# What does the probabilistic interpretation of QM claim?

by A. Neumaier
Tags: claim, interpretation, probabilistic
PF Gold
P: 1,942
 Quote by meopemuk the most spectacular failure is related to electrons registered by a photographic plate. If you describe the incident electron by a plane wave or other continuous charge density field, you will have a hard time to explain how this distributed charge density condenses to a single location of one emulsion grain.
Mott even explains how a complete particle track appears in a bubble chamber - caused by a classical external electromagnetic field reaching the detector from a particular direction.
PF Gold
P: 1,942
 Quote by meopemuk I agree with you that parameter x in quantum field $$\psi(x,t)$$ has absolutely no relationship to physically measurable position.
We completely disagree. There is indeed no relation to an alleged particle position.

But the parameters x and t in a quantum field have the definite meaning of position and time - not of a particle, but of the point where the field strength is measured. The rate of response of the detector at position x at time t is for a photon proportional to the intensity <|E(x,t)|^2>, where E(x,t) is the complex analytic signal of the electric field operator, and for an electron proportional to the intensity <|Psi(x,t)|^2>, where Psi(x,t) is the Dirac field operator.
 Quote by meopemuk You were correct to point out that in the case of indistinguishable particles this does not allow to form a Hermitian "particle position" operator. But the above construction of n-particle localized states is sufficient to describe position measurements in the Fock space.
The radial wave produced by a double slit is not a localized state.
 Quote by meopemuk Another point is that refusing the measurability of positions you are are not saving yourself from the "weird" quantum collapse. You've mentioned elsewhere that the momentum-space wavefunction $$\psi(p)$$ does have a measurable probabilistic interpretation. So, it does require a collapse. This time in the momentum space.
There is only an apparent collapse due to changing the description before and after reaching the detector. Discontinuities caused by changes in the description level are ubiquitous in physics - whether classical or quantum.
 Quote by meopemuk Our difference is that I believe that the blackening of silver atoms or the formation of bubbles are direct local effects of incident particles. So, by measuring positions of exposed grains of photoemulsion or bubbles we measure (albeit indirectly) positions of particles, which created these effects.
I know. Whereas I interpret it in terms of quantum fields, which have a much more benign intuitive interpretation, and also apply to electromagnetic radiation, where your interpretation breaks down.
 Quote by meopemuk If I understand correctly, your position is that the blackened grain of photoemulsion or the formed bubble is not a proof that the particle really hit that spot.
Instead, it is proof that there is an incident quantum field.
 Quote by meopemuk creation of the local photographic image or a small bubble is "explained" by a sequence of non-trivial condensation events happening in the bulk of the detector. These events require migration of charge to macroscopic distances
Of charge density. But charge density migrates over macroscopic distances also during the flight from the source to the detector - there is nothing strange about it.
 Quote by meopemuk If I understand correctly, your motivation for applying these non-trivial models of particle detection is to avoid using the quantum-mechanical wave function collapse.
No. My motivation is to have a consistent intuitive view of quantum field theory, which since over half a century is regarded as the correct description of microscopic physics, with ''particles are just bundles of energy and momentum of the fields'' (Weinberg).

That one doesn't need the collapse is just a welcome byproduct of this view.
P: 1,744
 Quote by A. Neumaier I know. Whereas I interpret it in terms of quantum fields, which have a much more benign intuitive interpretation, and also apply to electromagnetic radiation, where your interpretation breaks down.
Is there a single example, where the corpuscular interpretation "breaks down", as you say?

 Quote by A. Neumaier Of charge density. But charge density migrates over macroscopic distances also during the flight from the source to the detector - there is nothing strange about it.
I can understand a charge wave that propagates and spreads out. However, I have a difficulty to imagine a wave that collapses to a point spontaneously. Which physical mechanism can be responsible for such a collapse?

 Quote by A. Neumaier That one doesn't need the collapse is just a welcome byproduct of this view.
I think that the discovery of the quantum nature of things, sometimes dubbed "collapse", was the single most important discovery in 20th century physics. I know that we disagree about that.

Eugene.
PF Gold
P: 1,942
 Quote by meopemuk Is there a single example, where the corpuscular interpretation "breaks down", as you say?
Photons have no position; they disappear upon the attempt to measure any of their properties. It is only continuous brain washing that calls such ghost-like objects particles.
 Quote by meopemuk I can understand a charge wave that propagates and spreads out. However, I have a difficulty to imagine a wave that collapses to a point spontaneously. Which physical mechanism can be responsible for such a collapse?
In my understanding there is no collapse and there need not be one. The collapse is an artifact of the point particle interpretation of quantum mechanics.
 Quote by meopemuk I think that the discovery of the quantum nature of things, sometimes dubbed "collapse", was the single most important discovery in 20th century physics. I know that we disagree about that.
Collapse is not the quantum nature of things, but the least understood aspects of quantum mechanics. QM is an extremely successful description of Nature no matter whether one believes in collapse.
P: 1,744
 Quote by A. Neumaier Photons have no position; they disappear upon the attempt to measure any of their properties. It is only continuous brain washing that calls such ghost-like objects particles.
There is nothing strange in the fact that photons can be created and absorbed easily. This is described naturally in QFT, which is a theory designed to work with systems, where the number of particles can change.

 Quote by A. Neumaier In my understanding there is no collapse and there need not be one. The collapse is an artifact of the point particle interpretation of quantum mechanics. Collapse is not the quantum nature of things, but the least understood aspects of quantum mechanics. QM is an extremely successful description of Nature whether one believes in collapse.
I think we agree that in the double-slit setup the locations of marks on the photographic plate are random. I hope we also agree that the clicks produced by a Geiger counter attached to a piece of radioactive material occur at random times. At least, it is fair to say that nobody was able to predict locations of individual marks or timings of individual clicks.

In my understanding, quantum mechanics says that these kinds of events are not predictable as a matter of principle. Nature has an inherently random component, which cannot be explained. The best we can do is to calculate probabilities of these random events. That's what quantum mechanics is doing and it is doing it brilliantly. Once we agreed on the fundamental randomness of quantum events, there is no other way, but to accept the idea of collapse: The outcomes are not known to us before observations, they are described only as probability distributions. After the observation is made a single outcome emerges, so the probability distribution collapses.

There is nothing there to understand about the collapse. Things that are fundamentally random cannot be explained or understood any better than simply saying that they are random.

From my discussions with you I've understood that you have a different view on the origin of randomness. You basically believe that nature obeys deterministic field-like equations. The appearance of a mark on the photographic plate has a mechanistic explanation in which the impacting electron field interacts with the fields of atoms in the plate. This interaction leads to some physical migration of the field energy and charge density to one specific point, which appears to us as a blackened AgBr microcrystal. These migration processes involve huge number of atoms, so they are "stochastic" or "chaotic", and their outcomes cannot be predicted at our current level of knowledge. Nevertheless, you maintain that at the fundamental level there are knowable field equations as opposed to the pure chance.

These are two different philosophies, two different world views, which could be completely equivalent as far as specific experimental observations are concerned. In general, I find it not fruitful to argue about ones philosophy, religion or political preferences. These kinds of convictions cannot be changed by logical arguments. So, perhaps we should agree to disagree.

Eugene.
PF Gold
P: 1,942
 Quote by meopemuk There is nothing strange in the fact that photons can be created and absorbed easily. This is described naturally in QFT, which is a theory designed to work with systems, where the number of particles can change.
An entity about which we can say nothing at all during its flight from the source to the detector, which never has a position, produces a spot on a plate and at this moment disappears forever. This is a perfect description of a ghost, whereas calling it a particle is an unfortunate historical accident. Everyone beginning to study quantum mechanics finds this extremely strange and un-particle-like. Not to find that strange is the result of years of indoctrination by famous and less famous kindergarden storytellers. That the most famous of them had won a Nobel prize helped in making the brainwashing more efficient.
 Quote by meopemuk Nature has an inherently random component, which cannot be explained.
I explain is as microscopic chaos in the detector.
 Quote by meopemuk The best we can do is to calculate probabilities of these random events. That's what quantum mechanics is doing and it is doing it brilliantly. Once we agreed on the fundamental randomness of quantum events, there is no other way, but to accept the idea of collapse: The outcomes are not known to us before observations, they are described only as probability distributions. After the observation is made a single outcome emerges, so the probability distribution collapses.
Nobody but you calls the change of prior probabilities into posterior certainties a collapse.

Collapse _always_ refers to the collapse of the state - that after the measurement, the state of the measured system is in an eigenstate of the measured observable!!!
 Quote by meopemuk You basically believe that nature obeys deterministic field-like equations.
No. Nature obeys the rules of QFT, and all macroscopic information arrives in the form of expectation values of appropriate fields, as given by statistical thermodynamics, the quantum theory of macroscopic matter. This is enough to explain everything without assuming a collapse of the state. (What you call collapse, but what others label a change of probabilities into certainties is fully explained by the subjective inability to predict a chaotic system with zillions of degrees of freedom.)
 Quote by meopemuk Nevertheless, you maintain that at the fundamental level there are knowable field equations as opposed to the pure chance.
Field equations are operator equations. What is knowable are the field expectations at macroscopic resolutions. Engineers measure them routinely.
 Quote by meopemuk These are two different philosophies, two different world views, which could be completely equivalent as far as specific experimental observations are concerned. In general, I find it not fruitful to argue about ones philosophy, religion or political preferences.
I find it _very_ fruitful to argue about ones philosophy, religion or political preferences.
This is the only way to influence people's convictions.
 Quote by meopemuk These kinds of convictions cannot be changed by logical arguments. So, perhaps we should agree to disagree.
We always agreed that we disagree, from the start of this thread. But we draw different consequences from this fact.
P: 1,744
 Quote by A. Neumaier Nobody but you calls the change of prior probabilities into posterior certainties a collapse. Collapse _always_ refers to the collapse of the state - that after the measurement, the state of the measured system is in an eigenstate of the measured observable!!!
I've forgotten to mention that I am not interested in the state of the quantum system after it has "interacted" with the measuring device and produced the measurement outcome. So, I am agnostic about the state after the measurement. Yes, I understand that there are situations when one can measure repeatedly different things on the same copy of the system. However, I would like to avoid discussions of such situations. So, I would prefer to think that after the measurement is done and its result is recorded, the system is discarded. Dealing only with such one-time measurements makes my life a bit easier.

So, I agree that collapse = "the change of prior probabilities into posterior certainties". However, I disagree that the collapse ever happens in classical physica, because in classical physics everything is determined and predictable. If somebody has encountered a "probability" in classical physics, that's only because this somebody was too lazy or ignorant to specify exactly all necessary initial conditions. Somebody's ignorance and laziness cannot be accounted for in a rigorous theory. "Zillions of degrees of freedom" is also not a good excuse to introduce probabilities, because we are talking about principles here, not about practical realizations.

Eugene.
 P: 661 It should be pointed out that A Neumaier's suggestion that a deterministic chaotic dynamics may underly quantum randomness is not the standard view, and to even be consistent with modern experimental results requires some additional weird assumptions such as explicit non-locality (Bohm) or information loss behind event horizons ('t Hooft). http://www.nature.com/news/2007/0704...s070416-9.html
PF Gold
P: 1,942
 Quote by meopemuk I've forgotten to mention that I am not interested in the state of the quantum system after it has "interacted" with the measuring device and produced the measurement outcome. So, I am agnostic about the state after the measurement.
But this means that you are agnostic about collapse, as the term is traditionally understood: ''In quantum mechanics, wave function collapse (also called collapse of the state vector or reduction of the wave packet) is the phenomenon in which a wave function—initially in a superposition of several different possible eigenstates—appears to reduce to a single one of those states after interaction with an observer.'' http://en.wikipedia.org/wiki/Wavefunction_collapse
 Quote by meopemuk So, I agree that collapse = "the change of prior probabilities into posterior certainties".
You only agree to your own nonstandard interpretation of the word ''collapse''. I don't agree at all with this usage.
 Quote by meopemuk "Zillions of degrees of freedom" is also not a good excuse to introduce probabilities, because we are talking about principles here, not about practical realizations.
''Unperformed experiments have no results'' (A. Peres, Amer. J. Phys. 46 (1978), 745).
This holds even more for unperformable measurements or preparations.
PF Gold
P: 1,942
 Quote by unusualname It should be pointed out that A Neumaier's suggestion that a deterministic chaotic dynamics may underly quantum randomness is not the standard view, and to even be consistent with modern experimental results requires some additional weird assumptions such as explicit non-locality (Bohm) or information loss behind event horizons ('t Hooft).
It is enough to assume information loss to the part of the universe not visible from our planetary system (where all our experiments are done). Radiation goes there all the time; so this assumption is satisfied.
 Quote by unusualname http://www.nature.com/news/2007/0704...s070416-9.html
The paper says nothing about requiring weird assumptions. But the author states: ''But for objects governed by the laws of quantum mechanics, like photons and electrons, it may make no sense to think of them as having well defined characteristics. Instead, what we see may depend on how we look.''

depend on how wee look = depend on the measurement apparatus (here our eye).

Thus his statement confirms my hypothesis.
P: 661
 Quote by A. Neumaier It is enough to assume information loss to the part of the universe not visible from our planetary system (where all our experiments are done). Radiation goes there all the time; so this assumption is satisfied.
Radiation travels via a local mechanism, are you saying your deterministic model is local and real?

 The paper says nothing about requiring weird assumptions. But the author states: ''But for objects governed by the laws of quantum mechanics, like photons and electrons, it may make no sense to think of them as having well defined characteristics. Instead, what we see may depend on how we look.'' depend on how wee look = depend on the measurement apparatus (here our eye). Thus his statement confirms my hypothesis.
Shouldn't you say that statement doesn't contradict your model, rather than asserting it confirms it.

But I'm not saying you're wrong, just that it's an an unusual model to be promoting.
P: 1,744
 Quote by A. Neumaier But this means that you are agnostic about collapse, as the term is traditionally understood: ''In quantum mechanics, wave function collapse (also called collapse of the state vector or reduction of the wave packet) is the phenomenon in which a wave function—initially in a superposition of several different possible eigenstates—appears to reduce to a single one of those states after interaction with an observer.'' http://en.wikipedia.org/wiki/Wavefunction_collapse
I've possibly created a confusion by using my own definition of collapse, which is different from the wikipedia's one. To clarify, I would like to mention that I am interested only in single measurements of observables. I am not interested in what is the state of the system after the measurement is completed. I am not sure if wave function is a good description for such states.

Eugene.
P: 1,723
 Quote by meopemuk I agree that some aspects of particle detection can be explained by Mandel & Wolf type arguments. However, there are situations, where these arguments fail completely. I think the most spectacular failure is related to electrons registered by a photographic plate. If you describe the incident electron by a plane wave or other continuous charge density field, you will have a hard time to explain how this distributed charge density condenses to a single location of one emulsion grain. I think it is well established that after "observation" the entire electron charge is located in the neighborhood of the blackened emulsion grain. Apparently, there should be a mechanism by which the distributed charge density condenses to a point and overcomes a strong Coulomb repulsion in the process. This doesn't look plausible even from the point of view of energy conservation.
In that case, what is wrong with Mott's or Schiff's analyses (which apply for incident
field carrying charge)? To me these seem adequate to account for the experimental
observations.
PF Gold
P: 1,942
 Quote by unusualname Radiation travels via a local mechanism, are you saying your deterministic model is local and real?
My interpretation is not deterministic, since it is based on standard QFT. But like the latter it is local.

By the way, there are no no-go theorems against deterministic field theories underlying quantum mechanics. Indeed, local field theories have no difficulties violating Bell-type inequalities. See http://www.mat.univie.ac.at/~neum/ms/lightslides.pdf , starting with slide 46.

 Quote by unusualname Shouldn't you say that statement doesn't contradict your model, rather than asserting it confirms it.
If a key statement that wasn't known to the proposer of some model doesn't contradict this model, it is usually considered as a confirmation of the model. In the present case, since you brought the paper as argument to caution readers against my views, and my main assumption was that the results of measurements depend on the detector, and the author of the paper made precisely this point (for the special detector called us - or our yes), it is a significant confirmation.
 Quote by unusualname I added that link to a mainstream science article to point out the mainstream view on quantum interpretation, just in case people think your "science advisor" tag adds credibility to your nonstandard view.
I am not reponsible for having this tag.
 Quote by unusualname But I'm not saying you're wrong, just that it's an an unusual model to be promoting.
I am only taking quantum field theory seriously. It is not that unusual: People working on dynamic reduction models have a very similar view:

G. Ghirardi,
Quantum dynamical reduction and reality:
Replacing probability densities with densities in real space,
Erkenntnis 45 (1996), 349-365.
http://www.jstor.org/stable/20012735

My only new point compared to them is that one doesn't need the dynamic reduction once one has the field density ontology.
 P: 661 @A. Neumaier, I don't understand you, make it simple for me, is deterministic chaotic dynamics the fundamental mathematical description of reality in your model?
PF Gold
P: 1,942
 Quote by unusualname is deterministic chaotic dynamics the fundamental mathematical description of reality in your model?
The fundamental mathematical description of reality is standard quantum field theory, _not_ deterministic chaos. The latter is an emergent feature.

In my thermal interpretation of quantum physics, the directly observable (and hence obviously ''real'') features of a macroscopic system are the expectation values of the most important fields Phi(x,t) at position x and time t, as they are described by statistical thermodynamics. If it were not so, thermodynamics would not provide the good macroscopic description it does.

However, the expectation values have only a limited accuracy; as discovered by Heisenberg, quantum mechanics predicts its own uncertainty. This means that <Phi(x)> is objectively real only to an accuracy of order 1/sqrt(V) where V is the volume occupied by the mesoscopic cell containing x, assumed to be homogeneous and in local equilibrium. This is the standard assumption for deriving from first principles hydrodynamical equations and the like. It means that the interpretation of a field gets more fuzzy as one decreases the size of the coarse graining - until at some point the local equilibrium hypothesis is no longer valid.

This defines the surface ontology of the thermal interpretation. There is also a deeper ontology concerning the reality of inferred entities - the thermal interpretation declares as real but not directly observable any expectation <A(x,t)> of operators with a space-time dependence that satisfy Poincare invariance and causal commutation relations.
These are distributions that produce exact numbers when integrated over sufficiently smooth localized test functions.

Approximating a multiparticle system in a semiclassical way (mean field theory or a little beyond) gives an approximate deterministic system governing the dynamics of these expectations. This system is highly chaotic at high resolution. This chaoticity seems enough to enforce the probabilistic nature of the measurement apparatus. Neither an underlying exact deterministic dynamics nor an explicit dynamical collapse needs to be postulated.
 P: 661 Sorry, but chaotic dynamics is an exact mathematical model, that's the whole point of it, you can't say it's "emergent". Sensitive dependence at infinitesimally small changes in the the dynamical parameters is part of the definition of chaotic dynamics. If you have a stochastic dynamics then you have stochastic dynamics, if you have deterministic dynamics then you have deterministic dynamics, there's no inbetween "emergent" type system.