Spooky action at a distance and various interpretations of QT

[J O Linton]

TL;DR Summary

By analysing a simple experiment involving entangled photons I wish to explore which, if any, of the various interpretations of QT violate locality (i.e. involve spooky action at a distance)

By analysing a simple experiment involving entangled photons I wish to explore which, if any, of the various interpretations of QT violate locality (i.e. involve spooky action at a distance)
Consider the following idealised scenario: A source S produces entangled pairs of photons A and B such that the plane of polarisation is random but the two photons are at right angles. Photon A impinges on a vertical polaoid P1 placed 1m from the source, a short distance behind which is a detector D1. Photon B impinges on a similar vertical polaroid P2 placed 2 m from the source behind which is a second detector D2. We are interested in the times and circumstances under which both detectors respond together.
I believe that I am right in saying that if the photons were purely classical (i.e. not entangled) both detectors would fire 12½% of the time but since the two photons are entangled and their planes of polarisation are at right angles, QT predicts that the two detectors will never fire together.
On a very simplistic interpretation of QT this effect may be explained as follows: When the two photons leave the source they are in a superposition of an infinite number of states with their polarisation planes in all possible directions (but always at right angles to each other); when photon A reaches P1 a 'measurement' of its plane of polarisation is made and the wavefunction partially collapses into just two possible states with equal propbability: in one, photon A passes through P1 with it plane of polarisation vertical while photon B is absorbed because its plane of polarisation is horizontal; in the other, photon A is absorbed and photon B passes through P2. Later when the detectors are examined, the wavefunction collapses still further into one or other of these possibilities.
To my mind, this interpretation does not violate locality because all that happens when photon A passes through or is absorbed by P1 is that a large number of alternative possibilities are simply ruled out. No information is passed from A to B.
First, is my analysis of the situation correct and second, how would the experiment be explained by an adherent of the Many Worlds interpretation or indeed any other popular interpretation of QT?

 

[PeterDonis]

By analysing a simple experiment involving entangled photons I wish to explore which, if any, of the various interpretations of QT violate locality (i.e. involve spooky action at a distance)

First you need to define what you mean by "violate locality". If you mean "violate the Bell inequalities" then all interpretations of QM violate locality because QM itself does.

 

[Christian Thom]

I agree with you, but with the mechanism you propose, the measurement you make changes the reality. In fact, reality is determined by the measurement you make and that is the reason Bell's theorem doesn't apply.

 

[J O Linton]

I had hoped to avoid all mention of Bell's inequality despite the apparent similarity in the experimental setup. I am not trying to assert or deny the existence of 'hidden variables'. I am trying to focus on locality - a property which depends on your metaphysical interpretation of QT and is quite independent of whether or not QT violates Bells inequality.

 

[PeterDonis]

I am trying to focus on locality

And in order to do that, you have to tell us what you mean by that term. Anyone with even a passing familiarity with the literature in this area should know that the term "locality" has at least as many different definitions as there are quantum physicists. Given the many different definitions of this term, you cannot use it as though it had a single accepted meaning. You have to tell us what you mean by it.

 

[PeterDonis]

reality is determined by the measurement you make

This statement, to the extent that it is meaningful at all, is only true for certain QM interpretations.

 

[Christian Thom]

I had hoped to avoid all mention of Bell's inequality despite the apparent similarity in the experimental setup. I am not trying to assert or deny the existence of 'hidden variables'. I am trying to focus on locality - a property which depends on your metaphysical interpretation of QT and is quite independent of whether or not QT violates Bells inequality.

Precisely, I just wanted to emphasize that it is a way to reconcile locality and the fact that Bell's inequalities are violated, but at the cost of admitting that reality is determined by measurements.

 

[PeterDonis]

I just wanted to emphasize that it is a way to reconcile locality and the fact that Bell's inequalities are violated

For one definition of "locality", yes. But not for others. As I have already pointed out, one definition of "locality" is simply "the Bell inequalities are not violated", so on this definition any violation of the Bell inequalities is a violation of locality.

 

[Christian Thom]

For one definition of "locality", yes. But not for others. As I have already pointed out, one definition of "locality" is simply "the Bell inequalities are not violated", so on this definition any violation of the Bell inequalities is a violation of locality.

I would say that for me locality is that there is a place and a time where the state of the system is determined.

 

[PeterDonis]

I would say that for me locality is that there is a place and a time where the state of the system is determined.

It seems impossible for any spatially extended entangled quantum system (of which the system described in the OP is an example) to satisfy this definition, since in an entangled system, the individual subsystems have no definite state by themselves, only the full system does, and the full system does not have a single location.

 

[J O Linton]

I agree with you both that 'locality' is not well defined - but then it is a metaphysical concept, not a physical one. What I have in mind when I speak of a violation of locality is what Einstein would have objected to - namely the transfer of information instantaneously (or at superluminal speed) from one place to another. I have also tried to suggest that the 'simplistic' interpretation of QT does not violate this concept of 'locality' because it involves the removal of possible states/outcomes/worlds etc rather than the transfer of information.

What I would like to know is how adherents of other interpretations of QT would explain the results of the experiment I have described and whether their explanations would or would not require some form of superluminal communication between the two photons.

 

[Christian Thom]

It seems impossible for any spatially extended entangled quantum system (of which the system described in the OP is an example) to satisfy this definition, since in an entangled system, the individual subsystems have no definite state by themselves, only the full system does, and the full system does not have a single location.

Locality means precisely (for me) that there is a time where it has. For example the interaction having produced the particles. After that, the state(s) evolve deterministically following the Schrödinger equation and may spread out largely, but is determined.

 

[PeterDonis]

I agree with you both that 'locality' is not well defined

That's not what I said. I said there are different possible definitions, so you need to tell us which one you are using. "Not well defined" implies that there is no possible definition, which is not the case.

then it is a metaphysical concept, not a physical one

The different "locality" definitions in the literature are all physical concepts; they're just different physical concepts.

What I have in mind when I speak of a violation of locality is what Einstein would have objected to - namely the transfer of information instantaneously (or at superluminal speed) from one place to another.

That just shifts the problem to what your definition of "information" is; there are different definitions in the literature, so you need to tell us which one you are using. (And no, that does not mean "information" is not well defined, any more than "locality".)

 

[Christian Thom]

I agree with you both that 'locality' is not well defined - but then it is a metaphysical concept, not a physical one. What I have in mind when I speak of a violation of locality is what Einstein would have objected to - namely the transfer of information instantaneously (or at superluminal speed) from one place to another. I have also tried to suggest that the 'simplistic' interpretation of QT does not violate this concept of 'locality' because it involves the removal of possible states/outcomes/worlds etc rather than the transfer of information.

What I would like to know is how adherents of other interpretations of QT would explain the results of the experiment I have described and whether their explanations would or would not require some form of superluminal communication between the two photons.

If I may answer in their place, I think they would evoke the collapse of the common wave-function of the two photons; it seems to me that is the standard answer to this situation.

 

[PeterDonis]

What I would like to know is how adherents of other interpretations of QT would explain the results of the experiment I have described

If this is the question you want answered, you can just ask it, without any of the other baggage about ambiguous terms like "locality" and "information". (Some QM interpretations do not even use those terms at all.)

and whether their explanations would or would not require some form of superluminal communication between the two photons.

"Superluminal communication" as a term has the same issue as "information", since it means communicating information faster than light.

 

[PeterDonis]

Locality means precisely (for me) that there is a time where it has.

So as long as the photons originally came from a common source at a single location, they satisfy "locality"? This is a clear definition, but it's also useless, since it says nothing whatever about what happens to the photons after they separate spatially, and that's where the issue is.

 

[Christian Thom]

So as long as the photons originally came from a common source at a single location, they satisfy "locality"? This is a clear definition, but it's also useless, since it says nothing whatever about what happens to the photons after they separate spatially, and that's where the issue is.

As I said : "After that, the state(s) evolve deterministically following the Schrödinger equation and may spread out largely, but is determined."

 

[PeterDonis]

all that happens when photon A passes through or is absorbed by P1 is that a large number of alternative possibilities are simply ruled out.

This looks like an intepretation in which the quantum state (or wave function) represents our knowledge about the system, rather than the system itself; ruling out possibilities is something that happens in our mental model.

how would the experiment be explained by an adherent of the Many Worlds interpretation

In the MWI, all possible results happen for both measurements; the entanglement between the photons just determines which results coexist in each world. So there would be a world in which photon A passes through P1 and is detected at D1, and photon 2 is absorbed at P2; and there would be a world in which photon A is absorbed at P1, and photon 2 passes through P2 and is detected at D2.

 

[PeterDonis]

As I said : "After that, the state(s) evolve deterministically following the Schrödinger equation and may spread out largely, but is determined."

If you adhere to this strictly, you end up with the many worlds interpretation, in which there is never any wave function collapse at all, it's just unitary evolution (which is indeed deterministic) all the time. Is that the position you are taking? That only the MWI is consistent with your definition of "locality"?

 

[PeterDonis]

I think they would evoke the collapse of the common wave-function of the two photons

Not in the many worlds interpretation, since there is no collapse in that interpretation.

 

[Christian Thom]

If you adhere to this strictly, you end up with the many worlds interpretation, in which there is never any wave function collapse at all, it's just unitary evolution (which is indeed deterministic) all the time. Is that the position you are taking? That only the MWI is consistent with your definition of "locality"?

Not at all. I should have added "until another interaction happens" interaction (or measurement) which in fact determines the reality as proposed in this thread.

 

[J O Linton]

I accept that it is important to define what we are talking about and that terms like 'locality' and 'information' can be defined in different ways so let us talk about SAAAD (Spooky action at a distance) which is quite deliberately left a bit vague. From what you have both said I am inclined to believe that both my 'simplistic' interpretation of QT and the MWI avoid having to postulate anything like SAAAD because each state of the system (or 'world') already contains all the information needed to determine its subsequent evolution.
On the other hand an interpretation such as the Pilot Wave interpretation quite explicitly uses what I have called SAAAD. What language would an adherent of the Pilot Wave Interpretation use to explain my experiment?

 

[PeterDonis]

I should have added "until another interaction happens"

At which point the system is spatially extended so your definition of "locality" doesn't apply to the system as a whole any more; it only applies to individual subsystems.

 

[martinbn]

I agree with you both that 'locality' is not well defined - but then it is a metaphysical concept, not a physical one. What I have in mind when I speak of a violation of locality is what Einstein would have objected to - namely the transfer of information instantaneously (or at superluminal speed) from one place to another. I have also tried to suggest that the 'simplistic' interpretation of QT does not violate this concept of 'locality' because it involves the removal of possible states/outcomes/worlds etc rather than the transfer of information.

What I would like to know is how adherents of other interpretations of QT would explain the results of the experiment I have described and whether their explanations would or would not require some form of superluminal communication between the two photons.

This might be of interest to you.
https://en.wikipedia.org/wiki/No-communication_theorem

 

[PeterDonis]

let us talk about SAAAD (Spooky action at a distance) which is quite deliberately left a bit vague

If it's vague I don't see the point of talking about it.

From what you have both said I am inclined to believe that both my 'simplistic' interpretation of QT and the MWI avoid having to postulate anything like SAAAD because each state of the system (or 'world') already contains all the information needed to determine its subsequent evolution.

In the MWI this is true. However, I don't see how it's true in your "simplistic" interpretation because in that interpretation the "state" does not determine anything; it's just our knowledge about the system. The state changes when a measurement is made because our knowledge about the system changes, not because the state has determined any evolution of anything.

 

[PeterDonis]

On the other hand an interpretation such as the Pilot Wave interpretation quite explicitly uses what I have called SAAAD.

If you can say which interpretations use SAAAD and which don't, it isn't vague.

What language would an adherent of the Pilot Wave Interpretation use to explain my experiment?

They would say that the pilot wave enforces the perfect anti-correlation between the measurements on the two photons by connecting the two measurements.

 

[Christian Thom]

At which point the system is spatially extended so your definition of "locality" doesn't apply to the system as a whole any more; it only applies to individual subsystems.

That's true, because at that point the entanglement is broken.

 

[PeterDonis]

at that point the entanglement is broken.

On certain interpretations, yes. But not, for example, in the MWI.

 

[Christian Thom]

On certain interpretations, yes. But not, for example, in the MWI.

I place myself in the interpretation of this thread, not MWI.

 

[PeterDonis]

I place myself in the interpretation of this thread, not MWI.

The OP was not just asking about his "simplistic" interpretation. He was asking how other interpretations explain the experiment he described.

 

[Christian Thom]

The OP was not just asking about his "simplistic" interpretation. He was asking how other interpretations explain the experiment he described.

Yes, you are right, sorry.

 

[J O Linton]

In #28 Peter Donis implied that in the MWI the entanglement was not broken by the passage of photon A through P1. Can you explain what you meant by this? As I understand the MWI when the photon is emitted there are an infinite number of possible or parallel worlds in which the plane of polarisation takes all possible values. These worlds are 'coherent' and can potentially 'interfere' with each other. I am interested to know what happens to the world in which the photon is at, say, 30 degress to the vertical when A passes through P1. Does the photon in this world suddenly become vertical? Or is it that this world cesaes to exist is some sense? If the former we end up with an infinite number of identical worlds; if the latter, what is the difference between this and wavefunction collapse?

 

[Nugatory]

As I understand the MWI when the photon is emitted there are an infinite number of possible or parallel worlds in which the plane of polarisation takes all possible values.

That is not right, and may be the source of your confusion here. When the photon is emitted there is one world in which the plane of polarization has no value. When the polarization is measured this world splits into two, one in which the photon passed through the polarizer and one in which it didn’t. In the world in which the photon passed through the polarizer, the entangled partner will not pass through a polarizer at the same angle if we try.

I am interested to know what happens to the world in which the photon is at, say, 30 degress to the verticalwhen A passes through P1.

You are reasoning as if the polarization has some definite value even though it hasn’t been measured. That doesn’t work in any interpretation.

 

[J O Linton]

Thanks for this. Your answer makes a lot of sense. Clearly, however, the MWI adopts a different definition of what I consider a 'world' to be (i.e. an entity in which objects exist which have unique and measurable properties.)

You also say that there is no interpretation in which the polarisation has a definite value before it has been measured. I was under the impression that the Pilot Wave interpretation assumed that all the entities involved had definite properties at all times, but that their behaviour was in some way 'guided' by the pilot wave. More specifically, a 30 degree photon encountering a vertical polaroid would either pass or not according to the usual cos^2 rule and that the pilot wave would 'inform' photon B what had happened to A.

 

[PeterDonis]

These worlds are 'coherent' and can potentially 'interfere' with each other.

The "worlds" in the MWI cannot interfere with each other; splitting into multiple "worlds" only occurs after a measurement has been made and decoherence has occurred. The "worlds" are the different decoherent branches of the wave function and cannot interfere.