wrote: "... studying the foundations of quantum mechanics is a waste of time"<br /><br />What a sad conclusion! Did Sabine start a sterile discussion here?<br /><br />Yet there is an important lesson to be learned here: by taking the extreme view that there is only unitary evolution, MWI reduces this to an absurdity. Fermi's golden rule certainly is one of the most Wernerhttps://www.blogger.com/profile/08502954437062856468noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-73015595203792362002019-10-17T15:27:07.211-04:002019-10-17T15:27:07.211-04:00Dear Sabine,
Sorry to come back to this, but I th...Dear Sabine,<br /><br />Sorry to come back to this, but I think your reasoning misses the essential point of (modern) MWI approaches.<br /><br />Their intention is not to provide this kind of equivalence and nothing else, but to provide a mathematically (or phenomenologically) equivalent formalism and a "better interpretation".<br /><br />So no matter whether you argue for or against TomShttps://www.blogger.com/profile/02042174216519413501noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68654958277886596112019-10-17T05:41:04.246-04:002019-10-17T05:41:04.246-04:00Sabine: You wrote, "But – and that’s the impo...Sabine: You wrote, "But – and that’s the important point – once you have measured the particle, you know with 100% probability where it is. This means that you have to update your probability and with it the wave-function. This update is also called the wave-function collapse."<br /><br />Could you be more specific on why the post hoc update of probability implies an update of the wave single_world_interpreterhttps://www.blogger.com/profile/02370138516934087366noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10254547796794281102019-10-16T21:17:28.705-04:002019-10-16T21:17:28.705-04:00Sabine wrote
“I don't care about interpretatio...Sabine wrote<br />“I don't care about interpretations. I am saying that for what making predictions are concerned, MWI isn't any better than Copenhagen, that's all.“<br /><br />The problem with Copenhagen is that it requires a wave-function collapse that is non-local. MWI does not suffer from this non-locality.<br /><br />Beyond that, I agree with you that interpretations have no Udi Fuchshttps://www.blogger.com/profile/02529460830838964526noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-17414131854529042472019-10-15T15:47:02.905-04:002019-10-15T15:47:02.905-04:00Sabine,
I fully agree, absolutely consistent - fr...Sabine,<br /><br />I fully agree, absolutely consistent - from your instrumentalist position.<br /><br />For me the whole many worlds story is only necessary if one wants to interpret quantum mechanics, state vectors etc. in an ontic manner (which I prefer).<br /><br />Your answer clarifies a lot, e.g. your dispute with Peter Shor re counting assumptions vs. discussing their meaning and TomShttps://www.blogger.com/profile/02042174216519413501noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-18159684684523698212019-10-15T09:00:51.944-04:002019-10-15T09:00:51.944-04:00That it is formally the same is exactly what I mea...That it is formally the same is exactly what I mean.<br /><br />I didn't say it's a collapse (in MWI) and don't think that's a good way to describe it, but really it doesn't matter. Look, I am an instrumentalist. For me a theory is a thing in which you put assumptions and get out predictions. I don't care about interpretations. I am saying that for what making predictions Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10081632262001819672019-10-15T08:23:57.464-04:002019-10-15T08:23:57.464-04:00Sabine,
I agree, formally this looks like the me...Sabine, <br /><br />I agree, formally this looks like the measurement postulate. <br /><br />I do not agree that this is a collapse b/c subsequent time-evolution is based on the unprojected state. <br /><br />The projection is used in an epistemic context, not affecting ontology. Selecting objects from a parent population for statistical purposes is not identical to pick these objects physically TomShttps://www.blogger.com/profile/02042174216519413501noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-14593265483837626032019-10-15T07:45:09.749-04:002019-10-15T07:45:09.749-04:00Tom,
The measurement postulate is a combination o...Tom,<br /><br />The measurement postulate is a combination of projection and normalization. You are using a projection and have thrown away the normalization because you say you will stuff it into the definition. This means you are still using the measurement postulate. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-62777318638969724002019-10-14T09:43:20.503-04:002019-10-14T09:43:20.503-04:00Dear Sabine,
the way I see it I am the first one ...Dear Sabine,<br /><br />the way I see it I am the first one to do this explicitly. So here‘s my proposal. I would be happy if you could tell me where it produces wrong predictions or where I introduce a hidden assumption like E-5. <br /><br />Given a state |ψ> and one projector E for one specific branch |E>, generated via a previous measurement, the calculation for a subsequent measurement TomShttps://www.blogger.com/profile/02042174216519413501noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-44420241139634380942019-10-14T03:40:29.714-04:002019-10-14T03:40:29.714-04:00Thomas,
I have explained this in my blogpost and ...Thomas,<br /><br />I have explained this in my blogpost and several times above already. In Many Worlds, the time-evolution of a detector is not a detector. You need an additional assumption to define something else as detector. Just try to write down what it takes to actually arrive at a prediction that's compatible with observation.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-31698655203673036912019-10-14T03:26:56.538-04:002019-10-14T03:26:56.538-04:00Sabine, thanks for the reply.
I didn‘t forget vN-...Sabine, thanks for the reply.<br /><br />I didn‘t forget vN-5. My understanding of MWI is that there isn‘t anything like E-5. I haven’t seen any hidden assumption or formula where something like that has been used.<br /><br />The detector is nothing else but a quantum mechanical device living in many worlds and generating unitary time transformation across all branches. These branches could TomShttps://www.blogger.com/profile/02042174216519413501noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-691133522809619112019-10-13T23:39:55.152-04:002019-10-13T23:39:55.152-04:00Hi Tom,
Yes, you misunderstood this. You entirely...Hi Tom,<br /><br />Yes, you misunderstood this. You entirely forgot about the relevant axiom which is vN-5. I am saying you still need to replace this. You do replace it by postulating what you mean by detector, hence you have an equivalent 5th postulate in many worlds. I am not talking about the Born rule. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70580944643982240762019-10-13T15:42:49.472-04:002019-10-13T15:42:49.472-04:00Dear Sabine,
I am afraid that I misunderstood som...Dear Sabine,<br /><br />I am afraid that I misunderstood some of your considerations, therefore I propose first to compare axioms of quantum mechanics in the spirit of von Neumann and Everett (and successors).<br /><br />vN-1: a physical system ist described by a state vector (ray) in a separable Hilbert space <br />vN-2: physical quantities / observables are described by self-adjoint operators <TomShttps://www.blogger.com/profile/02042174216519413501noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53381505420397340722019-10-11T03:49:40.970-04:002019-10-11T03:49:40.970-04:00Does it help if I point out that the Born rule is ...Does it help if I point out that the Born rule is not the measurement postulate? Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-17585826700271511692019-10-11T02:51:01.711-04:002019-10-11T02:51:01.711-04:00A measurement is just another physical process, a ...A measurement is just another physical process, a collision of some sort. Why pretend that the measuring apparatus is something classical and separate from the rest of reality in our math? Why not just just apply the Schrödinger equation to the whole process and include the measuring device in the equations? Just applying the standard dynamics to the whole system (say we are measuring the z-spin Nathan Leehttps://www.blogger.com/profile/09539788447914313742noreply@blogger.com