Undergrad Is quantum collapse an interpretation?

[zonde]

A have seen here on forum statements that quantum collapse is interpretation dependent, for example:

Collapse isn't a phenomenon, it's an interpretation. There are many interpretations of QM, and not all of them have collapse.

So I would like to ask for explanation that does not relay on quantum collapse for this phenomena:
We have unpolarized beam of light. It goes trough two orthogonally oriented polarizers. Assuming idealized polarizers we detect no light after second polarizer. Then we insert third polarizer oriented at 45 degrees between the first two. Now we observe 1/8 of the intensity of original beam of light.

How such experiment is treated in no collapse interpretations?

 

[Demystifier]

In the minimal interpretation, you just change the wording. Instead of saying "collapse" you say "update of information". Everything else remains the same as in collapse interpretation.

 

[zonde]

In the minimal interpretation, you just change the wording. Instead of saying "collapse" you say "update of information". Everything else remains the same as in collapse interpretation.

But certainly "update of [experimenter's] information" can not explain this experiment as something different has to happen with beam of light between two setups as we observe different results.

 

[tom.stoer]

In Everett's Interpretation we accept that the system including the measuring device, the laboratory, the observer and the environment really is in a state consisting of classically incompatible "branches" which are mutually "invisible". The mathematical description i.e. the state vector with superposition of components (called "branches", "worlds", ...) results from the unitary time evolution w/o collapse; the "invisibility" follows from decoherence. So we simply accept this prediction which fits to our observations - instead of introducing an ad-hoc collapse via an additional postulate. That means we believe that the state vector and its unitary time evolution describe the structure and th dynamics of the "real system out there" and does not only encode information we have access to and which we have to adjust based on measurement outcomes.

Without this realistic philosophical position there is no need to believe in Everett's Interpretation.

How such experiment is treated in no collapse interpretations?

Mathematically it's standard quantum mechanics w/o ever referring to collaps, projection etc. Born's rule must follow as a theorem from other axioms; we do have indications that this works, but afaik there is no consensus on this issue.

 

[Demystifier]

But certainly "update of [experimenter's] information" can not explain this experiment as something different has to happen with beam of light between two setups as we observe different results.

That's correct, the minimal interpretation does not answer such questions. That's why it is called minimal.

One of non-minimal interpretations without collapse is the Bohmian interpretation. To see how it explains phenomena like the one you described in the first post, see
https://arxiv.org/abs/1305.1280

 

[vanhees71]

But certainly "update of [experimenter's] information" can not explain this experiment as something different has to happen with beam of light between two setups as we observe different results.

It's indeed very simple. An ideal polarizer is realizing an ideal von Neumann filter measurement, i.e., you prepare photons (or if you experiment with usual light, coherent states of the em. field) with a determined polarization. You just through away the unwanted stuff, not polarized in the direction given by the polarizer. You don't need to assume an instantaneous collapse, it's just a local (!) interaction between the em. radiation and the polarizer. The minimal interpretation just doesn't make the unnecessary assumption of an esoteric mechanism that "collapses" something instantaneous in entire space(time)!

 

[vanhees71]

That's correct, the minimal interpretation does not answer such questions. That's why it is called minimal.

One of non-minimal interpretations without collapse is the Bohmian interpretation. To see how it explains phenomena like the one you described in the first post, see
https://arxiv.org/abs/1305.1280

Well, as to be expected it's about non-relativistic particles. I'm not aware that there is a satisfactory BM treatment of photons. Note that photons don't even have a position observable to begin with. So how can BM with its preference for trajectories in position space ever describe photons?

 

[Demystifier]

Note that photons don't even have a position observable to begin with.

They do, it's just not Lorentz covariant. But that's not a problem for instrumental approaches to QM, because you can always define the position observable with respect to the Lorentz frame in which the detector is at rest. After all, the photon detector determines the photon position, doesn't it?

So how can BM with its preference for trajectories in position space ever describe photons?

There are many ways to do it, if you are ready to postulate a preferred Lorentz frame in a manner which does not contradict existing experiments.

 

[zonde]

In Everett's Interpretation we accept that the system including the measuring device, the laboratory, the observer and the environment really is in a state consisting of classically incompatible "branches" which are mutually "invisible". The mathematical description i.e. the state vector with superposition of components (called "branches", "worlds", ...) results from the unitary time evolution w/o collapse; the "invisibility" follows from decoherence. So we simply accept this prediction which fits to our observations - instead of introducing an ad-hoc collapse via an additional postulate. That means we believe that the state vector and its unitary time evolution describe the structure and th dynamics of the "real system out there" and does not only encode information we have access to and which we have to adjust based on measurement outcomes.

Thanks, for your explanation.
I would like to ask, at what point do you place "decoherence"? Does it happens in polarizers or in detector?

Mathematically it's standard quantum mechanics w/o ever referring to collaps, projection etc. Born's rule must follow as a theorem from other axioms; we do have indications that this works, but afaik there is no consensus on this issue.

Are you not mixing up collapse with Born's rule? Collapse "happens" at polarizers while Born's rule is applied at the end (at detector). Isn't this so?

 

[vanhees71]

They do, it's just not Lorentz covariant. But that's not a problem for instrumental approaches to QM, because you can always define the position observable with respect to the Lorentz frame in which the detector is at rest. After all, the photon detector determines the photon position, doesn't it?There are many ways to do it, if you are ready to postulate a preferred Lorentz frame in a manner which does not contradict existing experiments.

The photon detector defines the position of a local interaction between the photon and the detector material. Indeed you can live with a preferred frame as far as it is determined by the physical situation, and the detector-rest frame makes sense.

 

[zonde]

It's indeed very simple. An ideal polarizer is realizing an ideal von Neumann filter measurement, i.e., you prepare photons (or if you experiment with usual light, coherent states of the em. field) with a determined polarization. You just through away the unwanted stuff, not polarized in the direction given by the polarizer. You don't need to assume an instantaneous collapse, it's just a local (!) interaction between the em. radiation and the polarizer. The minimal interpretation just doesn't make the unnecessary assumption of an esoteric mechanism that "collapses" something instantaneous in entire space(time)!

So your approach is to ditch particles (photons) up until detection. Well, I suppose it works for my example.
But if I replace photons with gold atoms and polarizers with SG apparatuses? What about this modified experiment?

 

[vanhees71]

I don't understand what you mean. Of course, I have an incoming photon, prepared in its state. Then it's interacting with the polarizer, either being absorbed or let through (with probabilities given by Born's rule for the prepared state). Those photons that come through are polarized accordingly to the polarizer's orientation. That's what a polarizer does after all!

There's no fundamental difference between this example and the SG aparatus. The only difference is that the SG apparatus doesn't filter (except you put some blocking material in some of the partial beams) but entangles position with the spin component in direction of the magnetic field. This is even described by unitary time evolution of QT since it's not an "open system" from the point of view of the gold atoms as in the case of the photons, where you have treated the polarizer effectively.

 

[zonde]

I don't understand what you mean. Of course, I have an incoming photon, prepared in its state. Then it's interacting with the polarizer, either being absorbed or let through (with probabilities given by Born's rule for the prepared state). Those photons that come through are polarized accordingly to the polarizer's orientation. That's what a polarizer does after all!

Hmm, probably I misunderstood you.
But then the idea of collapse is that polarization of photon "collapses" from it's initial polarization to the new polarization state according to polarizer's orientation if it goes through the polarizer. Isn't this the same what you are saying?

 

[vanhees71]

I don't know, what you understand by the term "collapse". Usually, it's understood that the quantum state after a measurement goes instantaneously into a corresponding eigenstate of the operator representing the measured observable. This process is clearly outside of the quantum dynamics based on local interactions (in relativistic QFT), and it violates Einstein causality. However, if you look more closely at it, this assumption is simply not needed and by nothing justified from the formalism and its application to the desription of real-world observations and experiments. That's why I am a proponent of the minimal statistical interpretation, which is practically just the flavor of the class of Copenhagen interpretations without a collapse assumption. I think, it's pretty close to what Bohr meant, although I cannot say with certainty what he really thought, because his papers are pretty enigmatic (too much philosophy for my taste). The same holds for Heisenberg's writing. I prefer Dirac and Pauli, which give a clear picture of quantum theory without philosophical additions that obscure the scientific content of the theory!

 

[martinbn]

@vanhees71 : But going from a superposition of vertical and horisontal polarization to let's say vertical, is not unitary. It is a projection. How do you explain things without it.

 

[vanhees71]

That's exactly what I said. The reason is that we treat the polarizer effectively, i.e., not as part of the quantum dynamics.

 

[martinbn]

That's exactly what I said. The reason is that we treat the polarizer effectively, i.e., not as part of the quantum dynamics.

That only pushes the projections further down the chain. The question, the way I understood it, was can you have an interpretation that never needs to use a projection (or something equivalent)?

 

[vanhees71]

I don't think so, because we "project" all the time by sorting out the states we want.

 

[zonde]

I don't know, what you understand by the term "collapse". Usually, it's understood that the quantum state after a measurement goes instantaneously into a corresponding eigenstate of the operator representing the measured observable.

Yes, this is my understanding. Did I said something different?

This process is clearly outside of the quantum dynamics based on local interactions (in relativistic QFT), and it violates Einstein causality.

It's local in my example. Well, if you consider EPR experiment then yes, it's non-local.

However, if you look more closely at it, this assumption is simply not needed and by nothing justified from the formalism and its application to the desription of real-world observations and experiments.

You have my simple example. So show how do you treat it without collapse.

The reason is that we treat the polarizer effectively, i.e., not as part of the quantum dynamics.

Basically you are saying that "collapse" is emergent from more complete unitary dynamics. But that does not change the fact that it represents real physical phenomena.

 

[vanhees71]

If you have a collapse, it's never local, because it claims that the quantum state changes instantaneously, in the extreme case from a mixed to a pure state. That's outside the quantum dynamics based on local interactions.

Which real physical phenomena does the collapse represent? I don't know a single example!

 

[zonde]

Which real physical phenomena does the collapse represent? I don't know a single example!

Surely you know that photon beam interacts with polarizer. And I suppose that you consider it real physical phenomena.
So you don't think that collapse prepresents this phenomena, right?

 

[Mentz114]

Surely you know that photon beam interacts with polarizer. And I suppose that you consider it real physical phenomena.
So you don't think that collapse prepresents this phenomena, right?

Surely you can call it what you like ? We know it is a projective measurement between light and polarizer and the physics is well understod. As with SG apparatus.
You think it should be part of the unitary evolution but there is no reason to expect that.

 

[vanhees71]

Surely you know that photon beam interacts with polarizer. And I suppose that you consider it real physical phenomena.
So you don't think that collapse prepresents this phenomena, right?

No, the local interaction between between the em. field and the matter in the polarizer represent this phenomenon, according to QED. I don't need a collapse to describe it.

 

[vanhees71]

Surely you can call it what you like ? We know it is a projective measurement between light and polarizer and the physics is well understod. As with SG apparatus.
You think it should be part of the unitary evolution but there is no reason to expect that.

On a microscopic level, it is part of the unitary evolution, but it is completely sufficient to describe the interaction of the photon with the polarizer effectively with the polarizer being described classically. Then the polarizer is reduced to a projection operator to the polarization state given by its orientation.

This effective description is of course not given by unitary S-matrix since we don't resolve the microscopic interaction between the photon and the polarizer.

 

[Mentz114]

On a microscopic level, it is part of the unitary evolution, but it is completely sufficient to describe the interaction of the photon with the polarizer effectively with the polarizer being described classically. Then the polarizer is reduced to a projection operator to the polarization state given by its orientation.

This effective description is of course not given by unitary S-matrix since we don't resolve the microscopic interaction between the photon and the polarizer.

The part I've emphasized is a source of controversy, is it not ?

If a measurement has to extract /add information from/to a system then it cannot avoid changing the state of some part in a non-unitary (dissipative) way. This is implied by thermodynamics which seems to apply at macroscopic and microscopic scales.

 

[martinbn]

I don't think so, because we "project" all the time by sorting out the states we want.

So you think that you cannot have an interpretation without projection and at the same time you are convinced that you can have one without collapse. Aren't these two the same?

 

[mikeyork]

But certainly "update of [experimenter's] information" can not explain this experiment as something different has to happen with beam of light between two setups as we observe different results.

"Update of information" does not require an experimenter. It requires only interaction for information to be updated in the knowable (not necessarily known) universe. Once information exists in the knowable universe it cannot be contradicted by experiment. This is a basic assumption of science.

 

[tom.stoer]

Are you not mixing up collapse with Born's rule? Collapse "happens" at polarizers while Born's rule is applied at the end (at detector). Isn't this so?

The collapse happens at no specific point in space. The quantum state does neither depend on space, nor does it live in space; it lives in an abstract Hilbert space.

The collapse simply happens whenever and wherever it seems appropriate to project the full quantum state - after unitary time evolution - to some subspace selected via observation. If you measure an eigenvalue then you project the full state to the corresponding subspace. So the collapse happens in your mind when you readjust the information you have.

The collapse is nothing real; it cannot be real in the same sense as the unitary time evolution b/c it contradicts this unitary time evolution.

So the consequence is either to use the collapse as a tool for calculations giving up a realistic interpretation of the quantum state, or to insist on a realistic interpretation of the quantum state and giving up the idea of a collapse.

 

[atyy]

The collapse happens at no specific point in space. The quantum state does neither depend on space, nor does it live in space; it lives in an abstract Hilbert space.

The collapse simply happens whenever and wherever it seems appropriate to project the full quantum state - after unitary time evolution - to some subspace selected via observation. If you measure an eigenvalue then you project the full state to the corresponding subspace. So the collapse happens in your mind when you readjust the information you have.

The collapse is nothing real; it cannot be real in the same sense as the unitary time evolution b/c it contradicts this unitary time evolution.

So the consequence is either to use the collapse as a tool for calculations giving up a realistic interpretation of the quantum state, or to insist on a realistic interpretation of the quantum state and giving up the idea of a collapse.

The unitary time evolution occurs after the collapse. So neither collapse nor unitary time evolution are real.

 

[atyy]

So I would like to ask for explanation that does not relay on quantum collapse for this phenomena:
We have unpolarized beam of light. It goes trough two orthogonally oriented polarizers. Assuming idealized polarizers we detect no light after second polarizer. Then we insert third polarizer oriented at 45 degrees between the first two. Now we observe 1/8 of the intensity of original beam of light.

There is no collapse here. Collapse is needed when you make a sequence of measurements (ie. more than one measurement per setup). However, you have only described two different setups, each with a single measurement.