Nobody understands quantum physics?

[vanhees71]

Does it? Hasn't it widened the field (quantum cryptography, quantum "teleportation", quantum computing)?

Sure, but all this is just standard quantum theory, and it's possible, because entanglement is the correct description of Nature and not "realism", i.e., all Bell experiments prove (together with the validity of locality in the sense of microcausality of relativistic QFT) that the observable don't take determined values, i.e., there's "true randomness" in Nature, which is not due to our ignorance of the exact state as in classical statistical physics. E.g., if you have a two-photon Bell state (say the polarization-singlet state) then the single-photon polarizations are maximally random, i.e., the reduced statistical operator describing them is $\hat{1}/2$, i.e., the single photons are perfectly unpolarized.

My conclusion is the exact opposite. I'd rather give up the "sacred" locality than realism. For me, realism means accepting the results of experiments as real; it does not mean we have to believe in the existence of photons with definite polarization states.

You can't give up locality without giving up the most successful quantum theory ever, i.e., local (microcausal) relativistic QFT the Standard Model is based on, which is more successful than the HEP community wishes for!

I agree that we are in the posession of a very good description, but I doubt that we have found the best formulation. Obviously you can't conceive of the possibility that quantum theory (after almost a century!) may be in a situation similar to that of electrodynamics before 1905.

Of course, the current understanding is incomplete, but not because of some philosophical quibbles about the interpretation of the present QT formalism but because we don't have a satisfactory quantum theory of gravitation and/or a quantum theory of spacetime. It's well possible that a future solution of this problem will result in a completely new paradigm with the classical spacetime model(s) used in our contemporary physics being an "emergent phenomenon".

 

[WernerQH]

We're obviously talking past each other!

 

[A. Neumaier]

After the magnet the spin component in direction of the field is (almost completely) entangled with position, i.e., in each of the two partial beams "selected" by the magnet you have a well-prepared spin component.

No. You have two well-prepared spin components, one in the up beam and one in the down beam. Prepared is a single particle in a superposition, so immediately before the measurement is taken, it "is in" both partial beams. The measurement decides on one of the two spots where the particle can possibly be recorded. (More precisely, after - and not before - the measurement has been taken, it is known at which of the two spots the screen responded.)

The question is when, in a quantum description of the detector, the definite value is obtained.

Thus: in the quantum description of the measurement device, at which time is the up spot (or the down spot) obtained?

We should be careful not to attribute a property like $+1$ to the object of measurement. would only be attributed to the classical datum post-measurement.

But after the measurement, this property is a definite property of the quantum measurement device. Thus it must have been somehow obtained dynamically!

 

[vanhees71]

No. You have two well-prepared spin components, one in the up beam and one in the down beam. Prepared is a single particle in a superposition, so immediately before the measurement is taken, it "is in" both partial beams. The measurement decides on one of the two spots where the particle can possibly be recorded.

Yes, and if I select a particle from this beam, it's in a definite spin state.

Thus: in the quantum desxcription of the measurement device, at which time is the up spot (or the down spot) obtained?

It's obtained when the detector registers the particle, when else?

But after the measurement, this property is a definite property of the quantum measurement device. Thus it must have been somehow obtained dynamically!

Sure, how else?

 

[gentzen]

The measurement decides on one of the two spots where the particle can possibly be recorded.

In which interpretation? Do you have a reference to back up that assertion (with respect to a specific interpretation)?

 

[A. Neumaier]

Yes, and if I select a particle from this beam, it's in a definite spin state.

The SG experiment has no selection. How do you select a particle without adding a filter before the screen?

It's obtained when the detector registers the particle, when else?

Sure, how else?

The classical behavior of macroscopic matter, including matter used for measurements, can be understood from quantum many-body theory.

What is to be explained is how the definite macroscopic measurement result read from the measurement device, interpreted by quantum many-body theory, is obtained through the interaction with the silver atom prepared in a superposition.

The measurement decides on one of the two spots where the particle can possibly be recorded.

In which interpretation? Do you have a reference to back up that assertion (with respect to a specific interpretation)?

My statement just means that after (and not before) the measurement has been taken, it is known at which of the two spots (where the particle can possibly be recorded) the screen responded. This comes directly from the observational facts, and is independent of quantum physics, hence independent of any interpretation of it.

 

[vanhees71]

The SG experiment has no selection. How do you select a particle without adding a filter before the screen?

I select a particle by taking it simply from the region, where the particles have the spin component I like to get. Of course, I can also block the other beam with some filter. That doesn't make a difference.

What is to be explained is how the definite macroscopic measurement result read from the measurement device, interpreted by quantum many-body theory, is obtained through the interaction with the silver atom prepared in a superposition.

It's a silver atom reacting with the detector material by e.g. kicking out an electron, which then is electronically registered or in the original setup it got stuck on the plate and could then be made visible with photographic development (or initially due to the smoke from cheap cigars ;-)).

My statement just means that after (and not before) the measurement has been taken, it is known at which of the two spots where the particle can possibly be recorded, the screen responded. This comes directly from the observational facts, and is independent of quantum physics, hence independent of any interpretation of it.

Of course, you don't know where each individual silver atom will end up, but you know that if you select silver atom from one of the regions that you have a particle with a determined spin component. That's always the case with probabilistic statements. This indeed has nothing specifically to do with quantum physics.

 

[gentzen]

My statement just means that after (and not before) the measurement has been taken, it is known at which of the two spots (where the particle can possibly be recorded) the screen responded. This comes directly from the observational facts, and is independent of quantum physics, hence independent of any interpretation of it.

That way to put it is fine for me. My worry with the initial statement that "the measurement decides" is that people tend to interpret that literally. And I was also unsure whether you did this too, with the intention to ridicule some specific interpretation. (That is why I asked for a reference.)

 

[A. Neumaier]

I select a particle by taking it simply from the region, where the particles have the spin component I like to get. Of course, I can also block the other beam with some filter. That doesn't make a difference.

If you have only one particle prepared, you don't know which region to choose.

It's a silver atom reacting with the detector material by e.g. kicking out an electron, which then is electronically registered or in the original setup it got stuck on the plate and could then be made visible with photographic development (or initially due to the smoke from cheap cigars ;-)).

But in the quantum description, the silver atom is in a superposition of position states, whose quantum evolution in the interacting system particle+detector does not lead to a definite macroscopic measurement result of the detector.

 

[vanhees71]

If you have only one particle prepared, you don't know which region to choose.

I know which region to choose, given the spin state I want to investigate. Of course, I don't know beforehand, whether I get something to experiment with or not. I only know the probabilities whether I get a particle or not. It's a conditional probability: If I get a particle, then I know it's in the wanted spin state. If I don't get a particle, I know it's in the other region with the other possible spin state. As you said before, that's as with any probabilistic description, not specific to "quantum probabilities". Obviously, I don't understand, where you think the problem is.

But in the quantum description the silver atom is in a superposition of position states, whose quantum evolution in the interwacting system particle+detector does not lead to a definite measurement result of the detector.

Let's describe the particle with a wave function for simplicity. You can of course also describe it by a general mixed state, but the qualitative results we are discussing here are the same.

You prepare a silver atom to have a pretty well defined momentum in one direction and be quite well located in the transverse direction (you can use a Gaussian wave packet). When released it's running through the magnetic field. The wave function develops such that after the magnet it's peaked around two positions due to the deflection by the inhomogeneous magnetic field dependent on the spin component in direction of the magnetic field. This implies that particles in the region around either peak has a well-determined spin component in the direction of the magnetic field, i.e., that the spin component and the position is entangled. That's all that's need to select either of the spin states, i.e., you consider simply only particles in the one or the other region.

A definite "outcome" at the detector is of course described by the interaction of the atom with the detector material, and it's random for each single atom whether the one or the other detector will detect the particle. I still don't get where the problem might be.

 

[LittleSchwinger]

A definite "outcome" at the detector is of course described by the interaction of the atom with the detector material, and it's random for each single atom whether the one or the other detector will detect the particle. I still don't get where the problem might be.

I think A. Neumaier might be asking how the outcome being definite at the detector is modelled quantum mechanically, i.e. where in the model of the atom+detector is this definiteness seen.

 

[Fra]

I think A. Neumaier might be asking how the outcome being definite at the detector is modelled quantum mechanically, i.e. where in the model of the atom+detector is this definiteness seen.

It must be modelled by the hamiltonian of the detector+atom. ie. the process of measurement, is a physicainteraction in the hamiltonian of the bigger hilbert space. This is IMO a simple motivator for trying to unify physics and inference in deeper way (which is what I keep nagging about).

At each level the hamiltonian is somehow "given". But what if we ould understand the measurement process, shouldnt that help us with emergent hamiltonian of bigger systems?

It seems to me the definitness of an outcome is defined relative to the orignal observer (to which the term "outcome" refers to). Which is the detector in this example. Thus in the quantum mechanical description of detector+atom, there notion of definite outcome of an internal parts is undefined.

/Fredrik

 

[vanhees71]

But an "outcome" is due to a macroscopic observable, e.g., a visible spot on a photoplate and as such via coarse graining a classical description is adequate. As any classical phenomenon it's the result of sufficient coarse graining and the associated decoherence which leads to a "definite classical outcome".

 

[Fra]

But in the QM description of atom+detector, the "macroscopic variable" is now also a quantum system.

/Fredrik

 

[Fra]

as any classical phenomenon it's the result of sufficient coarse graining and the associated decoherence which leads to a "definite classical outcome".

Taking this ultimate method of explanation, you are force to effective consider the whole universe as a quantum system. But, how do you confirm that model by experiment? (ie as per the high standards you are used to from particle experiments?) THIS is my problem. I understand the idea of decoherence, but that does not solve the problem.

/Fredrik

 

[Morbert]

But after the measurement, this property is a definite property of the quantum measurement device. Thus it must have been somehow obtained dynamically!

"Therefore, the program of computing what the effect of the [measurement] disturbance was and correcting for it is, in general, impossible. Accordingly, the two basic tenets of the theory of macroscopic measurement are both violated. Either the interactions cannot be made arbitrarily weak because of the phenomenon of atomicity, or if we wish to accept this and correct for it, we cannot do so because we do not have a detailed, deterministic theory of each individual event" --Schwinger

Using QM to resolve the "somehow" in your message above is presumably impossible. We can't use quantum theory to predict in detail what events will occur. We can assign probabilities to possible alternative histories of events during the measurement process, but no dynamics will ever explain why one history actually occurs over other alternatives.

 

[A. Neumaier]

But an "outcome" is due to a macroscopic observable, e.g., a visible spot on a photoplate and as such via coarse graining a classical description is adequate. As any classical phenomenon it's the result of sufficient coarse graining and the associated decoherence which leads to a "definite classical outcome".

Please point to a paper describing this in a measurement context.

Standard coarse-graining would produce a definite classical outcome independent of the atomic input. Thus something more must be going on that turns the unitary dynamics into a bistable system with two macroscopic outcomes that depend stochastically on the state of the atom.

 

[A. Neumaier]

Using QM to resolve the "somehow" in your message above is presumably impossible.

You presume this, but many don't. The quest for answering this is the measurement problem.

We can't use quantum theory to predict in detail what events will occur. We can assign probabilities to possible alternative histories of events during the measurement process, but no dynamics will ever explain why one history actually occurs over other alternatives.

According to some interpretations that you take as being unquestionably assumed - but not in all!

In Bohmian mechanics, one can (at least in principle) predict in detail what events will occur, at the expense of introducing degrees of freedom that are nowhere used in practice.

In my thermal interpretation, one can also (at least in principle) predict in detail what events will occur, without introducing degrees of freedom that are nowhere used in practice.

 

[Morbert]

You presume this, but many don't. The quest for answering this is the measurement problem.

I'm not going to disparage any academic projects that attempt to ground QM's probabilistic character in something more deterministic, but I will defend QM against the charge of having a measurement problem insofar as I think a probabilistic interpretation can be discussed and applied in a consistent and unambiguous manner.

In my thermal interpretation, one can also (at least in principle) predict in detail what events will occur, without introducing degrees of freedom that are nowhere used in practice.

I will have a look at this again. iirc The thermal interpretation frames the response rate of a detector as an "event" in and of itself, as opposed to a statement about the likelihood of events?

 

[Demystifier]

Perhaps in lack of interest in dealing with the real problems?

You mean like world peace and cancer cure?

 

[vanhees71]

I meant of course real problems in physics as a natural science... Looking for cancer cure is also much more promising using biology and medicine rather than philosophy. Concerning world peace, I guess one can use philosophy for both making it better or worse... But that's now way off-topic!

 

[LittleSchwinger]

But an "outcome" is due to a macroscopic observable, e.g., a visible spot on a photoplate and as such via coarse graining a classical description is adequate. As any classical phenomenon it's the result of sufficient coarse graining and the associated decoherence which leads to a "definite classical outcome".

Yeah I agree, just thought I'd clarify what I saw the question to be.

 

[A. Neumaier]

I will defend QM against the charge of having a measurement problem insofar as I think a probabilistic interpretation can be discussed and applied in a consistent and unambiguous manner.

There is no measurement problem as long as one treats the detector as a classical object, as in the Copenhagen interpretation.

But once one jointly claims (as vanhees71 does)

• that there is no split between quantum and classical, and
• that coarse-graining explains everything about the detector,

the measurement problem becomes unavoidable.

For then (and only then) one must explain in particular why when fed with a single particle in a spatial superposition, the detector produces a definite outcome that depends stochastically on the superposition. Decoherence is of no help here, as Max Schlosshauer (who wrote the definitive book about the matter) acknowledged.

I will have a look at this again. iirc The thermal interpretation frames the response rate of a detector as an "event" in and of itself, as opposed to a statement about the likelihood of events?

No. Each single response is an event, and the rate of events is predicted by POVMs, as everywhere in quantum mechanics. What is different in the thermal interpretation is that the necessary link between measurement and eigenvalues is denied - POVMs work independent of any eigenvalue analysis.

 

[Morbert]

(My emphasis)

But once one jointly claims (as vanhees71 does)

• that there is no split between quantum and classical, and
• that coarse-graining explains everything about the detector,

the measurement problem becomes unavoidable.

For then (and only then) one must explain in particular why when fed with a single particle in a spatial superposition, the detector produces a definite outcome that depends stochastically on the superposition. Decoherence is of no help here, as Max Schlosshauer (who wrote the definitive book about the matter) acknowledged.

Modelling the interaction between the particle in spatial superposition and the detector array with a quantum theory will result in a new superposition state that entangles the particle with the detector. Decoherence and Quantum darwinism would select the sample space of outcomes most robust to observation, but would not select a definite outcome.

My question: Why is this a problem? Why must we explain in particular the definite outcome? Why can't we accept QM as always treating all possible outcomes on equal footing apart from their probabilities? Perhaps vanhees71 has some stronger sense of "explains everything about the detector" in mind, or perhaps not.

Each single response is an event, and the rate of events is predicted by POVMs, as everywhere in quantum mechanics. What is different in the thermal interpretation is that the necessary link between measurement and eigenvalues is denied - POVMs work independent of any eigenvalue analysis.

Ok, but it is the rate of events that is explained/predicted, as opposed to the particular event of a single run right?

 

[Demystifier]

Why must we explain in particular the definite outcome? Why can't we accept QM as always treating all possible outcomes on equal footing apart from their probabilities?

Because one of the goals of theoretical physics is to explain observations. We observe definite outcomes, we don't observe all possible outcomes on an equal footing.

 

[Demystifier]

I meant of course real problems in physics as a natural science... Looking for cancer cure is also much more promising using biology and medicine rather than philosophy.

Show me one important paper in theoretical physics that does not contain any philosophy! There is no such paper, some philosophy is always there, at least in Introduction. What bothers you is not some philosophy, but too much philosophy. I don't like too much philosophy either, but my "definition of too much" is different. There is no universal criterion of how much philosophy in a scientific work is OK, and how much is not. It's completely subjective.

 

[Demystifier]

"Therefore, the program of computing what the effect of the [measurement] disturbance was and correcting for it is, in general, impossible. Accordingly, the two basic tenets of the theory of macroscopic measurement are both violated. Either the interactions cannot be made arbitrarily weak because of the phenomenon of atomicity, or if we wish to accept this and correct for it, we cannot do so because we do not have a detailed, deterministic theory of each individual event" --Schwinger

Using QM to resolve the "somehow" in your message above is presumably impossible. We can't use quantum theory to predict in detail what events will occur. We can assign probabilities to possible alternative histories of events during the measurement process, but no dynamics will ever explain why one history actually occurs over other alternatives.

Impossible ... we cannot do ... no dynamics will ever explain. Do you mean by QM as we understand it today, or by any theory that will ever be developed?

 

[haushofer]

Show me one important paper in theoretical physics that does not contain any philosophy! There is no such paper, some philosophy is always there, at least in Introduction. What bothers you is not some philosophy, but too much philosophy. I don't like too much philosophy either, but my "definition of too much" is different. There is no universal criterion of how much philosophy in a scientific work is OK, and how much is not. It's completely subjective.

Take General Relativity; the main motivation behind its development was Einstein's conviction that there should be more behind the equivalence principle than Newton makes us believe.

Other people would shrug their shoulders and tell us to shut up and calculate.

I never understood how people can do science without philosophy.

 

[Fra]

I never understood how people can do science without philosophy.

Self denial. Just like Popper tried to deny all except deduction.

/Fredrik

 

[Fra]

Other people would shrug their shoulders and tell us to shut up and calculate.

The other obvious difference is that application of existing corroborated theories (which are the "fruits" of the scientific process) is easy to do without philosophy. But to think that one would progress science and create new theories/fruits with "shutting up and calculate" attitude seems to lack insight in how creativity and learning works.

This is why "pure interpretations" that does not aspired to eventually progress into a new better theory, are not very interesting for me, I see interpretations as a sign of your own expectations of in which direction we think we will find the next generation of theory.

/Fredrik