- #176
Jarvis323
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What do you mean? Intuitively I wouldn't think that is any less of a "thing" than anything else.WernerQH said:The click of a Geiger counter is not a "thing".
What do you mean? Intuitively I wouldn't think that is any less of a "thing" than anything else.WernerQH said:The click of a Geiger counter is not a "thing".
The Geiger counter is the existing thing of which the click is a property. If the quantum system called Geiger counter does not exist, neither the click exists.WernerQH said:The click of a Geiger counter is not a "thing".
No. I found something innovative this way whereas you only repeat an abstruse tradition. I consider the former to be more lucky than the latter.WernerQH said:If you think that, fundamentally, physics must necessarily be about "things", you are out of luck.
I would say that "centre of mass" is a fuzzy concept. With a large number of particles, the concept is hardly fuzzy at all, but as you remove particles, the property becomes fuzzier and fuzzier.A. Neumaier said:Certain quantum systems consisting of N particles exist and for large N they have properties (such as the center of mass) even when nobody is looking at them - detectors are an example. Decreasing N by one doesn't change the property of existing. By induction, quantum systems exist and have properites (such as the center of mass) down to N=0 (the vacuum), at which point we cannot decrease N anymore.
An object that has a fuzzy property surely exists, no matter how fuzzy the property. And the amount of fuzziness of a property is another (fuzzy) property of the object.PeroK said:I would say that "centre of mass" is a fuzzy concept. With a large number of particles, the concept is hardly fuzzy at all, but as you remove particles, the property becomes fuzzier and fuzzier.
Yes.PeroK said:That's certainly true of other properties: what if we replace "centre of mass" with "age", and apply the same logic?
When you are dead. There are objective procedures to decide that.PeroK said:A human being has a well-defined age that cannot be induced into its constituent particles. If you remove the particles that make up me, at what point does what's left stop being 58 years old?
That's a clever answer, but it doesn't get you round the fundamental problem.A. Neumaier said:When you are dead. There are objective procedures to decide that.
My question for a simple model of a screen accidentally skipped an important intermediate step: The description of the "quantum-result" of the Stern-Gerlach experiment can be given without relying on any specific model of a screen: As a wavefunction (or a collection of wavefunctions with suitable positive weights summing to 1 if details of the thermal state of the source should be included in the model too) that still depends on spin and linear moment of the silver atoms in addition to their (x,y) position just above the surface of the screen. (The linear momentum includes both energy and direction. It will be strongly correlated with spin and (x,y) position as a result of the Stern-Gerlach "quantum-measurement setup".)gentzen said:Two questions:
1) ...
2) What would be a simple reasonable model for a screen? A 2D non-relativistic quantum field? That seems simple enough, but may be "too coherent".
A 2D array of photosensitive pixels that can change their color through interaction with an incident particle.gentzen said:What would be a simple reasonable model for a screen?
A. Neumaier said:Information is not real. The things the information is about are!
As I noted in post #186, the term "real" is not a physics term, and should be avoided in this discussion.EPR said:The things you naively seem to define as real cannot withstand scrutiny
No. Information about something that does not exist is fiction, not science.EPR said:I am not sure I follow. Chasing this path leads to contradiction and confusion.
This is only part of what is real, i.e., of what exists. The Geiger counters, i.e., the quantum objects that make up the equipment used for experiments are as real. This uses 'real' in the everyday sense, without any sophisticated philosophy.PeterDonis said:I suspect that the "things information is about" that @A. Neumaier was referring to as "real" are things like clicks in Geiger counters, i.e., experimental data, and by "real" he simply meant that if we do not take the experimental data we have as given, we cannot do physics at all since we have no basis on which to test our models.
Since measurement takes a huge variety of concrete manifestations, this empirical fact about certain complex arrangements of atoms should (as all other empirical facts) be provable from the fact that measurement devices are quantum objects.vanhees71 said:the fact that there are single well-defined outcomes when measuring some observable is simply taken as an empirical fact
This is neither implied by Bell's theorem nor by any empirical investigations. The results of Bell-type experiments exclude noncontextual local observables propagating according to classical dynamics of local hidden variables. Nonlocal hidden variables or nonlocal observables are not covered by Bell's assumptions.vanhees71 said:What Bell's theorem and it's empirical investigation shows is that such a determinsitic HV theory would not be one with local interactions (local in the sense of the standard relativistic QFTs),
Why a large quantum system interacting with a spin has definite outcomes even though the unitary dynamics produces a nonthermal state. More specifically:vanhees71 said:So what do you think has to be "proven" with regard to definite outcomes when measuring, and which assumptions would you accept for such a proof?
A. Neumaier also uses philosophy. The main part of my review of his paper Foundations of quantum physics II. The thermal interpretation begins:Jarvis323 said:How you can build solid philosphical foundations of QM without using philosophy is only a question that philosophy can address.
However, this is a physics forum, so he focuses on the physics here.The presentation is easy to read, and contains many remarks and observations that are spot on both practically and philosophically.
Agreed. I was actually hoping you would expand on what I wrote, so thanks!A. Neumaier said:This is only part of what is real, i.e., of what exists.
Agreed. It would also be consistent to say that electrons themselves are not real, only observations like dots on a detector screen are, and that the information in the wave function is about those observations.A. Neumaier said:Those who claim reality of 'information about an electron' contained in a wave function are inconsistent if they don't treat the electron as being real and having real properties that we can sometimes have some information (know something) about.
Yes. But something real goes somehow from the source to the detector. Nobody would be interested in the dots on a detector if they wouldn't give information about this something. Thus this something (whether called electrons or electron field) must be real and have real properties.PeterDonis said:It would also be consistent to say that electrons themselves are not real, only observations like dots on a detector screen are, and that the information in the wave function is about those observations.
I'm not sure how one could justify this claim experimentally, except in the obvious special case where we continuously measure something in between. In any other case, this claim, however plausible it seems, cannot, I think, be taken as a necessary axiom that any interpretation of QM (or physical theories in general) must include. It can, of course, be taken as an axiom for a particular interpretation if the person building the interpretation wants to (I assume, for example, that you would take it as an axiom in your thermal interpretation).A. Neumaier said:something real goes somehow from the source to the detector
Just point the source in a direction different from the detector, and you don't get a response, while pointing it towards it gives a response.PeterDonis said:I'm not sure how one could justify this claim experimentally
To be clear, the claim I am saying can't be justified experimentally is not "there is some kind of physical effect between the source and the detector", but the narrower claim that "something real goes somehow from the source to the detector", which I am reading as claiming that something real, in the same sense as the source and the detector are real, has to travel continuously through the intervening space (or spacetime if we are using a relativistic interpretation).A. Neumaier said:Just point the source in a direction different from the detector, and you don't get a response, while pointing it towards it gives a response.
It can be demonstrated experimentally that, even when source and detector are far away, the source can reliably send signals that the detector responds to. These signals are surely as real as anything we consider real, though they are not material. For everything we consider real is known to us only through such signals.PeterDonis said:"something real goes somehow from the source to the detector", which I am reading as claiming that something real, in the same sense as the source and the detector are real, has to travel continuously through the intervening space (or spacetime if we are using a relativistic interpretation).
If this is equivalent to saying that it can't be demonstrated experimentally that the signals are there between the source and the detector (since by definition there are no measurements being made between them), then you are agreeing with me.A. Neumaier said:What cannot be demonstrated experimentally is that there is a medium carrying these signals.
The signals can be measured everywhere we place a detector (in the direction of emission). Thus we know experimentally that what is emitted is detectable anywhere on its path of transmission.PeterDonis said:If this is equivalent to saying that it can't be demonstrated experimentally that the signals are there between the source and the detector
Open quantum systems, and measurement devices are always open quantum systems, are never described by unitary time evolution but by a coarse-grained effective description of macroscopic observables. There are many approaches to this, among them the projection-operator formalism (Zwanzig et al), Lindblad equations for reduced stat. ops., quantum Langevin approaches, the influence functional, Kadanoff-Baym.A. Neumaier said:Since measurement takes a huge variety of concrete manifestations, this empirical fact about certain complex arrangements of atoms should (as all other empirical facts) be provable from the fact that measurement devices are quantum objects.
Instead, the notion of measurement is in the foundations of the statistical interpretation.
This is neither implied by Bell's theorem nor by any empirical investigations. The results of Bell-type experiments exclude noncontextual local observables propagating according to classical dynamics of local hidden variables. Nonlocal hidden variables or nonlocal observables are not covered by Bell's assumptions.Why a large quantum system interacting with a spin has definite outcomes even though the unitary dynamics produces a nonthermal state. More specifically:
Since the state is the maximum that can be known about a system, and since the definite outcome is definitely known, any objective macroscopic property of the quantum system (including the definite outcome) must be somehow encoded in the state of the system. Thus one has to be able to describe in mathematical detail a notion of state such that, and how,
One can assume some stochasticity to simplify the complexity of the system (with the same kind of arguments as in Brownian motion or classical statistical mechanics).
- the outcome is determined by the state of the system., and
- the unitary dynamics of detector + spin (+ whatever else is needed) ensures that, from a well-defined initial state of this combined system, one arrives at this definite outcome.
But one cannot assume anything about measurement, since the latter should be a consequence of the structure of the measurement device.
If one includes enough of the environment they can be treated as closed systems. Indeed, a very traditional way to do the coarse graining is to start from a unitary evolution and to derive the coarse-grained dynamics of a subsystem.vanhees71 said:Open quantum systems, and measurement devices are always open quantum systems, are never described by unitary time evolution but by a coarse-grained effective description of macroscopic observables.
All of these start with a unitary dynamics of a more detailed description to obtain the coarse grained dynamics using suitable approximations.vanhees71 said:There are many approaches to this, among them the projection-operator formalism (Zwanzig et al), Lindblad equations for reduced stat. ops., quantum Langevin approaches, the influence functional, Kadanoff-Baym.
In view of the above, the tasks are:vanhees71 said:So what do you think has to be "proven" with regard to definite outcomes when measuring, and which assumptions would you accept for such a proof?
No. Decoherence does not contribute to the unique outcome problem - it fails on point 3 of my list of requirements. Schlosshauer, the most competent physicist on decoherence, is very explicit about this, and I agree with him.vanhees71 said:Your points 1.-3. are indeed the true goal, and I think this has been achieved by all the work on "decoherence" in the past 2-3 decades.
Decoherence produces decay of phase information but does not explain how a single measurement results. It produces an improper diagonal density matrix instead of one where position has a fixed value. The improper mixture is not resolved into single results.vanhees71 said:Ok, then what's wrong with the model treatments for position mesurements in
Joos et al, Decoherence and the Appearance of a Classical World in Quantum Theory ?
vanhees71 said:then the goal you have in mind is not to reach within standard quantum mechanics
This amounts to taking the existence of unique macroscopic outcomes as an additional axiom in addition to the statistical interpretation.vanhees71 said:I always considered it sufficient to prove precisely that you get what you call an "improper diagonal density matrix", i.e., the probability distribution for the position (taken as a "pointer observable"). Since this is anyway all we can expect to measure and to predict with our theories, for me that's sufficient.
Based on what the proponents of consistent histories wrote about related matters, my guess is that one cannot disprove Many-Worlds (within standard quantum mechanics), but that one cannot prove it either, nor can one prove the need for something like GRW.vanhees71 said:Ok, then the goal you have in mind is not to reach within standard quantum mechanics, i.e., one has to extent quantum mechanics, i.e., something like the GRW theory with "explicit collapse".
And R. Omnès is also pretty clear that he is convinced that there is only a single world, even if the “consistent histories” doesn’t explain it, and cannot even disprove Many-Worlds. His defense is to admit that there is still a disagreement left between Reality and quantum theory, but that it would be hubris to expect otherwise. At least that is how I interpret the words on page 214:
… have reproached quantum physics for not explaining the existence of a unique state of events. It is true that quantum theory does not offer any mechanism or suggestion in that respect. This is, they say, the indelible sign of a flaw in the theory, … Those critics wish at all costs to see the universe conform to a mathematical law, down to the minutest details, and they certainly have reason to be frustrated.
…
I embrace, almost with prostration, the opposite thesis, the one proclaiming how marvelous, how wonderful it is to see the efforts of human beings to understand reality produce a theory fitting it so closely that they only disagree at ...
Not if you adopt the many-worlds interpretation.EPR said:even though all 'stuff' is quantum mechanical in nature and there is nothing in the evolution of the quantum system that mandates to bring about single outcomes, there must be something non-quantum that destroys superpositions and renders them into definite outcomes.
Yes. The variant of MWI where wavefunctions are somehow real, yet they extend throughout spacetime.PeterDonis said:Not if you adopt the many-worlds interpretation.