The Fundamental Difference in Interpretations of Quantum Mechanics - Comments

[Fra]

The issue I was talking about when I coined "soft contradiction" wasn't really inductive versus deductive. It was really about how we reason with huge numbers (or very tiny numbers). Let me give a toy example: Suppose I say that
...
I feel that the rules of thumb for using QM may be a similar type contradiction. That recipe consists of

1. A rule for how microscopic systems evolve (Schrodinger's equation)
2. A rule for how measurements produce outcomes (Born's rule)

They work very well. However, since measurement devices are themselves quantum systems (even if very complex) and measurements are just ordinary interactions between measurement devices and the systems being measured, these two rules may very well be contradictory. But a detailed analysis of the measuring process as a quantum interaction may infeasible, so actually deriving a contradiction (that everyone would agree was a contradiction) may never happen.

So by "soft contradiction" you mean a contradiction that occur only during certain - possible, but improbable - conditions.
Thus the softly flawed inference is justified?

If so, i would still think that relates to the discussion of general inference. I see your case as a possible kind of rational inference of a deductive rule but in an subjectively probabilistic inductive way, and it´s also something that can be generalized.

A possible driving force for a rational agent to abduct an approximative deductive rule, to base expectations and thus action on, is that of limited resources.

A deductive rule that are right "most of the time", can increase the evolutionary advantage of the agent in competition. As the agent can not store all information, it has not choice but to choose, what to store and how, and what to discard.

This is perfectly rational, but it also brings deep doubts on the timeless and observer invariant character of physical law.

I have also considered that this might even be modeled as an evolving system of axioms, where evolution selects for the consistent systems. An agent is associated with its axioms or assumptions. And different systems of axioms can thus be selected among, in terms of effiency of keeping their host agent in business. Meaning, efficient coding structurs for producing expectations of their environment.

This is also a way to see how deductive systems are emergent, and there is then always an evolutionary argument for WHY these axioms etc. Ie. while axioms in principle are CHOICES, the choices are not coincidental. This would then assume a one-2-one mapping between the axioms in the abstraction, and the physical postualtes which are unavoidably part in the mud of reality. In this view, the axioms are thus simply things that "seem to be true so far" but arent be proved, and they serve the purposes of a efficient betting system, but they can be destroyed/deleted whenever inconsistent evidence arrives.

These ideas gives a new perspecive into the "effectiveness of mathematics". Like Smolin also stated in this papers and talks, the deductive systems are effective precisely because they are limited. But to really appreciate this, you must also understand how and why deductive systems are emergent.

/Fredrik

 

[rubi]

There is nothing contradictory about having two types of time evolution (unitary evolution, collapse). In fact, this happens in classical Newtonian physics as well and we fully understand it. Whenever we have a stochastic description of some perfectly classical system, we have a probability distribution at $t=0$, which is then evolved by a probability conserving time evolution, then collapsed upon measurement, then evolved again, etc. The standard example is Brownian motion, which arises from completely classical equations like $F=ma$. In fact, this scheme applies to all classical probabilistic theories with time evolution, so it would actually be more mysterious if time evolution didn't work analogously in quantum mechanics, which too is probabilistic after all. In classical probability theory, the collapse too isn't an emergent phenomenon that just needs to be derived from a better theory. Instead, it's an elementary ingredient in the theory of stochastic processes, which can't be removed from the theory. And since classical probability theory is a special case of quantum theory (when all observables commute), there must be something within quantum theory that reduces to classical collapse if only commuting observables are considered.

Moreover, the notion of measurement in quantum theory is no less well-defined than in classical probability theory. A measurement in classical probability theory has occured, when the experimenter somehow has learned the measurement result. It's up to the experimenter to decide when this happened. However, the mathematical formulation is perfectly rigorous and if the experimenter knows the time of measurement and the measurement precision very well, then the theory will produce numbers that match the measured data very well.

Also, collapse is not in conflict with relativity. One can also have classical probabilitic theories of classical relativistic systems and of course they too will include collapse.

The only misconception about collapse is that it is often considered to be a type of time evolution. In fact, it's not a type of time evolution. It's just a necessary mathematical ingredient if you want to compute the probability distribution on the space of paths of a stochastic process. So by analogy, it's unlikely a type of time evolution in quantum theory either, because again, in the case of commuting observables, quantum theory reduces to classical probabilty theory.

The real quation about quantum theory is: Why does it make sense to have non-commuting observables in the first place?

 

[Stephen Tashi]

In classical probability theory, the collapse too isn't an emergent phenomenon that just needs to be derived from a better theory. Instead, it's an elementary ingredient in the theory of stochastic processes, which can't be removed from the theory.

A measurement in classical probability theory has occured, when the experimenter somehow has learned the measurement result.

I make the mild objection that "collapse" in the sense of a "realization" of some event in a probability space is not a topic treated by probability theory. It is a topic arising in interpretations of probability theory when it is applied to specific situations. What you are calling "classical probablity theory" is, more precisely, "the classical interpretations used when applying probability theory".

I agree that applications of probability theory to model macroscopic events like coin tosses involves the somewhat mysterious assumption that an event can have a probability of 1/2 of being "possible" and then not happen. (If it didn't happen, why can we assert it was possible?).

It would be interesting to hear opinions about whether there are problems in interpreting the "collapse" of the wave function that are distinct from the elementary metaphysical dilemma of applying probability theory to coin tosses.

One can also have classical probabilistic theories of classical relativistic systems and of course they too will include collapse.

What's an example of that?

 

[rubi]

I make the mild objection that "collapse" in the sense of a "realization" of some event in a probability space is not a topic treated by probability theory. It is a topic arising in interpretations of probability theory when it is applied to specific situations. What you are calling "classical probablity theory" is, more precisely, "the classical interpretations used when applying probability theory".

No, I really mean the rigorous measure theoretical formulation of probability theory and collapse is just the projection of the probability distribution. Of course, nobody in probability theory calls it "collapse". I don't like the word either, because it suggests that it is a physical process, but I just used it in order to stick with the terminology of the thread. Mathematically, it refers to the insertion of the projectors in the construction of probability measures on the space of paths of a stochastic process, i.e. the fact that these probability distributions have the form $UPUPUP \rho_0$, when evaluated on cylindrical sets. ($U$ denotes time evolution, $P$ denotes projection.)

I agree that applications of probability theory to model macroscopic events like coin tosses involves the somewhat mysterious assumption that an event can have a probability of 1/2 of being "possible" and then not happen. (If it didn't happen, why can we assert it was possible?).

I don't really understand how that relates to my post.

What's an example of that?

For example, you could study the Brownian motion of relativistic particles instead of Newtonian particles.

 

[RUTA]

These two options are generally called “psi-epistemic” and “psi-ontic,” respectively, in the foundations community. Psi-epistemic interpretations do not necessarily entail that QM is incomplete, see http://www.ijqf.org/wps/wp-content/uploads/2015/06/IJQF2015v1n3p2.pdf for example. I didn’t have time to read all 8 pp of this thread, so my apologies if something along these lines was already posted.

 

[Stephen Tashi]

No, I really mean the rigorous measure theoretical formulation of probability theory and collapse is just the projection of the probability distribution.

Projection of which probability distribution upon what?

For example, you could study the Brownian motion of relativistic particles instead of Newtonian particles.

I can imagine that as a topic in special relativity.

 

[rubi]

Projection of which probability distribution upon what?

When you have a distribution $\rho(x)$ at time $t$, then if you have gotten to know that $x\in A$, you multiply $\rho(x)$ by the characteristic function $\chi_A(x)$. The projection is $\rho(x)\rightarrow \chi_A(x)\rho(x)$. It's the same thing that happens to the quantum state $\psi(x)$ if you have learned the result of a measurement of the position operator $\hat x$. In quantum theory you have the additional complication that observables might not commute. If you have an observable in a different basis, you have to perform this multiplication in that basis, so for instance a measurement of the momentum $\hat p$ results in a multiplication of the Fourier transformed state $\tilde \psi(p)$ by the characteristic function $\chi_A(p)$. Every projector $\hat P$ in quantum theory is a multiplication operator by some characteristic function in the eigenbasis of the corresponding observable, so for the special case of commuting observables, the mathematics of projections is exactly the same as in classical probability theory.

I can imagine that as a topic in special relativity.

I probabily don't have time for a long discussion about this, but this seems to be a good overview article: https://arxiv.org/abs/0812.1996

 

[vanhees71]

Philosophy need not be speculation. Neither Bohr's nor Einstein's nor Heisenberg's nor Feynman's philosophy was wild speculation.

Well, Einstein was at least understandable in his very clear criticism against QT. As is clear since Bell's work and the confirmation of the violation of the Bell inequality he was wrong, and QT came out right. Bohr has his merits in clearly stating the idea of complementarity and that QT is about what can be prepared and measured in a atomistic world. Unfortunately he was not very clear in his own writings but had to be translated by others into understandable statements. I'm not too keen about Heisenberg. E.g., he didn't get his own finding of the uncertainty principle right and was corrected by Bohr. Also his version of QT, matrix mechanics, had to be clarified in the "Dreimännerarbeit" by him, Bohr, and Jordan.

 

[A. Neumaier]

So whatever is keeping us from making deterministic predictions about the results of quantum experiments, it isn't chaos due to nonlinear dynamics of the quantum state.

However, it is chaos in the (classical) part of the quantum state that is accessible to measurement devices.

Can you elaborate?

What we directly observe in a measurement device are only macroscopic (what I called ''classical'') observables, namely the expectation values of certain smeared field operators. These form a vast minority of all conceivable observables in the conventional QM sense. For example, hydromechanics is derived in this way from quantum field theory. It is well-known that hydromechanics is highly chaotic in spite of the underlying linearity of the Schroedinger equation defining (nonrelativistic) quantum field theory from which hydromechanics is derived. Thus linearity in a very vast Hilbert space is not incompatible with chaos in a much smaller accessible manifold of measurable observables.

 

[A. Neumaier]

I'm not too keen about Heisenberg.

His philosophy (what you called ''wild speculations'') lead him in 1925 to the discovery of the canonical commutation relations.

 

[Fra]

I think this discussion has many components, not sure which post rubis post related to.

For the record My core issue is not the principle of information update as such, i agree is the most natural component. My issues are different and more subtle.

My previous point of emergence did not refer to the collapse, it rather referred to to the rule of evolution in between updates.

The real quation about quantum theory is: Why does it make sense to have non-commuting observables in the first place?

The way i see it (inference interpretation), is because it allows the the observer to maximise its predictive power, when computing the expectation from the state of information. And this stabilises the observer from the destabilising environment.

It can be understood as datacompression of the abduced rules of expectation.

I think this can be described mathematically as well. But it requires a different foundation and framework pf physics, which is not yet in place. But developing that goes hand in hand with understanding and vague insight.

/Fredrik

 

[stevendaryl]

What we directly observe in a measurement device are only macroscopic (what I called ''classical'') observables, namely the expectation values of certain field operators. These form a vast minority of all conceivalbe observables in the conventional QM sense. For example, hydromechanics is derived in this way from quantum field theory. Everyone knows that hydromechanics is highly chaotic in spite of the underlying linearity of the Schroedinger equation defining (nonrelativistic) quantum field theory from which hydromechanics is derived. Thus linearity in a very vast Hilbert space is not incompatible with chaos in a much smaller accessible manifold of measurable observables.

I think that's a very interesting subject--the reconciliation of the linearity of Schrodinger's equation with the chaotic nonlinearity of macroscopic phenomena. But I really don't think that chaos in the macroscopic world can explain the indeterminism of QM. In Bell's impossibility proof, he didn't make any assumptions about the complexity of the hidden variable $\lambda$, or the difficulty of computing measurement outcomes from $\lambda$, or the sensitivity of the outcomes to $\lambda$.

 

[A. Neumaier]

I really don't think that chaos in the macroscopic world can explain the indeterminism of QM. In Bell's impossibility proof, he didn't make any assumptions about the complexity of the hidden variable $\lambda$, or the difficulty of computing measurement outcomes from $\lambda$, or the sensitivity of the outcomes to $\lambda$.

Bell doesn't take into account that a macroscopic measurement is actually done by recording field expectation values. Instead he argues with the traditional simplified quantum mechanical idealization of the measurement process. The latter is known to be only an approximation to the quantum field theory situations needed to be able to treat the detector in a classical way. Getting a contradiction from reasoning with approximations only shows that at some point the approximations break down.

 

[vanhees71]

His philosophy (what you called ''wild speculations'') lead him in 1925 to the discovery of the canonical commutation relations.

This was Born, and it was not philosophy but good knowledge of the math behind Heisenberg's ingenious idea. Not everything what Heisenberg did was philosophical gibberish but he also did some physics for which he rightfully got the Nobel prize in the early 1930ies.

 

[A. Neumaier]

This was Born, and it was not philosophy but good knowledge of the math behind Heisenberg's ingenious idea.

It was Born and Jordan. But Heisenberg had the ingenious idea - a philosophical feat!

Die mathematische Grundlage der H e i s e n b e r g s c h e n Betrachtung ist das M u l t l p l i k a t i o n s g e s e t z der quantentheoretisehen Größen, das er dutch eine geistreiche Korrespondenzbetrachtung erschlossen hat.

Bei dieser Sachlage scheint es geratener, jene Hoffnung auf eine Beobachtung der bisher unbeobachtbaren Größen (wie Lage, Umlaufszeit des Elektrons) ganz aufzugeben, gleichzeitig also einzuräumen, daß die teilweise Übereinstimmung der genannten Quantenregeln mit der Erfahrung mehr oder weniger zufälllig sei, und zu versuchen, eine der klassischen Mechanik analoge quantentheoretische Mechanik auszubilden, in welcher nur Beziehungen zwischen beobachtbaren Größen vorkommen.

This is pure philosophy - the attempt to reframe concepts to make sense of formerly apparently meaningless (or contradictory) concepts.

 

[vanhees71]

Well, this "Korrespondenzbetrachtung" I'd still call physical rather than philosophical heuristics (as Einstein's "heuristic aspect" of the photoelectric-effect paper was physical rather than philocophical), but that's a bit semantics. Thanks anyway for pointing to the paper. It's interesting that this was written without Heisenberg. There's also the "Dreimännerarbeit", which is the 2nd part written with Heisenberg:

https://link.springer.com/article/10.1007/BF01379806

 

[Stephen Tashi]

When you have a distribution $\rho(x)$ at time $t$, then if you have gotten to know that $x\in A$, you multiply $\rho(x)$ by the characteristic function $\chi_A(x)$.

What that mathematical procedure represents is a matter of interpretation. It looks like we are computing an un-normalized version of a probability density representing "the probability that $X =x$ given $X \in A$". and interpreting "$X \in A$" to mean "the event $X \in A$ actually happened".

My point is that the interpretation of a conditional probability in terms of some event actually happening is not formalized in the mathematical theory of probability. The same comment applies to when we apply probability theory to compute "The probability a fair die lands a 6 given the event that the die showed an even number". The measure theoretic approach to probability theory does not formally define what it means for an event to "actually happen" or "be observed" etc. What the measure theoretic approach gives us is a definition of conditional probabilities in terms of a procedure for computing them from other probabilities.

Whether we are talking about the fancy mathematics of stochastic processes or the mathematics of a coin toss, there is the common theme of a "collapse" of a probability distribution - to a single outcome or to a conditional probability distribution when we interpret the mathematics. So one question is: Why is such a collapse of a probability distribution in a "classical" setting any more of a conceptual problem than the collapse of a wave function in QM?

I'm not asking that as a rhetorical question. I'd really like to hear about other problematic aspects of wave function collapse - if there are any.

----

As to how the interpretation of collapse meets relativity, I don't see that special relativity adds any new conceptual problems. General relativity adds the conceptual problem that collapse must be defined without some reference to absolute time. One article that deals with this is Collapse of Probability Distributions in Relativistic Spacetime by Hans Ohanian https://arxiv.org/pdf/1703.00309.pdf. It's notable that the solution given for where probability distributions collapse is the same for wave function collapse. So I don't see that wave function collapse added any conceptual problems that weren't already present in probability distribution collapse - at least from the viewpoint of that paper.

 

[A. Neumaier]

Thanks anyway for pointing to the paper. It's interesting that this was written without Heisenberg.

I added to the previous post a link to Heisenberg's first paper that started it all,and a quote from it that shows the philosophy that went into Heisenberg's ingenious idea.

 

[vanhees71]

Well, sure. So if you call this philosophy rather than physics then there's indeed some valuable physics originating from philosophy ;-)). Amazing!

 

[lightarrow]

The real quation about quantum theory is: Why does it make sense to have non-commuting observables in the first place?

Can something like "because a measurement, in general, changes the quantum sistem's state, so if we measure the observable A and then the observable B, the final state will be different than when we measure B first and then A" be an answer, given that there is "some relation" from a measurement of the observables A or B on a state |psi> and computing A|psi> and B|psi>?

--
lightarrow

 

[junjunjun233]

Bell doesn't take into account that a macroscopic measurement is actually done by recording field expectation values. Instead he argues with the traditional simplified quantum mechanical idealization of the measurement process. .

Will there be any implication of the observed value? Especially the dynamics on Quantum state. Like some limitations on visual aspect.

Like for instance. Quantum clock in a superposition of energy eigenstates, the mass–energy equivalence leads to a trade-off between the possibilities for an observer to define time intervals at the location of the clock and in its vicinity. In the sense that it does not depend on the particular constitution of the clock, and is a necessary consequence of the superposition principle and the mass–energy equivalence. In SR, some aspect of observations changes visually under extreme gravitational field or traveling at certain speed.

Are their any particular visual phenomenon in QM when observing quantum level?

https://phys.org/news/2017-03-blurred-quantum-world.html#jCp

..aslav Brukner from the University of Vienna and the Institute of Quantum Optics and Quantum Information demonstrated a new effect at the interplay of the two fundamental theories. According to quantum mechanics, if we have a very precise clock its energy uncertainty is very large. Due to general relativity, the larger its energy uncertainty the larger the uncertainty in the flow of time in the clock's neighbourhood. Putting the pieces together, the researchers showed that clocks placed next to one another necessarily disturb each other, resulting eventually in a "blurred" flow of time. This limitation in our ability to measure time is universal, in the sense that it is independent of the underlying mechanism of the clocks or the material from which they are made. "Our findings suggest that we need to re-examine our ideas about the nature of time when both quantum mechanics and general relativity are taken into account", says Esteban Castro, the lead author of the publication.

 

[meopemuk]

There is nothing contradictory about having two types of time evolution (unitary evolution, collapse).

I totally agree. There is no deep difference between quantum collapse and classical "collapse" of probability distributions. The only difference is that in classical physics the probabilities are results of our ignorance or laziness (the underlying dynamics is supposed to be predictable), but in quantum physics there is an additional fundamental and irreducible component of randomness/probabilities in the underlying dynamics (God plays dice).

The real quation about quantum theory is: Why does it make sense to have non-commuting observables in the first place?

I am very happy with the explanation provided by "quantum logic". I think this is a very beautiful piece of theory, which answers all questions about quantum foundations in the most satisfying way.

The original paper:
G. Birkhoff and J. von Neumann, "The logic of quantum mechanics", Ann. Math. 37 (1936), 823

A recent review:
L. Curcuraci, "Why do we need Hilbert spaces?", arXiv:quant-ph/1708.08326

Eugene.

 

[bhobba]

but in quantum physics there is an additional fundamental and irreducible component of randomness/probabilities in the underlying dynamics (God plays dice).

This is exactly why people should study a few interpretations - to stop falling for errors like that,

Its truth is entirely interpretation dependent.

Thanks
Bill

 

[PeterDonis]

in quantum physics there is an additional fundamental and irreducible component of randomness/probabilities in the underlying dynamics (God plays dice).

To amplify @bhobba's response a bit, this is only the case for "collapse" interpretations; it is not the case for the MWI and similar interpretations. So this statement is interpretation dependent.

 

[meopemuk]

To amplify @bhobba's response a bit, this is only the case for "collapse" interpretations; it is not the case for the MWI and similar interpretations. So this statement is interpretation dependent.

Isn't it true that in M(any)W(orlds)I(nterpretation) each measurement creates new Universes -- as many as there are possible measurement outcomes? But you -- the observer -- must jump into one of these Universes to get on with your life. And this jump is random. So, we are back to the fundamental irreducible randomness, this time with all the bells and whistles of multiple worlds.

As far as I can see, all QM interpretations have a random element in them. The crucial question: is this the true irreducible randomness (like in QM) or just the ignorance/laziness randomness (like in statistical mechanics)? In the former case, our interpretation hasn't added anything of value to the regular shut-up-and-calculate quantum mechanics. There is no experiment that can prove/disprove our interpretation. The latter case is more interesting, as it promises that one day we will learn how to handle these "hidden variables". Then we will be able to predict the exact timing sequence of clicks in the Geiger counter or the pattern of electron hits on the scintillating screen.

I choose the former answer, with the full understanding that this choice is based on belief, not on knowledge.

Eugene.

 

[stevendaryl]

Bell doesn't take into account that a macroscopic measurement is actually done by recording field expectation values.

Well, that is certainly far from being an accepted resolution. I don't see how anything in his argument depends on that.

 

[PeterDonis]

Isn't it true that in M(any)W(orlds)I(nterpretation) each measurement creates new Universes -- as many as there are possible measurement outcomes?

No. There is only one universe and one quantum state. But it contains subsystems that are entangled in a way that invites the ordinary language term "many worlds" because our ordinary language assumes that things are in ordinary classical states.

For example, if you watch a Stern-Gerlach experiment measuring the spin of an electron, your quantum state becomes entangled with the electron's so that, heuristically, in the universal wave function, there are two terms, one a product of "electron spin up" and "you observe electron spin up", and the other a product of "electron spin down" and "you observe electron spin down". Since our ordinary language concept of "you" assumes that you observed only one result, this kind of quantum state invites the ordinary language description that there are now two "copies" of you (and the electron).

And since stuff like this is happening all the time, everywhere in the universe, it invites the ordinary language description that there are multiple "universes". But there is still only one quantum state--one wave function--and as far as the math of QM is concerned, that is one universe, one "world", not many. Nothing gets "created" during a measurement, because everything is unitary, and unitary operations don't create or destroy anything.

 

[meopemuk]

No. There is only one universe and one quantum state.

But still, even within MWI, there should be a point where a choice must be made, which copy of myself -- the observer -- is the real one. Whether the electron went up or down in this particular instance? So, we are back to the random collapse. I am not sure what is collapsing in this case, but Nature makes a random choice between a number of possibilities. I call it a collapse.

So, all the elaborate mechanisms of MWI have bought us absolutely nothing. We are still staring at the fundamental irreducible randomness of Nature.

Eugene.

 

[PeterDonis]

But still, even within MWI, there should be a point where a choice must be made, which copy of myself -- the observer -- is the real one

Nope. In the MWI, the wave function is real, and there is one wave function. There is no choice to be made.

Whether the electron went up or down in this particular instance?

It went in both directions. In the MWI, measurements do not have single results; they have all possible results.

all the elaborate mechanisms of MWI have bought us absolutely nothing. We are still staring at the fundamental irreducible randomness of Nature.

You are incorrect. The MWI is not easy to come to terms with, but what it says is perfectly straightforward, and it completely eliminates randomness. The question is whether it is true; we don't know since we have no way of testing it experimentally (since it makes the same predictions for the results of measurements as all other interpretations of QM--which it must since it uses the same math).

 

[meopemuk]

In the MWI, measurements do not have single results; they have all possible results.

But in reality measurements do have single results. Then how does MWI relates to reality?

Eugene.

 

[PeterDonis]

in reality measurements do have single results.

Not if the MWI is true. If the MWI is true, all possible results happen, and each result is entangled with the appropriate state of measuring devices, observers, etc., so that, for example, all possible experiences you can have observing the different possible results of a measurement also happen. You are only aware of one result because your awareness depends on your brain state, and the different branches of the wave function have different states for your brain, just as they have different states for the measured system.

In other words, according to the MWI, we don't actually know that "in reality", measurements have single results. All we know is that we observe measurements to have single results. The MWI accounts for this the way I explained just above--by treating our observations, our experiences, as part of the universal wave function, so they get entangled, superposed, etc., just like everything else.

I understand that this is difficult to wrap your mind around. But any discussion of interpretations of QM has to take it into account.

 

[meopemuk]

In other words, according to the MWI, we don't actually know that "in reality", measurements have single results. All we know is that we observe measurements to have single results. The MWI accounts for this the way I explained just above--by treating our observations, our experiences, as part of the universal wave function, so they get entangled, superposed, etc., just like everything else.

Thank you for explaining this. If so, then the MWI is the weirdest kind of philosophy. I remember learning that there are two major schools of philosophy:

Materialism says that things happen in the real world, and our perceptions are just reflections of these objective events. I am cool with that.

Idealism says that the real world is just an illusion, and all we have are our perceptions. I can accept even that.

But now, there is the MWI, which says that even our perceptions are illusions, and there is nothing but the super-entangled MWI wave function. How weird is that!

Eugene.

 

[PeterDonis]

there is the MWI, which says that even our perceptions are illusions

No, they're not illusions. When you perceive an electron coming out of the "spin up" side of a Stern-Gerlach device, that's because the branch of the electron's wave function that that branch of your brain is entangled with is coming out of the "spin up" side of the device. All of the entanglements match up just the way they should for the results you perceive to be correct.

there is nothing but the super-entangled MWI wave function

The MWI doesn't say there is "nothing but" the wave function. It just says that everything is "made of" the wave function, the same way classical atomic theory said everything was made of atoms. The MWI does not say that you and I don't exist. It just says that what we are actually referring to by the words "you" and "I" is not as simple as our naive intuitions tell us it is.

 

[meopemuk]

When you perceive an electron coming out of the "spin up" side of a Stern-Gerlach device, that's because the branch of the electron's wave function that that branch of your brain is entangled with is coming out of the "spin up" side of the device.

Sorry for being so slow. But I just don't get it.

I understand that there is a part of my brain's wave function entangled with the "spin up" electron. I can also accept that there is another piece/part/branch of my brain entangled with the "spin down" electron. But when I looked at the Stern-Gerlach device last time I can swear I saw only the "spin up". So, where did the other ("spin down") portion disappeared? In another Universe?

I don't care whether the "spin down" portion disappeared in reality, or only in my perception. For me it did disappear, because I didn't see it. I don't care if both up and down portions exist in the MWI wave function (maybe in different Universes). I would like to have a theory that describes for me what I see (or what I think I see) in my Universe. And I see that the up and down outcomes occur in some random pattern, if I keep repeating my experiment. But the MWI tries to convince me that both outcomes coexist. Then who is right, me -- the observer -- or the MWI?

Eugene.

 

[PeterDonis]

when I looked at the Stern-Gerlach device last time I can swear I saw only the "spin up". So, where did the other ("spin down") portion disappeared? In another Universe?

The problem is that word "I". You are using it like it refers to the entire "piece" of the wave function that corresponds to your brain. But it doesn't. It only refers to the part of your brain's wave function that got entangled with the spin up part of the electron's wave function. There is another part of your brain's wave function that got entangled with the spin down part of the electron's wave function; and that part of your brain's wave function uses the word "I" to refer to itself, just as the part you are thinking of in the quote above, the part that got entangled with the spin up part of the electron's wave function, uses the word "I" to refer to itself. So the problem is that there is only one word, "I", but it's doing double duty. This is very confusing, of course, but it's what we get for trying to use ordinary language to describe something that ordinary language has no proper terminology to describe.

I would like to have a theory that describes for me what I see (or what I think I see) in my Universe.

The MWI does do that; it tells "you" that "you" will observe whatever is consistent with the states of the observed object that "your" brain gets entangled with. But the referent of the word "you" has to be clearly understood. See above.