Particle vs Wave Interpretations of QM

[PeterDonis]

The idea was that the tube walls are coated with absorbing material, so it is not intended to behave as a conventional waveguide with reflecting boundaries.

In that case the tube will simply reduce the overall wave intensity as well as confining the wave to within the tube.

Perhaps a simpler version of the thought experiment would be to remove the tube entirely and place a small detector or screen further downstream, centered on the propagation axis, so that only particles arriving near the middle are recorded.

Ok, but if what you want to know is whether electrons diffract, what's the point of all the double slit apparatus? Why not just pass electrons through an aperture, or something that acts like one? Which brings me to...

would electrons behave in the same way as photons and exhibit diffraction from the aperture

We've known that electrons diffract since 1927. Look up the Davisson Germer experiment.

 

[Roberto Pavani]

I am not asking whether electrons diffract. We know they do.

My point is that placing the detector much farther downstream (L2 >> L) makes the measured count rate highly sensitive to the angular distribution of the electrons transmitted through the central aperture.

Different angular distributions would lead to very different intensities at the downstream detector.

Here, L is the distance from the double slits to the first screen (where the central aperture is located), while L2 is the distance from that screen to the downstream detector.

For example, L2 = 10L.

In that case, even relatively small differences in the angular distribution after the aperture could produce significantly different count rates at the downstream detector.

 

[PeterDonis]

placing the detector much farther downstream (L2 >> L) makes the measured count rate highly sensitive to the angular distribution of the electrons transmitted through the central aperture.

What is the physical significance of this angular distribution? Why would we want a sensitive measurement of it?

 

[Roberto Pavani]

What is the physical significance of this angular distribution? Why would we want a sensitive measurement of it?

This relates back to the discussion about replacing the screen with a piece of plastic or a bubble chamber and asking what kind of tracks would be observed.

We already discussed that @renormalize's picture in post #95 proposes possible green, blue, and red paths (or a combination of them).

In the modified setup of post #203, the plastic/bubble chamber is replaced by an additional aperture located at the central interference maximum and a downstream detector placed at a distance L2 >> L.

The measured count rate should therefore be sensitive to the angular spread of the transmitted electrons.

So my interest is not whether electrons diffract in general, but whether the downstream count rate can provide information about which of the propagation pictures discussed around post #95 is closer to what actually happens.

 

[Lord Jestocost]

As there is no concept of trajectories in quantum mechanics, particles cannot be pictorially represented in the way of classical physics.

 

[Roberto Pavani]

Even if standard QM does not assign a unique trajectory prior to measurement, a cloud chamber placed between the single-slit plane and the downstream screen does reveal localized tracks. Those tracks are not randomly distributed throughout space; they exhibit a statistical angular distribution.

My question is whether the downstream brightness profile is related to that statistical distribution of observed track directions.

Furthermore, even in the absence of a cloud chamber, the shape and intensity of the pattern observed on the downstream screen clearly contain information about the propagation from the slit to the detector. The question is what information about that propagation can be inferred from the observed distribution.

 

[PeterDonis]

whether the downstream count rate can provide information about which of the propagation pictures discussed around post #95 is closer to what actually happens.

I still don't understand. You say:

a cloud chamber placed between the single-slit plane and the downstream screen does reveal localized tracks.

Which already tells you "which of the propagation pictures discussed around post #95 is closer to what actually happens". Why would you then add another single slit and a downstream tube? What information can that give you that isn't already in the cloud chamber tracks?

 

[Roberto Pavani]

Pretty much. My prediction is that, for each individual run of the experiment, we'll see either one of the blue tracks or one of the green tracks, and over a large number of runs, the tracks will fall into two bundles, a blue one and a green one.

My prediction is that the electron tracks will be strictly vertical there (shown in red), whereas you predict that we'll see either the blue or the green tracks (presumably with equal probability) that point back to the individual slits. Do I have that right?

My reasoning is that the blue/green picture and the red picture appear to imply different angular distributions immediately after the central aperture.

The downstream screen is intended to probe that angular distribution.

The idea is not to compare a cloud chamber with no cloud chamber. The downstream screen would always remain in place. The comparison would be between the same setup with vacuum in the intermediate volume and the same setup with the cloud-chamber gas present.

If the blue/green and red pictures correspond to physically different propagation mechanisms, I would expect the downstream pattern to respond differently to the introduction of the medium.

 

[Sambuco]

Interestingly, this paper claims that its conclusions are independent of any interpretation--in other words, Zurek is claiming to derive the Born Rule period, not just to derive it for the MWI. He uses the "relative state" framework only for convenience.

In this paper, Schlosshauer and Fine discuss that particular point:

"(...) Zurek’s aim is clearly to derive Born’s rule from within standard quantum mechanics, where the state vector is assumed to provide a complete description of the physical system. The assumption, of course, might well be questioned in a hidden variable or modal interpretation."

Lucas.

 

[PeterDonis]

Schlosshauer and Fine discuss that particular point:

Hm. Their statement "standard quantum mechanics, where the state vector is assumed to provide a complete description of the physical system" doesn't seem right to me; "standard" QM, without any particular interpretation adopted, makes no such claim. Certain interpretations do, but others explicitly deny it--not just a hidden variable or modal interpretation, but, for example, a statistical or ensemble interpretation, such as the one used by Ballentine.

If Zurek indeed does make the assumption that the state vector is a complete description of the physica system, then to me that clearly means his argument is not interpretation dependent. However, I'm not sure if he actually does make that assumption.

 

[Sambuco]

If Zurek indeed does make the assumption that the state vector is a complete description of the physica system, then to me that clearly means his argument is not interpretation dependent. However, I'm not sure if he actually does make that assumption.

While the proof itself, mathematically speaking, is independent of interpretation, since it only assumes universality, I get the impression that would be necessary to assume that the state vector represents the complete description of the system in order to link the proof with the notion of probabilities of observed events. That is, within Bohmian mechanics, Zurek's calculations remain valid, but they don't represent probabilities, since these are associated with hidden variables, such as the position of the particles.

Lucas.

 

[PeterDonis]

I get the impression that would be necessary to assume that the state vector represents the complete description of the system in order to link the proof with the notion of probabilities of observed events.

I don't see why, since QM interpretations that do not say that the state vector represents the complete description of the system still use the Born Rule.

within Bohmian mechanics, Zurek's calculations remain valid, but they don't represent probabilities, since these are associated with hidden variables, such as the position of the particles.

No, this is not correct. The results of measurements in Bohmian mechanics are determined by the positions of the particles, but the probabilities of different possible results are still predicted by the wave function and the Born Rule.

 

[bhobba]

Hobson is wrong. QFT does not remove the need for quantum interpretations.

Since his book, which I often recommend, is itself an interpretation of QFT, it obviously can't.

Its advantage is that QFT is more fundamental than ordinary QM, which is the theory most interpretations apply to.

Nor is the book without issues, but IMHO, it should be in the library of those seriously interested in the foundations of QM.

Thanks
Bill

 

[Demystifier]

Since his book, which I often recommend, is itself an interpretation of QFT, it obviously can't.

Its advantage is that QFT is more fundamental than ordinary QM, which is the theory most interpretations apply to.

Nor is the book without issues, but IMHO, it should be in the library of those seriously interested in the foundations of QM.

Thanks
Bill

What does it say, in anything, about the measurement problem in QFT?

 

[bhobba]

I hear sometimes that somehow QFT resolves the measurement problem or it resolves quantum weirdness but I have never actually seen a rigorous treatment that made things any more natural and intuitive for me. QFT has always just made things more complicated in general since the mathematical machinery is quite a bit more complicated. If such a rigorous treatment exists, I would certainly like to see it.

It doesn't.

It does resolve some things, like what a particle is.

Its main advantage is that we know ordinary QM is wrong, e.g., it can't account for spontaneous emission. So interpreting ordinary QM can only be seen as a sort of 'warm-up' exercise.

Thanks
Bill

 

[bhobba]

I am curious whether undergrad physics students take courses on the philosophy of science. For example, does your syllabus include reading the seminal book, The Structure of Scientific Revolutions by Thomas Kuhn.

Physics degrees come in a number of flavours.

General physics degrees are often done in a double major with applied math. This can leave little room for philosophy of science.

But a double major in both physics and philosophy is not uncommon, especially with those not interested in doing experimental subjects. Sean Carroll, for example, did a degree in Astrophysics and Philosophy from Villanova University.

Regarding Kuhn's famous book, I don't think it ever really caught on among most physicists. Weinberg's view seems typical:

https://www.cs.utexas.edu/~vl/notes/weinberg.html

Thanks
Bill

 

[bhobba]

What does it say, in anything, about the measurement problem in QFT?

Basically nothing other than some dubious waffle about the simplest solution. No appeal to even Gleason, which is surprising. As I said, it has issues, but I still like it.

Thanks
Bill

 

[jeffn1]

Basically nothing other than some dubious waffle about the simplest solution. No appeal to even Gleason, which is surprising. As I said, it has issues, but I still like it.

Thanks
Bill

At some point I would like to start (or participate in) a thread dedicated to Art Hobson's views in Fields and their Quanta and his other writings (strengths/weaknesses).

At one point he "seems" to assert there is a causal relationship between entangled particles (even millions of miles apart). I believe he said this does not violate GR because they are not in the same "light cone".

Of course, he prefaces these point by saying it should be clear at this point that quantum physics is not constrained by locality (referring to the work of Bell and others). He said this is logical when viewed within the context QFT.

I would tend to think there is no need to assert a causal relationships between (distant) entangled particle. I did a bit of research and it was significant to me that when one quantum particle interacts with a detector (is "measured") nothing appears different with the entangled particle until it is measured. (There is no spontaneous collapse of the entangled particle, etc.).

Once the distant particle is measured, however, the relationship between the two entangled particles (e.g., spin) is apparent.

To my way of thinking, the most logical way of viewing this is that the two entangled particles retain their same relationship to their (let's say) infinite quantum field. This creates the correlation. So, when one particle is detected/measured, it does not "cause" anything to happen with regard to the entangled particle. But, it just reveals information about the entangled particle millions of miles away. [I always wonder if I am saying something that will get me in trouble here, hah].

 

[Sambuco]

No, this is not correct. The results of measurements in Bohmian mechanics are determined by the positions of the particles, but the probabilities of different possible results are still predicted by the wave function and the Born Rule.

What I'm referring to is not whether or not Born's rule is used in Bohmian mechanics, but how Born's rule is demonstrated from the other postulates. In that sense, within Bohmian mechanics, it's not possible to demonstrate that the outcome probabilities follow Born's rule solely from the wave function, as Zurek does. In fact, to prove Born's rule in Bohmian mechanics, you need both equivariance and the quantum equilibrium hypothesis.

Lucas.

 

[bhobba]

What I'm referring to is not whether or not Born's rule is used in Bohmian mechanics, but how Born's rule is demonstrated from the other postulates. In that sense, within Bohmian mechanics, it's not possible to demonstrate that the outcome probabilities follow Born's rule solely from the wave function, as Zurek does. In fact, to prove Born's rule in Bohmian mechanics, you need both equivariance and the quantum equilibrium hypothesis.

Bohmian Mechanics (BM) is an interesting case. @Demystifier may like to chime in, but Gleason's theorem evidently does not apply to BM, as its assumption of non-contextuality does not necessarily hold. The physicist Antony Valentini has been investigating the implications of this (which of course means BM is a separate theory and not an interpretation):
https://en.wikipedia.org/wiki/Antony_Valentini

His ideas, aside from the professional literature detailed in the Wikipedia article above, can also be found in a book I am currently reading (nearly finished):
Beyond the Quantum: A Quest for the Origin and Hidden Meaning of Quantum Mechanics
https://www.amazon.com.au/Beyond-Quantum-Origin-Meaning-Mechanics/dp/0198853742

It examines an idea called quantum death, where the Born Rule is something left over from the early universe.

Thanks
Bill

 

[Sambuco]

The physicist Antony Valentini has been investigating the implications of this (which of course means BM is a separate theory and not an interpretation)

It is worth noting that Valentini's proposal is not the only justification for Born's rule within Bohmian mechanics. There is also the typicality argument put forward by Dürr-Goldstein-Zanghì.

Lucas.

 

[selfsimilar]

like what a particle is

If in QFT the particle is generated from the field so the ultimate question then is what is a field made of. And how is it that multiple fields exist in the same space ....etc

 

[martinbn]

If in QFT the particle is generated from the field so the ultimate question then is what is a field made of. And how is it that multiple fields exist in the same space ....etc

The fields could be the fundamental entities. They wouldn't be made of anything else.

Why is it a problem to have many fields in the same space?

 

[jeffn1]

Basically nothing other than some dubious waffle about the simplest solution. No appeal to even Gleason, which is surprising. As I said, it has issues, but I still like it.

Thanks
Bill

My impression: He highlights that the pre-detection state is non-local. He stresses repeatedly that failure to accept this historically has pushed the physics community in wrong directions. He references the Rarity-Tapster-Ou (RTO) Experiments (and of course Bell's and Anspect's work). In referencing Schrodinger's Cat and the "meaurement problem" (which he calls the "detection problem"), he states: "contrary to prior beliefs, the predetection state is not a problematic macroscopic superposition, but is instead just what one suspects before detection: a nonlocal entanglement between a superposed quantum system and the quantum component of a detector. This understanding will resolve the detection problem."

He goes on to say: "the cat state is not a superposition of states.....It is a superposition of correlations between states."

All this is a bit (more than a bit) above my paygrade. But, I'll keep trying. Hah!

 

[iste]

I am curious whether undergrad physics students take courses on the philosophy of science. For example, does your syllabus include reading the seminal book, The Structure of Scientific Revolutions by Thomas Kuhn. Physics is sort of the cutting edge for these type of issues. Of course, Newton to Einstein was a classic example of a Kuhnian "paradigm shift". I might tend to argue that particle oriented quantum mechanics to field-based quantum field theory represents a similar shift. Thoughts?

Thomas Kuhn lecture about early days of quantum physics.

 

[Roberto Pavani]

The fields could be the fundamental entities. They wouldn't be made of anything else.

Why is it a problem to have many fields in the same space?

If fields are regarded as fundamentally distinct entities, what is it that allows them to interact with each other at all?
I'm not suggesting that they must be made of a common substance. However, the very possibility of energy transfer between distinct entities makes me wonder whether they share some deeper common framework.

 

[weirdoguy]

some deeper common framework

If there exists some deeper common framework one can ask the same question: what is it that allows this deeper common framework to exist and work the way it works?

When this pyramid of "ever deeper common frameworks" will end?

 

[Roberto Pavani]

Fair enough. Every theory has to stop somewhere and take something as fundamental.

My only point is that, when several distinct fundamental fields are required, I naturally wonder whether a more economical framework might exist, one in which those fields emerge from a smaller set of fundamental assumptions.

 

[martinbn]

If fields are regarded as fundamentally distinct entities, what is it that allows them to interact with each other at all?
I'm not suggesting that they must be made of a common substance. However, the very possibility of energy transfer between distinct entities makes me wonder whether they share some deeper common framework.

How is it classical physics? You have all kineds of different entitiwa that interact. The sun emits light, it gets to earth and interacts with it.

 

[Roberto Pavani]

Sure, classical physics also contains different interacting entities.

My point was not that interactions are mysterious or impossible. Rather, the history of physics contains several examples where entities initially regarded as distinct turned out to be different aspects of a more unified structure.

For example, electric and magnetic fields were once viewed as separate entities and are now understood as components of the electromagnetic tensor.

So when I see several fundamental fields interacting with each other, I naturally wonder whether they are truly fundamental and distinct, or whether a more economical underlying description might exist.