Steven Weinberg on the interpretation of quantum mechanics

[EPR]

That's not what is "certain". The only thing that is "certain" is that Alice's and Bob's measurements must commute; but that does not require that there are no causal influences between them. It only requires that any such influences must not have a preferred direction, i.e., that the "Alice to Bob" and "Bob to Alice" causal directions must both be consistent with the results.

They are certain in as much as the theory of relativity is certain. There are no ftl influences.

 

[PeterDonis]

Without realism/determinism, how can there be causal influences?

Without realism, how can there be anything? If you're going to say realism has been "broken", you need to explain how you exist before asking how there can be anything else.

As you can see from the above, we can go round and round forever making assertions about "realism" by simply using different definitions of what that word means. That will get us nowhere.

A new, bigger theory superceded qm of single objecta(qft) as a more comprehensive explanation of reality.

QFT still predicts violations of the Bell inequalities (as it must, since it agrees with experiment and such violations have been observed experimentally), so it does nothing at all to solve whatever problems people have with explaining the fact of such violations.

Gerrymandering definitions of words like "realism" does not help with those problems at all.

They are certain in as much as the theory of relativity is certain. There are no ftl influences.

The theory of relativity does not say "no ftl influences". It only says that any such "influences" must be independent of the time order of the events involved.

 

[EPR]

The theory of relativity does not say "no ftl influences". It only says that any such "influences" must be independent of the time order of the events involved.

But all events pertaining to the Alice/Bob's measurements are causal. And time ordered. How can there be ftl influences without breaking SR?

 

[vanhees71]

You have been focusing on the easy case, where Alice and Bob both measure spin in the same direction, so there is perfect anti-correlation and a "Bertlmann's socks" type of argument, which is the argument you are making, is workable.

However, such an argument is not workable for the hard cases, where Alice and Bob measure spin in different directions, at angles for which the correlations violate the Bell inequalities. For those cases, I don't think you can claim that "it's all just due to the previously prepared state", since the whole point of the Bell inequalities being violated is that there is no possible "previously prepared state" (no set of local hidden variables) that can account for the correlations. The QM "state" can do it, but only by being nonlocal, i.e., giving correlation probabilities that do not factorize as described in Bell's paper.

Why is not workable? To the contrary as you say, the correlations violate the Bell inequalities precisely as predicted by QT, i.e., also in these cases the quantum formalism describes the outcomes correctly, and of course you have prepared just the entangled state you need to do this experiment. The preparation does not predetermine which measurements you do afterwards of course, but the correlations are described by these Bell states, including the violation of Bell's inequalities.

The state itself you may call nonlocal, but that's somewhat misleading, because it seems to imply non-local interactions. That's why I prefer Einstein's more precise word "inseparability", i.e., the entanglement describes precisely what's nowadays confirmed with these Bell tests, which at Einsteins lifetime was not yet known. So relativistic QFT is a theory which in a consistent way describes both the locality of interactions (implemented by the construction of local quantum fields obeying the microcausality condition, i.e., the commutation of local observables with the Hamiltonian density at space-like distances) as well as the inseparability of entangled subsystems leading to the long-ranged correlations between measurements, including the violation of Bell's inequalities for the appropriate measurements (in this example of course by choosing a certain set of measurements of spin components (or, for photons, polarizations) in non-collinear directions). Of course, and this is Bell's great achievement, it rules out any local deterministic hidden-variable models.

 

[vanhees71]

Quote someone to back up your position. Other than yourself. As requested too many times to count.

Read any experimental paper on Bell tests. There it's described, that the entangled state is prepared and then the measurements are done. It's demonstrated that the correlations are there even when measuring with space-like separated measurement events ("detector clicks"). I think this has been achieved first in the late 1990ies by Zeilinger's group with polarization entangled photons. Today we have several "loophole-free" measurements too.

 

[PeterDonis]

all events pertaining to the Alice/Bob's measurements are causal. And time ordered.

Not for both measurements taken together. For each measurement taken individually, yes. But the whole point is that we are looking at the correlations between the measurement results; that means we have to look at the complete set of events involved in both measurements. And that set includes events that are spacelike separated from each other and therefore do not have an invariant time ordering.

 

[PeterDonis]

Why is not workable?

Because, as Bell explained in his "Bertlmann's socks" paper, that kind of argument implicitly presupposes local hidden variables, and local hidden variables are ruled out if the Bell inequalities are violated.

The preparation does not predetermine which measurements you do afterwards of course

Exactly. So the preparation by itself is not sufficient to account for the actual measurement results. But you have been asserting the contrary.

 

[PeterDonis]

Read any experimental paper on Bell tests.

Referring to the experimental results does not support any particular claim about interpretation, since all interpretations make the same predictions for experimental results. So you are not responding to the request @DrChinese made by referring to the experimental results. We all know the experimental results violate the Bell inequalities, and we all know the math of QFT correctly predicts that. Stating those things as though they supported your particular position is not responsive.

 

[vanhees71]

Indeed, and it has been demonstrated that the correlations are as predicted even when the measurement events at Alice's and Bob's place are space-like separated. Any experimental paper I know takes this as an indication that thus the correlations can NOT be caused by an influence of A's measurement on B's subsystem when measured by him and vice versa. DrChinese knows all these papers very well. We have discussed about them at length for some time in this forum.

 

[DrChinese]

Without realism/determinism, how can there be causal influences?

Well, ask Weinberg. As I repeatedly request, a citation saying something different than what HE says would be helpful. By the lack of presentation of such, which I think we know is not around, one might conclude these are your personal opinions and not backed by scientific consensus.

But to address your question: as best I can: As I said earlier, there is no causal direction that is evidenced by experiment. Nonetheless, experiment shows that anyone of a large number of measurements that can be performed lead to an exact prediction of the result of a similar faraway measurement. So that is the type of influence Weinberg describes. It's not a coincidence, and you can't wave your hand and make it disappear. It can't be predetermined, as Bell showed us (excepting of course the Bohmian solution).

Again, waiting for your suitable quote* to counter Weinberg's well spoken words. (*Doesn't it make you wonder when such a quote cannot be located, and you must say it yourself?)

 

[DrChinese]

Indeed, and it has been demonstrated that the correlations are as predicted even when the measurement events at Alice's and Bob's place are space-like separated. Any experimental paper I know takes this as an indication that thus the correlations can NOT be caused by an influence of A's measurement on B's subsystem when measured by him and vice versa.

No. Everyone else says they are NOT coincidendal correlations, but evidence instead of a mechanism we do not at present understand. Ergo the reason for this thread, and what Weinberg says. Which AGAIN you FAIL to back up with words other than your own. Why cannot you supply the authoritative quote?

And they have a specific name for it: quantum nonlocality and it is generally accepted. QFT has provided no more useful predictions on this than the QT of 1935/1951/1964 etc.

 

[vanhees71]

Do you think, Weinberg claims there are causal actions at a distance? So how do you think is this then compatible with his earlier book on QFT (doubtlessly among the best textbooks in this topic)?

I've only the 1st edition of Weinberg's textbook. There's a Chpt. 12, where Weinberg discusses the physics of Bell-type measurements rather than "interpretational issues", and there he is saying (2nd paragraph on p. 340):

"Of course, Bob's measurement does not change the wave function for the part of the system observed by Alice - it just doesn't change the results of Alice's measurements. If it were possible for Alice to probe this wave function, other than by making measurements, then faster-than-light communication would be possible. As mentioned in Section 3.7, the phenomenon of entanglement thus poses an obstacle to any interpretation of quantum mechanics that attributes to the wave function or the state vector any physical significance other than as a means of predicting the results of measurements."

I prefer to conclude from this, given the fact that there are no better theories around than Q(F)T, that "the wave function of the state vector" (I would generalize this to any quantum state, i.e., any stat. op. representing also mixed states) is just a means of "predicting the results of measurements", i.e., the minimal statistical interpretation. Particularly there's no need for a collapse.

So I'd say, Chpt. 12 in Weinberg's QM book (at least in the 1st edition) puts all this in the right perspective. I'm still (or even more) puzzled by the final conclusion he draws in Sect. 3.7, i.e., that the issue or interpretation is not decided. For me it is in the sense, Weinberg himself gives in the above quote: The quantum state is just "a means of predicting the results of measurements".

 

[Morbert]

I prefer to conclude from this, given the fact that there are no better theories around than Q(F)T, that "the wave function of the state vector" (I would generalize this to any quantum state, i.e., any stat. op. representing also mixed states) is just a means of "predicting the results of measurements"

Yeah this is the issue I attempted to tease out in post #33

 

[vanhees71]

No. Everyone else says they are NOT coincidendal correlations, but evidence instead of a mechanism we do not at present understand. Ergo the reason for this thread, and what Weinberg says. Which AGAIN you FAIL to back up with words other than your own. Why cannot you supply the authoritative quote?

And they have a specific name for it: quantum nonlocality and it is generally accepted. QFT has provided no more useful predictions on this than the QT of 1935/1951/1964 etc.

Indeed, nobody thinks these are coincidental correlations, but everybody knows that the correlations are precisely included in the quantum description of the situation in terms of this entangled state. As I said, it's called nonlocality, but it's not meant to imply nonlocal interactions. Relativistic QFT has implemented this from the very beginning: local observables commute at spacelike separated arguments. It's also true that QFT is so far the only working quantum theory consistent with special relativity. It has been formulated already in 1926 by Jordan in a section of the famous "Dreimännerarbeit".

I quoted in #72 above Weinberg himself in the same book on quantum mechanics!

 

[Morbert]

As an aside, since Griffiths's consistent histories was mentioned previously:
If Alice measures spin x of her particle at time $t_1$ and Bob measures spin z of his particle at time $t_2$, we can extract the right probabilities from the family of four consistent histories $\{C_1,C_2,C_3,C_4\}$
$$\begin{eqnarray}
C_1 &=& \Pi^{\uparrow_x}_{t_1-\delta t}&\otimes&\Pi^{\uparrow_x,A_{\uparrow_x}}_{t_1}&\otimes&\Pi^{\uparrow_z}_{t_2-\delta t}&\otimes&\Pi^{\uparrow_z,B_{\uparrow_z}}_{t_2}\\
C_2 &=&\Pi^{\downarrow_x}_{t_1-\delta t}&\otimes&\Pi^{\downarrow_x,A_{\downarrow_x}}_{t_1}&\otimes&\Pi^{\uparrow_z}_{t_2-\delta t}&\otimes&\Pi^{\uparrow_z,B_{\uparrow_z}}_{t_2}\\
C_3 &=&\Pi^{\uparrow_x}_{t_1-\delta t}&\otimes&\Pi^{\uparrow_x,A_{\uparrow_x}}_{t_1}&\otimes&\Pi^{\downarrow_z}_{t_2-\delta t}&\otimes&\Pi^{\downarrow_z,B_{\downarrow_z}}_{t_2}\\
C_4 &=&\Pi^{\downarrow_x}_{t_1-\delta t}&\otimes&\Pi^{\downarrow_x,A_{\downarrow_x}}_{t_1}&\otimes&\Pi^{\downarrow_z}_{t_2-\delta t}&\otimes&\Pi^{\downarrow_z,B_{\downarrow_z}}_{t_2}
\end{eqnarray}$$
and nothing in these histories implies Alice's measurement impacts Bob's or vice versa.

* Note that I also added projectors for the spins just before measurements (e.g. $\Pi^{\downarrow_x}_{t_1-\delta t}, \Pi^{\downarrow_z}_{t_2-\delta t}$ etc). Griffiths uses these projectors to assert that measurements reveal pre-existing properties, and considers his account to be a realist account.

 

[EPR]

Well, ask Weinberg. As I repeatedly request, a citation saying something different than what HE says would be helpful. By the lack of presentation of such, which I think we know is not around, one might conclude these are your personal opinions and not backed by scientific consensus.

But to address your question: as best I can: As I said earlier, there is no causal direction that is evidenced by experiment. Nonetheless, experiment shows that anyone of a large number of measurements that can be performed lead to an exact prediction of the result of a similar faraway measurement. So that is the type of influence Weinberg describes. It's not a coincidence, and you can't wave your hand and make it disappear. It can't be predetermined, as Bell showed us (excepting of course the Bohmian solution).

Again, waiting for your suitable quote* to counter Weinberg's well spoken words.(*Doesn't it make you wonder when such a quote cannot be located, and you must say it yourself?)

This is why i stick to a minimalist, pragmatic view on these issues. Bohr said to keep assumptions to a minimum - you should do too. Weinberg's issue with entanglement stems from the same misplaced notion that objects in the quantum world must reflect our daily experience. Had he adopted a more pragmatic, working attitude of quantum fields, he wouldn't be as dissatisfied with quantum mechanics interpretations(dealing with single particles and alluding strongly of billiard balls). Most qm interpretations are faulty because of this precisely. Focusing on single particles(billiard balls) as if it is relevant for the larger picture. Fields are better suited and a more correct way to talk about reality. Not particles and qm interpetations of particles. The focus should be on fields, not particles if these conceptual issues are to be resolved.
The world is made up of fields and 'particles' and their interactions(incl. entanglement) are secondary iterations.

You can find experts who conjecture that space and distance are a secondary phenomenon arising from(upon) measurement.

You seem to think you have stumbled upon something Earth shattering that other people do not admit is as Earth shattering as it seems to you. This is because they have long departed from the cozy world you are sticking up for. In this new world, only observations can be considered 'real'(having a definite reality).
And entaglement is the smallest issue. It's explaining the existence of everything and anything that's at stake.

 

[PeterDonis]

Thread closed for moderation.