Comments about "Popper's experiment" in Wikipedia

This document contains comments about the article "Popper's experiment" in Wikipedia
In the last paragraph I explain my own opinion.



The article starts with the following sentence.

1. Popper's proposed experiment

Popper first proposed an experiment that would test indeterminacy in quantum mechanics in two works of 1934.
You must first define what indeterminacy is. Next you can demonstrate this as part of the physical reality.
Popper wrote:
I wish to suggest a crucial experiment to test whether knowledge alone is sufficient to create 'uncertainty' and, with it, scatter (as is contended under the Copenhagen interpretation), or whether it is the physical situation that is responsible for the scatter.
Whatever the outcome scatter is a physical effect and has nothing to do with any human activity i.e. knowledge.
Popper's proposed experiment consists of a low-intensity source of particles that can generate pairs of particles traveling to the left and to the right along the x-axis. The beam's low intensity is "so that the probability is high that two particles recorded at the same time on the left and on the right are those which have actually interacted before emission."
Because the particle are recorded at the same time i.e. symultaneous the assumption that the particles have a common source is reasonable.
Popper argued that because the slits localize the particles to a narrow region along the y-axis, from the uncertainty principle they experience large uncertainties in the y-components of their momenta.
Such an argument is "wrong". You can only argue (based on certain physical laws) that the behaviour of the particles is different comparing slit size, but that has nothing to do with the uncertainty principle.
Popper suggests that we count the particles in coincidence, i.e., we count only those particles behind slit B, whose partner has gone through slit A. Particles which are not able to pass through slit A are ignored.
My interpretation is that only in case two particles are counted simulataneous they are accounted for.
The Heisenberg scatter for both the beams of particles going to the right and to the left, is tested "by making the two slits A and B wider or narrower. If the slits are narrower, then counters should come into play which are higher up and lower down, seen from the slits. The coming into play of these counters is indicative of the wider scattering angles which go with a narrower slit, according to the Heisenberg relations.
The prediction that the total number of counters used is a function of the slit size.
This is because from the initial entangled EPR state we can calculate the position of the particle 2, once the position of particle 1 is known, with approximately the same precision.
The reader should study the whole text.
The point is that the word entangled is used. What does that mean in this context?
The central issue of the experiment is that only simultaneous events or counts are considered? Is this the same that the particles are entangled or does entanglement means more?. The article does not explain that.
Popper was inclined to believe that the test would decide against the Copenhagen interpretation, as it is applied to Heisenberg's uncertainty principle. If the test decided in favor of the Copenhagen interpretation, Popper argued, it could be interpreted as indicative of action at a distance.

2 The debate

Many viewed Popper's experiment as a crucial test of quantum mechanics, and there was a debate on what result an actual realization of the experiment would yield.
This debate does not make mauch sense. The whole issue is to perform actual experiments.
They showed that once the uncertainty in the position of the source is taken into account, the blurring introduced washes out the Popper effect.
All of this should be based on real experiments. Not on a thought experiment.

3. Realization of Popper's experiment

Popper's experiment was realized in 1999 by Kim and Shih using a SPDC photon source. Interestingly, they did not observe an extra spread in the momentum of particle 2 due to particle 1 passing through a narrow slit.
The full experiment is not clear. Apperently there are two particles involved. What is not clear what they mean with extra spread.
They write: "Indeed, it is astonishing to see that the experimental results agree with Popper’s prediction."
Also here it is not clear what the prediction of Karl Popper is.
The text continues with:
"Through quantum entanglement one may learn the precise knowledge of a photon’s position and would therefore expect a greater uncertainty in its momentum under the usual Copenhagen interpretation of the uncertainty relations."
The first issue what has "quantum entanglement" to do with this?
The problem is also the "Copenhagen interpretation" and to what extend we know the precise position of a photon.
See also "EPR paradox": 4.1 EPR paper

4. Criticism of Popper's proposal

Tabish Qureshi has published the following analysis of Popper's argument:
This implies perfect correlation, meaning, detecting particle 1 at position x0 will also lead to particle 2 being detected at x0. If particle 1 is measured to have a momentum p0 particle 2 will be detected to have a momentum - p0. The particles in this state have infinite momentum spread, and are infinitely delocalized. However, in the real world, correlations are always imperfect.
The first issue is how do you know that a particle has a momentum sec?
A different issue is how do you know that the two momenta are each other opposite?
A whole different issue is how are correlations calculated?

5 Popper's experiment and ghost diffraction

6 Popper's experiment and faster-than-light signalling

The expected additional momentum scatter which Popper wrongly attributed to the Copenhagen interpretation can be interpreted as allowing faster-than-light communication, which is thought to be impossible, even in quantum mechanics.
What is meant with expected additional momentum scatter? The issue is that first you actual have to perform some experiment under different conditions and monitor the results.
In Reflection 2: Faster than light communication the possible outcomes of 3 different setups of Popper's experiment are discussed. In setup 1 the scatter areas are both identical and called area1. In setup 2 they are called area2. Setup 3 is a combination and the scatter areas are called area1' and area2'. The expected results are that area1'is equal to area1 and area2' is equal two area2. In that case no faster than light signalling is involved.
In order to clarify the situation actual experiments should be performed. You have nothing with a thought experiment.

7. See also

Following is a list with "Comments in Wikipedia" about related subjects

Reflection 1: Popper's Experiment.

The issue at hand is the explanation of Popper's Experiment. The problem is that it only makes sense to study real experiments not thought experiments.

Reflection 2: Faster than light communication

When you study Popper's experiment "Faster than light communication" is an issue. The problem is how do you perform an experiment to demonstrate that?
The central part of Poper's experiment is that it is symmetric. The experiment starts from a source which creates pairs of particles which are entangled. At each side there is a slit and there is an array of detectors. Initial condition is that the size of the slit, the distance of the slit from the source and the distance of the detectors each are identical.


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Created: 20 January 2017

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