I Would studying MWI be a waste of time?

[AlexCaledin]

The problem is (the lack of) ontology.

Why not assume the "toy onthology" of the Great Game suggested by Feynman in the Character of Physical Law? It seems explaining absolutely everything. The collapse could be a game event ("move"), the branches of reality constituting the mathematical Tree of the Game, so we just safely return to FAPP-Copenhagen .

______________

Sorry! I've got that Feynman book in Russian

 

[Demystifier]

Why not assume the "toy onthology" of the Great Game suggested by Feynman? It seems explaining absolutely everything.

I have no idea what is toy ontology of the Great Game by Feynman. Reference?

 

[stevendaryl]

According to the minimal ensemble interpretation, the physical system cannot be an eigenstate simply because the physical system does not live in the Hilbert space. Only our knowledge is represented by a state in the Hilbert space.

That's fine. The wave function is just some kind of summary of our information about the system, and the information is incomplete, which is why it only gives probabilistic predictions. Now, we perform a measurement/observation and we learn something that we didn't know before--that an electron is spin-up in the z-direction, for example. The question then is: Is this information that already existed beforehand, and the observation just revealed it, or was the information created by the act of measurement? The first option seems like a hidden-variables-type assumption, while the second option seems like a collapse-type assumption.

 

[Demystifier]

The question then is: Is this information that already existed beforehand, and the observation just revealed it, or was the information created by the act of measurement? The first option seems like a hidden-variables-type assumption, while the second option seems like a collapse-type assumption.

The first option, that information existed before the measurement which only revealed it, corresponds to non-contextual hidden variables. It is certainly wrong (even in Bohmian mechanics) due to the Kochen-Specker theorem.

The second option, that measurement somehow created the information, can be realized even without the collapse. In fact, all consistent interpretations (Copenhagen, Bohm, MWI, ...) involve some kind of creation of information by measurement. They must, due to the Kochen-Specker theorem.

 

[stevendaryl]

There is nothing to understand about Born's rule. It's just a fundamental property of QT as a theory of nature.

That cannot possibly be true. A measurement is simply a special kind of interaction whereby microscopic properties are magnified to make a macroscopic difference. Presumably, facts about measurements should be derivable from facts about the particles making up the measuring devices. So a claim about measurements cannot possibly be fundamental.

 

[vanhees71]

I don't understand your argument. Born's rule is just telling the probabilistic meaning of the state (described as a statistical operator) concerning the outcome of measurements, and of course a measurement device works according to the laws of nature, i.e., QT. Born's rule is as far as I know not derivable from the dynamical laws of QT (see Weinberg, Lectures on Quantum Mechanis, Cambridge University Press). It's a postulate independent of the other postulates making up QT, and it's a postulate on the "kinematics" if you wish, not on the dynamics of quantum systems.

 

[stevendaryl]

The second option, that measurement somehow created the information, can be realized even without the collapse. In fact, all consistent interpretations (Copenhagen, Bohm, MWI, ...) involve some kind of creation of information by measurement. They must, due to the Kochen-Specker theorem.

If you consider an EPR-type experiment, and you want to say that Alice's measurement creates information about Bob's particle, that seems like an inherently nonlocal effect. Whether you want to call it a "collapse" or not, it seems like it amounts to the same thing.

In MWI, you would say that Alice's measurement doesn't reveal anything about Bob's particle.

 

[Demystifier]

That cannot possibly be true. A measurement is simply a special kind of interaction whereby microscopic properties are magnified to make a macroscopic difference. Presumably, facts about measurements should be derivable from facts about the particles making up the measuring devices. So a claim about measurements cannot possibly be fundamental.

Read more carefully what @vanhees71 said. He said "fundamental property of QT as a theory of nature." (my bolding). He did not say "a fundamental property of nature itself", nor he said "fundamental property of the final theory of nature".

 

[vanhees71]

If you consider an EPR-type experiment, and you want to say that Alice's measurement creates information about Bob's particle, that seems like an inherently nonlocal effect. Whether you want to call it a "collapse" or not, it seems like it amounts to the same thing.

In MWI, you would say that Alice's measurement doesn't reveal anything about Bob's particle.

It's not a nonlocal effect since Alice's measurement only reveals something about Bob's particle (and indeed according to standard QT it does), because of the entanglement of the measured observables, and the entanglement is due to the preparation of the two-particle system in this state. So the correlations (one-to-one correlations for some entangled observables on A's and B's particles) are inherent from the beginning before A's measurement and is not due to any action at a distance due to A's measurement!

 

[stevendaryl]

I don't understand your argument. Born's rule is just telling the probabilistic meaning of the state (described as a statistical operator) concerning the outcome of measurements

As I said, if a measurement is simply a complicated interaction, then no rule involving measurements can be fundamental, because measurements are not fundamental.

For an analogy, if Newton had given, in addition to his laws of motion, an additional law saying: "Cats always land on their feet", that couldn't possibly be a fundamental law. Any law involving cats in principle follows from the laws governing the particles they are made of.

Any statement about measurements (for instance, that they always produce an eigenvalue of the quantity being measured) cannot possibly be a fundamental law for the same reason.

 

[Demystifier]

If you consider an EPR-type experiment, and you want to say that Alice's measurement creates information about Bob's particle, that seems like an inherently nonlocal effect. Whether you want to call it a "collapse" or not, it seems like it amounts to the same thing.

Are you saying that collapse and, for instance, Bohmian non-locality, amount to the same thing?By the way, I do think that there are alternatives to quantum non-locality. See
https://arxiv.org/abs/1703.08341 Sec. 5.3.

 

[stevendaryl]

It's not a nonlocal effect since Alice's measurement only reveals something about Bob's particle (and indeed according to standard QT it does), because of the entanglement of the measured observables

To me, what you're saying is just nonsensical. When Alice measures spin-up for her particle along the z-axis, she knows that Bob will measure spin-down along the z-axis. So the statement

"Bob will not measure spin-up along the z-axis"

is new information about Bob that she didn't know prior to her measurement. Either that information was true before Alice performed her measurement, or it became true at the time she performed the measurement. What third possibility is there? (Well, MWI has the third possibility that the statement just isn't true--Bob might measure spin-up in a different "branch")

Saying that Alice's and Bob's particles are entangled is not an answer. That's the reason that Alice can confidently know that the statement is true. But it doesn't answer the question of whether it was true beforehand or became true as a result of Alice's measurement.

 

[vanhees71]

I don't understand, what you mean by fundamental here. I use the term only for theories/models, and I call something a "fundamental law" if it is not derivable from some other law (like an axiom in mathematics) but is assumed to define the theory in the first place. In physics "fundamental laws" are just formalized rules of experience, and Born's rule, in my opinion, is of this kind.

Also the other kinematical postulate you mention is fundamental in my opinion: An observable is described by a self-adjoint operator on a (separable) Hilbert space, and the possible values it takes are the eigenvalues of this operator. That I need to define the description of states and their relation in the first place, and it cannot be derived from some other assumptions. That's why I call it fundamental.

Whether or not you can measure an observable in the lab is another issue and only empirics can answer the question whether I can measure it with the one or other device and whether the measurements are in agreement or disagreement with the theory. For QT they are indeed in very good agreement, and that's why we bother about QT so much.

 

[stevendaryl]

Are you saying that collapse and, for instance, Bohmian non-locality, amount to the same thing?

For the purposes of this discussion, I think the differences are unimportant. The critical thing, which is true for both "collapse" interpretations and Bohm, is that Alice's measurement effects what happens to Bob.

 

[Demystifier]

The critical thing, which is true for both "collapse" interpretations and Bohm, is that Alice's measurement effects what happens to Bob.

Fine, but it's true even for MWI.

 

[vanhees71]

To me, what you're saying is just nonsensical. When Alice measures spin-up for her particle along the z-axis, she knows that Bob will measure spin-down along the z-axis. So the statement

"Bob will not measure spin-up along the z-axis"

is new information about Bob that she didn't know prior to her measurement. Either that information was true before Alice performed her measurement, or it became true at the time she performed the measurement. What third possibility is there? (Well, MWI has the third possibility that the statement just isn't true--Bob might measure spin-up in a different "branch")

Saying that Alice's and Bob's particles are entangled is not an answer. That's the reason that Alice can confidently know that the statement is true. But it doesn't answer the question of whether it was true beforehand or became true as a result of Alice's measurement.

My answer is completely sensical, and I've explained this very many times in this forum, in which sense I mean it, but here it's again. Obviously we discuss the following case:

A spin-0 particle at rest decays into two spin-1/2 particles. So the spin part of the two particles must be in the singlet state $|\psi \rangle \langle \psi|$ with
$$|\psi \rangle=\frac{1}{2} (|1/2,-1/2 \rangle - |-1/2,1/2 \rangle.$$
The single-particle spin state is described by the partial trace, and both partices are "unpolarized", i.e.,
$$\hat{\rho}_{A}=\hat{\rho}_B =\frac{1}{2} \hat{1}.$$
So the single-particle spins are completely undetermined.

However, there's the correlation that, if A measures $\sigma_z^{(A)}=+1/2$ then necessarily B measures $\sigma_a^{(B)}=-1/2$. This is due to the preparation of the two-particle system in the spin-entangled state as described and not due to any measurement A does on her particles. Of course, A gains information on her and thus due to the entanglement also B's particle, but nothing else happens at the instant A's detector registers her particle, particularly nothing happens instantaneously to B's particle which might be very far away if both experimenters are placed far away from each other and the place at which the decay of the original particle happened (i.e., the two-particle state was prepared).

 

[Demystifier]

It's not a nonlocal effect since Alice's measurement only reveals something about Bob's particle (and indeed according to standard QT it does), because of the entanglement of the measured observables, and the entanglement is due to the preparation of the two-particle system in this state. So the correlations (one-to-one correlations for some entangled observables on A's and B's particles) are inherent from the beginning before A's measurement and is not due to any action at a distance due to A's measurement!

So you disagree with Ballentine (see post #90 above).

 

[stevendaryl]

I don't understand, what you mean by fundamental here. I use the term only for theories/models, and I call something a "fundamental law" if it is not derivable from some other law (like an axiom in mathematics) but is assumed to define the theory in the first place.

No. To me, a fundamental law is one that is defined in terms of the fundamental objects of the theory: particles and fields and maybe geometry. A measurement is not a fundamental object of physics. Whether something is a measurement or not is a matter of engineering: You design a device so that the property being measured is reflected in a macroscopic property that can easily be read off and recorded.

 

[Demystifier]

No. To me, a fundamental law is

See my post #98. Or consider the statement:
"A fundamental property of non-relativistic mechanics is that velocity is not bounded from above".
Do you think that the word "fundamental" is used correctly in this statement? My point is, a non-fundamental theory may have internal fundamental properties which are not fundamental from the external point of view.

 

[vanhees71]

So you disagree with Ballentine (see post #90 above).

It may well be that I disagree with Ballentine. With locality of course I mean it in the usual sense of a local relativistic QFT, and I don't think that Ballentine disagrees about locality in this specific sense. Of course, there are long-range correlations, described by entanglement, in relativistic QFT as well as in non-relativistic QT, but the point is that they must not be misunderstood as "spooky actions at a distance". For me EPR's criticism is just about the collapse assumption but not about minimally interpreted QT.

Also this question about ontology is irrelevant for physicists to begin with, because they deal with real things in their lab or wherever they observe something in nature. Of course the moon is there, even if nobody observes it, because there are conservation laws ensuring that it doesn't simply vanish, and since the existence of the moon is well established from observations in the past, it's pretty save to predict that it is still there, and we even can predict where it is very accurately, but we don't need to even bother about QT here, because Newtonian classical mechanics is sufficient for that purpose.

 

[Demystifier]

A measurement is not a fundamental object of physics.

Depends on the definition of physics.

 

[Demystifier]

It may well be that I disagree with Ballentine.

But you still think that his book is one of the best books on QM, right?

 

[stevendaryl]

My answer is completely sensical, and I've explained this very many times in this forum, in which sense I mean it, but here it's again...

You're just repeating what it means to say that two systems are entangled, or that their values for particular observables are correlated. But that wasn't the question.

Once again, assuming that we have a source of anti-correlated pairs, one particle being sent to Alice and the other particle being sent to Bob. Assume that Alice and Bob agree ahead of time to measure the spin of their respective particle along the z-axis. For definiteness, let's assume that Bob measures his particle after Alice measures hers.

So suppose that Alice measures spin-up.

• Is the statement "Bob will measure spin-down" true, or not?
• If it is true, was it true before Alice's measurement?

 

[vanhees71]

I do. Also Weinberg's book is among the best books on QM, and he is obviously of a different opinion about the "interpretation problem" than Ballentine. So?

 

[vanhees71]

You're just repeating what it means to say that two systems are entangled, or that their values for particular observables are correlated. But that wasn't the question.

Once again, assuming that we have a source of anti-correlated pairs, one particle being sent to Alice and the other particle being sent to Bob. Assume that Alice and Bob agree ahead of time to measure the spin of their respective particle along the z-axis. For definiteness, let's assume that Bob measures his particle after Alice measures hers.

So suppose that Alice measures spin-up.

• Is the statement "Bob will measure spin-down" true, or not?
• If it is true, was it true before Alice's measurement?

I don't see any contradiction to what I said. The answer to your last question is very simply given by QT.

(a) If Alice finds her particle to have spin up, then Bob's particle with certainty has spin down.

(b) If Alice hasn't meaured here particle's spin, the only thing that's known about both measurements are the probabilities, and nothing else. The single-particle polarization is "maximally unknown" in the sense of Shannon-Jaynes-von Neumann entropy as a measure of the missing information. In other words: both single-particle spin-z components are as indetermined as they can be.

The observable determined here is, by the way the total spin, because it's in the $S=0$ (implying $\Sigma_z=0$ of course) of the two-particle system as a whole.

What is determined here due to the preparation is the 100% correlation of outcomes of spin-z measurements but not the value of the spin-z components themselves. I think that's the very difference between classical physics and quantum physics: While in the former all observables always have determined values, while in quantum physics you can have completely undetermined observables (like the single-particle spin in our example) but still have strong correlations about the outcome of measurements.

Bell's great achievement was to show that these correlations can be stronger than in any local deterministic model, and the great achievement of the experimentalists in the recent 3 decades is that they could measure this prediction of QT with astonishing accuracy. It's one of the rare occasions that a completely philosophical question could be put into the realm of hard facts in the sense of the natural sciences!

 

[Demystifier]

I do. Also Weinberg's book is among the best books on QM, and he is obviously of a different opinion about the "interpretation problem" than Ballentine. So?

So it's interesting that you disagree with some parts of your favored books. There is nothing wrong or inconsistent about it, that's just interesting.

But can you answer my questions in post #86? It seems that they could be central to our disagreement about the minimal ensemble interpretation.

 

[stevendaryl]

(a) If Alice finds her particle to have spin up, then Bob's particle with certainty has spin down.

So, you're saying that, in the case in which Alice measured spin-up along the z-axis, the statement

"Bob will measure spin-down along the z-axis"

is true. Good. The followup question is:

• Was it true before Alice performed her measurement?

(b) If Alice hasn't meaured here particle's spin, the only thing that's known about both measurements are the probabilities, and nothing else. The single-particle polarization is "maximally unknown" in the sense of Shannon-Jaynes-von Neumann entropy as a measure of the missing information. In other words: both single-particle spin-z components are as indetermined as they can be.

I interpret that as saying that it was not true before Alice performed her measurement, but was true afterward. So it sure seems to me that Alice had a nonlocal effect on Bob: The set of possible outcomes went from two possibilities before Alice's measurement to one possibility afterward. That's what "collapse" means.

 

[stevendaryl]

I interpret that as saying that it was not true before Alice performed her measurement, but was true afterward. So it sure seems to me that Alice had a nonlocal effect on Bob: The set of possible outcomes went from two possibilities before Alice's measurement to one possibility afterward. That's what "collapse" means.

It seems to me that you are assuming collapse, but are not taking it very seriously.

 

[Demystifier]

Bell's great achievement was to show that these correlations can be stronger than in any local deterministic model

And also stronger than any local non-deterministic (but ontic) model.

 

[vanhees71]

So, you're saying that, in the case in which Alice measured spin-up along the z-axis, the statement

"Bob will measure spin-down along the z-axis"

is true. Good. The followup question is:

• Was it true before Alice performed her measurement?

I interpret that as saying that it was not true before Alice performed her measurement, but was true afterward. So it sure seems to me that Alice had a nonlocal effect on Bob: The set of possible outcomes went from two possibilities before Alice's measurement to one possibility afterward. That's what "collapse" means.

No! The correlation comes from the fact that the two-particle system was prepared in the entangled state. That's the "cause" of the correlation but not A's measurement.