What does the probabilistic interpretation of QM claim?

Physics Monkey

I'm sorry, but this statement is simply false. I just gave an example where particle position is relevant. One can mention quantum fields all one wants, but that doesn't change the fact that as a practical matter particle positions can be meaningful and useful approximations. Even in quantum field theory.

A. Neumaier

I have nothing against particle positions as meaningful and useful semiclassical _approximations_, as is appropriate for particles assumed to have collapsed already, and hence described by an effective particle picture along a track. This is a change of the representation, simplifying the picture and the analysis in cases where the physics allows this.

Nevertheless, even in a track, one only has a measurement of the projection of the position on the plane transversal to the momentum.

But before the detector is reached, there is just a radially expanding quantum field for each particle kind involved in the decay (before and after), and Mott's analysis applies. The secondary bubble traces start at random positions along the track, for the same reason that the primary trace start at a random position anywhere at the surface of the detector where the field density is large enough and continues inside the detector.

A. Neumaier

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.

A. Neumaier

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.
The radial wave produced by a double slit is not a localized state.
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.
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.
Instead, it is proof that there is an incident quantum field.
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.
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.

meopemuk

Is there a single example, where the corpuscular interpretation "breaks down", as you say?

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?


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.

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.
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 no matter whether one believes in collapse.

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.

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.

A. Neumaier

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.
I explain is as microscopic chaos in the detector.
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!!!
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.)
Field equations are operator equations. What is knowable are the field expectations at macroscopic resolutions. Engineers measure them routinely.
I find it _very_ fruitful to argue about ones philosophy, religion or political preferences.
This is the only way to influence people's convictions.
We always agreed that we disagree, from the start of this thread. But we draw different consequences from this fact.

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. 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.

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).

http://www.nature.com/news/2007/0704...s070416-9.html

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
You only agree to your own nonstandard interpretation of the word ''collapse''. I don't agree at all with this usage.
''Unperformed experiments have no results'' (A. Peres, Amer. J. Phys. 46 (1978), 745).
This holds even more for unperformable measurements or preparations.

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.
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.

unusualname

Radiation travels via a local mechanism, are you saying your deterministic model is local and real?

Shouldn't you say that statement doesn't contradict your model, rather than asserting it confirms it.

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.

But I'm not saying you're wrong, just that it's an an unusual model to be promoting.

meopemuk

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.

strangerep

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.

A. Neumaier

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.

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.
I am not reponsible for having this tag.
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.

unusualname

@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?