Undergrad QM objects do not have properties until measured?

[rubi]

And by doing a measurement, Alice can instantly change the wave function at Bob's location, even though they are spacelike-separated. So if a pure quantum state is the complete information about the state of a system (eg. the entangled particles of Alice and Bob), then the quantum formalism is manifestly nonlocal.

Yes, but only if you make this strange attachment of quantum states to points in spacetime. It's like attaching the probability distribution $p_i=\frac{1}{6}$ to every point of space and then after finding that the die shows the number 5, changing it to $p_i = \delta_{i5}$ everywhere. Of course, there is not really a physical object called probability distributions that changes everywhere in the universe as soon as I look at the die. If I don't consider the probability distribution to be a physical object, nothing non-local happens. Of course, if you claim that there is actually a physical object with that property, then it changes non-locally, but why would you do that? Isn't it absurd?

 

[atyy]

Yes, but only if you make this strange attachment of quantum states to points in spacetime. It's like attaching the probability distribution $p_i=\frac{1}{6}$ to every point of space and then after finding that the die shows the number 5, changing it to $p_i = \delta_{i5}$ everywhere. Of course, there is not really a physical object called probability distributions that changes everywhere in the universe as soon as I look at the die. If I don't consider the probability distribution to be a physical object, nothing non-local happens. Of course, if you claim that there is actually a physical object with that property, then it changes non-locally, but why would you do that? Isn't it absurd?

But no prediction of the theory actually changes (no matter how absurd it is). So why would there be any problem?

And it seems that what you proposed for defining a local common cause would work here too. So even though the wave function is real, and eveything is manifestly nonlocal, we have no problem defining a common cause and keeping locality. So this is not an example of giving up realism to preserve locality - we can have locality in your definition, regardless of whether the wave function is real or not.

 

[rubi]

But no prediction of the theory actually changes (no matter how absurd it is). So why would there be any problem?

There is no problem. It's just not very reasonable.

And it seems that what you proposed for defining a local common cause would work here too. So even though the wave function is real, and eveything is manifestly nonlocal, we have no problem defining a common cause. So this is not an example of giving up realism to preserve locality - we can have locality in your definition, regardless of whether the wave function is real or not.

No, you have added an additional physical object to the formalism. The formalism without a real existing object called wave function is local, but since your new formalism contains an additional physical object that evolves non-locally, the new formalism becomes non-local. (Of course, if you add something non-local to a local theory, the new theory will contain non-local elements.)

 

[atyy]

No, you have added an additional physical object to the formalism. The formalism without a real existing object called wave function is local, but since your new formalism contains an additional physical object that evolves non-locally, the new formalism becomes non-local. (Of course, if you add something non-local to a local theory, the new theory will contain non-local elements.)

OK, but then how can the wave function be a "cause" if it is not physical?

 

[rubi]

OK, but then how can the wave function be a "cause" if it is not physical?

The wave function isn't a cause. The events $x$, $y$, $z$ in spacetime are causes (or effects). If you roll a die, then the event in spacetime where you rolled the die is the cause and an effect is an event in spacetime, where the die shows the number 5. The probability distribution $p_i=\frac{1}{6}$ didn't cause anything. It's just a container of information about what could happen.

 

[zonde]

However, one can of course add hidden variables to a non-simplicial theory without making the state space a simplex. One just can't represent them on a single probability space.

Why do you think that we can't represent them on a single probability space given locality?
Outcomes (sample spaces) are the same. So it leaves measurement settings. But if we enforce locality then measurement settings at one end can't have any effect at the other end. So different measurement settings would have to be modeled in the same probability space. What other possible reason do you see why we can't represent them on a single probability space?

 

[atyy]

The wave function isn't a cause. The events $x$, $y$, $z$ in spacetime are causes (or effects). If you roll a die, then the event in spacetime where you rolled the die is the cause and an effect is an event in spacetime, where the die shows the number 5. The probability distribution $p_i=\frac{1}{6}$ didn't cause anything. It's just a container of information about what could happen.

But nothing in your definition would fail if I made the wave function a real physical object.

 

[rubi]

Why do you think that we can't represent them on a single probability space given locality?
Outcomes (sample spaces) are the same. So it leaves measurement settings. But if we enforce locality then measurement settings at one end can't have any effect at the other end. So different measurement settings would have to be modeled in the same probability space. What other possible reason do you see why we can't represent them on a single probability space?

No, this doesn't follow. It's just a hard mathematical fact that non-commuting observables can't be modeled as random variables on one probability space and quantum theory just happens to be a theory with non-commuting observables. It's not the outcomes that need to represented on one probability space. Also the hidden variables need to represented on that space. (Moreover, I don't have the burden of proof. You are the one who claims that locality implies a simplicial state space, so you are the one who has the obligation to prove it.)

But nothing in your definition would fail if I made the wave function a real physical object.

Of course the definition fails, since it can be applied only to objects that are described by quantum theory. Your real wave function is not such an object (it is not represented by an observable on a Hilbert space). It's an additional object, external to the Hilbert space description, so the definition can't be applied to it.

 

[martinbn]

The wave function is always in Hilbert space. If one wants to, one can attach a copy of Hilbert space to every point on a spatial slice of spacetime (won't make any change to the predictions, but will make collapse manifestly nonlocal). So when one is saying that the wave function is real, one regards Hilbert space as real.

Can you explain more? What does this mean?

 

[atyy]

Of course the definition fails, since it can be applied only to objects that are described by quantum theory. Your real wave function is not such an object (it is not represented by an observable on a Hilbert space). It's an additional object, external to the Hilbert space description, so the definition can't be applied to it.

The wave function is still an object in Hilbert space. It's just that there is a copy of Hilbert space at every point in space.

 

[rubi]

The wave function is still an object in Hilbert space. It's just that there is a copy of Hilbert space at every point in space.

But it's not modeled as an observable on Hilbert space. That's what you need in order to get the projections I used in my definition. In a quantum theory, every physical object has a corresponding self-adjoint operator that models it. This is not the case for the "physical object" called wave function. The wave function in your theory of real wave functions is not a quantum object itself.

 

[zonde]

No, this doesn't follow. It's just a hard mathematical fact that non-commuting observables can't be modeled as random variables on one probability space and quantum theory just happens to be a theory with non-commuting observables.

Do I have to understand this as counterexample to my considerations? But whether or not QM is local is the topic of current discussion. So this can't be viewed as counterexample.

(Moreover, I don't have the burden of proof. You are the one who claims that locality implies a simplicial state space, so you are the one who has the obligation to prove it.)

I am not trying to prove anything. I am trying to understand your position.
If you want proof that QM is non-local then tell me and I will continue discussion from your response in post #20.

 

[martinbn]

The wave function is still an object in Hilbert space. It's just that there is a copy of Hilbert space at every point in space.

And? If the spatial slices are \mathbb R^3, then you consider \mathbb R^3\times\mathcal H. Fine, but what do you do with it? By the way is there a particular reason why you write "Hilbert space" without any article.

 

[rubi]

Do I have to understand this as counterexample to my considerations? But whether or not QM is local is the topic of current discussion. So this can't be viewed as counterexample.

Bell proved that classical local theories obey an inequality that is violated. He proved nothing about non-classical theories. We don't have any tool to decide upon the locality of quantum mechanics. A counterexample to your claim that non-simplicial states can't have hidden-variable descriptions is contained in this paper (section 4).

If you want proof that QM is non-local then tell me and I will continue discussion from your response in post #20.

None such proof exists. The book I mentioned in post #20 debunks all of them, be it a probabilistic proof or a proof using relative frequencies. I don't have enough time to discuss why proven mathematical theorems are not false. If you have another opinion, then please publish a paper, then we can discuss it.

 

[atyy]

But it's not modeled as an observable on Hilbert space. That's what you need in order to get the projections I used in my definition. In a quantum theory, every physical object has a corresponding self-adjoint operator that models it. This is not the case for the "physical object" called wave function. The wave function in your theory of real wave functions is not a quantum object itself.

OK. So in your proposal, in the Bell test, you would like to say that the preparation causes the correlations. But that means that the measurement choice of either Alice ore Bob is not a cause of the result?

(Typically, we say Alice's result is caused by the preparation as well as her measurement choice.)

 

[rubi]

OK. So in your proposal, in the Bell test, you would like to say that the preparation causes the correlations. But that means that the measurement choice of either Alice ore Bob is not a cause of the result?

Yes. The correlations are there, independent of whether they are measured or not, so they can't be caused by this choice. However, the measurement causes the particle to change its state after the measurement (be it through collapse or decoherence).

 

[atyy]

Yes. The correlations are there, independent of whether they are measured or not, so they can't be caused by this choice. However, the measurement causes the particle to change its state after the measurement (be it through collapse or decoherence).

Sure that's fine. I know you won't agree, but it basically proves Maudlin right - ones needs to change the idea of what "cause" means.

 

[rubi]

Sure that's fine. I know you won't agree, but it basically proves Maudlin right - ones needs to change the idea of what "cause" means.

I don't change the meaning of what "cause" means. $A$ causes $B$ if $B$ didn't happen without $A$ happening first. That seems to be a pretty classic definition of "cause". What is your definition of "cause"?

(Also independent of whether that's true, it doesn't prove Maudlin right. Maudlin has a mathematical error in his paper. Nothing can ever fix that.)

Edit: By the way, you have the same situation with a die: The act of rolling the die causes the die to yield some number consistent with the probability distribution $p_i=\frac{1}{6}$. The act of looking doesn't cause anything. If I roll the dice carefully, I might increase my chance to throw a 6. Whether this distribution can be calculated from a simplicial state space or not is unimportant. In fact, since we live in a quantum world, it can't. Yet, everyone would agree that the act of rolling the die caused the result.

 

[atyy]

I don't change the meaning of what "cause" means. $A$ causes $B$ if $B$ didn't happen without $A$ happening first. That seems to be a pretty classic definition of "cause". What is your definition of "cause"?

Edit: By the way, you have the same situation with a die: The act of rolling the die causes the die to yield some number consistent with the probability distribution $p_i=\frac{1}{6}$. The act of looking doesn't cause anything. If I roll the dice carefully, I might increase my chance to throw a 6. Whether this distribution can be calculated from a simplicial state space or not is unimportant. In fact, since we live in a quantum world, it can't. Yet, everyone would agree that the act of rolling the die caused the result.

It's fine, just different from the notion of cause used in Bell's theorem. I don't think it has much to do with quantum mechanics, because the preparation, and the measurement outcomes are all described by classical probability.

Cavalcanti and Lal have some comments on this approach http://arxiv.org/abs/1311.6852 (p11):
"Another way of dropping FP while keeping PCC would be to point out that correlations do not need to be explained in terms of a factorisability condition, but that the quantum state of the joint system in its causal past can itself be considered as the common cause of the correlations. An objection to this point of view, however, is that the precise correlations cannot be determined without knowledge of the measurements to be performed (the inputs x and y in Fig. 1), and these may be determined by factors not in the common past of the correlated events. A similar criticism may be made of the L-S approach. However, an advantage of the latter is that it does give an analogue of the factorisation condition (rather than simply dropping it), and thus could allow for a generalisation of Reichenbach’s Principle of Common Cause in understanding the implication of causal structure for probabilistic correlations, and be of potential application in areas such as causal discovery algorithms."

 

[N88]

… EPR arrives at this partial naive realism based explanation in EPR under condition of locality ["without in any way disturbing a system"]. Obviously rejecting "locality" renders EPR reasoning inapplicable (without rejecting realism).

On the other hand realism (in it's proper philosophical sense) can't be rejected if we hold on to scientific approach, as realism (in it's proper philosophical sense) is fundamental to science. Or more specifically science aims to explain reproducible certainty. And we favor such explanations over other types of explanations. So we (should) favor non-local explanation of reproducible certainty over local non-explanation of reproducible certainty. (Emphasis added.)

1. How do you define realism in its proper philosophical sense?
2. In your terms, but seeking greater generality, how about "science aims to explain reality"?

 

[zonde]

1. How do you define realism in its proper philosophical sense?

Describe not define.
Short description is that visible world has mind-independent existence.

2. In your terms, but seeking greater generality, how about "science aims to explain reality"?

No. You explain something in terms of something else. Explanations tie together different descriptions. Reality is very general term, so with what you would tie description of reality?

P.S. Pure philosophy is forbidden topic on PF. So if you have some questions about philosophy of science related to the topic keep it short and close to the topic.

 

[N88]

… On the other hand realism (in it's proper philosophical sense) can't be rejected if we hold on to scientific approach, as realism (in it's proper philosophical sense) is fundamental to science. ...

OK. I am here to study science, not philosophy. In scientific terms, and given your support for nonlocality, what do you mean by these phrases:
1. The scientific approach.
2. On the other hand realism (in it's proper philosophical sense) can't be rejected if we hold on to scientific approach.
3. Realism (in it's proper philosophical sense) is fundamental to science.

 

[zonde]

1. The scientific approach.

We use scientific method to reject some hypotheses and favor (accept for now) other hypotheses.

2. On the other hand realism (in it's proper philosophical sense) can't be rejected if we hold on to scientific approach.

To justify scientific method we have to assume realism.

3. Realism (in it's proper philosophical sense) is fundamental to science.

Scientific method is fundamental to science and therefore assumptions that justify scientific method are fundamental too.
https://en.wikipedia.org/wiki/Philosophy_of_science#Axiomatic_assumptions

 

[morrobay]

Nobody (I hope) is considering dropping locality as there is no philosophical framework for such a way of thinking. "Non-locality" of QM just means that QM approximates some physical mechanism that violates speed of light limit.

In terms of experimental results what differentiates between the " non locality" of QM approximating some physical mechanism violating the speed of light and particles not having definite properties before measurement ?
In other words are some describing non realism as non locality ?

 

[zonde]

In terms of experimental results what differentiates between the " non locality" of QM approximating some physical mechanism violating the speed of light and particles not having definite properties before measurement ?

These two things are not strictly related. It's because particles not having definite (certain) properties does not mean that local measurements can't have definite outcomes.
Let me give an analogy. We have a glass and we ask, does that glass have a property of being full or not after we pour some amount of water into it? Obviously glass does not have such a property because whether the glass will be full or not depends on the amount of water we are pouring into it. But if we can somehow fix the amount of water then the result will become certain.
So properties can be contextual and therefore not definite in absolute sense.

 

[Neandethal00]

QM objects do not have properties until measured?

Another way of saying this is
To a quantum particle nothing exists until it interacts with another 'object'.
From the 'object' point of view: a quantum particle doesn't exist until it interacts with the 'object'.

 

[morrobay]

These two things are not strictly related. It's because particles not having definite (certain) properties does not mean that local measurements can't have definite outcomes.
Let me give an analogy. We have a glass and we ask, does that glass have a property of being full or not after we pour some amount of water into it? Obviously glass does not have such a property because whether the glass will be full or not depends on the amount of water we are pouring into it. But if we can somehow fix the amount of water then the result will become certain.
So properties can be contextual and therefore not definite in absolute sense.

http://arxiv.org/pdf/quant-ph/0209123v2.pdf
See page 50.
5.2 locality and counterfactuality.
... while for others quantum non - locality is an artifact created by the introduction into QM notions which are foreign to it,
typically the EPR elements of reality.

 

[N88]

http://arxiv.org/pdf/quant-ph/0209123v2.pdf
See page 50.
5.2 locality and counterfactuality.
... while for others quantum non - locality is an artifact created by the introduction into QM notions which are foreign to it,
typically the EPR elements of reality.

From page 32: "A general way to express the Bell theorem in logical terms is to state that the following system of three assumptions (which could be called the EPR assumptions) is self-contradictory:
1. validity of their notion of “elements of reality”
2. locality
3. the predictions of quantum mechanics are always correct.
The Bell theorem then becomes a useful tool to build a “reductio ad absurdum” reasoning: it shows that, among all three assumptions, one (at least) has to be given up."

Can there be any doubt that #1 must be given up? And ONLY #1?

NB: #1 is the classicality assumption associated with EPR, Bell, d'Espagnat (see post #44 above) and was known to be false from the early days of QM.

 

[zonde]

http://arxiv.org/pdf/quant-ph/0209123v2.pdf
See page 50.
5.2 locality and counterfactuality.
... while for others quantum non - locality is an artifact created by the introduction into QM notions which are foreign to it,
typically the EPR elements of reality.

Yes, wording of EPR definition seems to exclude contextual HV while they are certainly realistic. But important question is whether quantum non-locality does not follow any more if we fix this problem and take contextual HV into consideration. And the thing is that Bell's inequalities are the same for contextual HV (Bell's $\lambda$ does not differentiate between the two). So it changes nothing in an idealized situation.

But there is difference for interpretation of experimental results. Prior to loophole free Bell inequality tests this distinction was important because fair sampling assumption (that opens detection loophole) applies to non-contextual HV but doesn't apply to contextual HV. So earlier Weihs experiment with fast switching polarization analyzers already excluded local non-contextual HV as a possible explanation.

 

[N88]

QM objects do not have properties until measured?

Another way of saying this is
To a quantum particle nothing exists until it interacts with another 'object'.
From the 'object' point of view: a quantum particle doesn't exist until it interacts with the 'object'.

Note that the opening sentence is a question, so another way to say it would also be a question. From the discussion so far, with its emphasis on Bell's theorem, I believe we can say this:
In Bell-tests, the correlated particles in a pair may have properties like opposite charge and identical spin, but they do not have an EPR element of physical reality as a property until measured.