Comments about "Popper's experiment" in Wikipedia

This document contains comments about the article "Popper's experiment" in Wikipedia

• The text in italics is copied from that url
  
• Immediate followed by some comments
  

In the last paragraph I explain my own opinion.

Contents

• 1. Popper's proposed experiment
• 2. The debate
• 3. Realization of Popper's experiment
• 4. Criticism of Popper's proposals
• 5 Popper's experiment and ghost diffraction
• 6 Popper's experiment and faster-than-light signalling
• 7. See also

Introduction

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.

• For a large slit size the number of counters used is small.
• For a small slit size the number of counters used is larger i.e. there is more scatter or spread.

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:

Okay.

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

• Copenhagen interpretation Comments
• Determinism Comments
• EPR paradox Comments
• Karl Popper Comments

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.

• Fig 4 shows the results of such an experiment. However the result is strange. What you would expect is that the surface of both curves should be the same. This is expected because both curves should be based on the same number of counts. My understanding should is that the height of the curve with the narrow slit (red dots) should be heigher as of the wide slit (blue dots). This is not the case. As such it is difficult to explain the results.
• IMO the explanation of Popper's experiment should be found in something physical. The experiment does not prove or disprove the Copenhagen interpretation nor has anything to do with the uncertainty theorem.
  In general when the slit is wide, most particles should pass through the slit undisturbed. Meaning a narrow band of particles which results in a small area where particles are detected.
  The smaller the slit the more the particles are influenced by the rim of the slit meaning a scatter of the particles. The result is a larger area where the particles are detected. This seems to me the most logical explanation.

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.

• The first test or correlation test is to establish that the particles are correlated. The details of such a correlation test are not discussed in the article. In the article the position x0 and the momemtum p0 are discussed but not how they are measured.
• Next we perform a sequence of three tests each with different setups. During each test the detector array is observed for a string of for example 100 particle pairs. In the first test the size of the slit in both sides 1 and 2 are large. In the second test the slit size is small and in the third test at side 1 the slit size is small and at side 2 large.
• Following are the expected results:
  1. In test 1 with a large slit size the detection area is small. The same for both sides (area1). This means that the particles more or less go undisturbed through the slit.
  2. In test 2 with a small slit size the detection area is "large". The same for both sides (area2). This means that the particles are influenced or recoiled by the rim of the slit.
  3. In test 3 we get the same result for side 1 (small slit size) as test 2 (area2') and for side 2 (large slit size) as test 1 (area1').

  Test	slit size 1	scatter 1	slit size 2	scatter 2
  1	large	small (area1)	large	small (area1)
  2	small	large (area2)	small	large (area2)
  3	small	large (area2')	large	small (area1')

  When you perform test 3 after test2, only at side 2 the slit size has to be changed. Only at side 2 the scatter area changes but not at side side 1.
  It is again important to remember this are the expected results.
  In test 3 there are two options:
  
  • The first option is that area2' is equal to area2 and area1' equal to area1.
    If this is the actual results then there is no communication involved between both sides 1 and 2.
  • The second option is that area2' is different from area2 and area1' is different from area1.
    Ofcourse such a result is strange because why should the scatter area of side 1 change when the slit size did not change. This requires a thorough investigation. Ofcourse the result of side 2 also requires additional research. area1' can be larger or smaller than area1. In any case you should perform test 2 again and observe that both scatter areas should become equal to area2.
    
  • Other options are that area2'is equal to area2 and only area1' is different from area1. The explanation is the "same"

  What is expected that Popper's experiment does not involve "Faster than light communication". What ever the outcome of these experiments they have no relation with the Copenhagen Interpretation.

Feedback

Go Back to Wikipedia Comments in Wikipedia documents
Back to my home page Index