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Quantum physics - Popper's experiment

Updated on June 8, 2009

Karl popper

philosopher of Science
philosopher of Science

Great Book on Carl popper

Philosophy and the Real World: An Introduction to Karl Popper
Philosophy and the Real World: An Introduction to Karl Popper

A very good book on, this and many other of Poppers Theorys and ideas, very readable and interesting, Highly recomended.



Before reading this hub I would advise reading these 2 hubs the sections stated as this will help with the understanding of this hub.

Karl Popper was a 20th centaury philosopher of Science and was a strict advocate of rigorous scientific method.  He like Einstein opposed the Copenhagen interpretation, to test quantum mechanics.

Poppers experiment is not as well known as the EPR paradox, due to the fact it is believed it was created on a flawed assumption. This means it is actually not a test of quantum mechanics, however it is still important and I have included it in this series because it demonstrates the problems with trying to “make sense” out of quantum mechanics.

Quantum mechanics is an extremely successful theory when it comes to predicting physical effects of atomic and subatomic interactions. There are various interpretations, which do not agree with each other but are almost experimentally indistinguishable in the results of there calculations.

The best known and most widely accepted version is the Copenhagen interpretation. The basics of this are that a quantum system is treated as a wavefunction which represents the entire system as a whole. Any disturbance or ‘measurement’ of the system causes the wavefunction to collapse over the entire system. This runs against common sense because it means that non-interacting systems are dependent upon each other no matter what the distance. This is what Popper didn’t like!

Initial set-up

Experiment with both slits equally wide. Both the particles should show equal scatter in their momentum.
Experiment with both slits equally wide. Both the particles should show equal scatter in their momentum.

The Experiment

So popper devised an experiment to test the Copenhagen Interpretation:

Popper proposed we set up the experiment you can see in the image on the right. We have a source in the centre which produces pairs of entangled photons; these pairs are then released in opposite directions known as the x-axis. The Y-axis runs at right angles to the y-axis and the photons are produced so that they have an entangled momentum along the y-axis of 0.  This is important because we are measuring the scattering of momentum along the y-axis after the slits. There is a sheet with a slit in it in the path of each beam of photons and behind this an arc of detectors to pick up the spread of momentum along the y-axis.

So the first thing that is done is we test to ensure that the photons are being released entangled, with a momentum of 0 along the y-axis, to do this we fire the photons towards slits of exactly the same width, there we measure the momentum spread. We do this by counting the particles which pass through B, whose entangled partner was also registered on the counter at A, the others are ignored. That way we only count the pairs who both particles of the pairs go through the slits. We then test that narrowing and widening the splits will affect the spread, which it does so now we are ready to perform the actual test.

Great book on This Experiment

Wittgenstein's Beetle and Other Classic Thought Experiments
Wittgenstein's Beetle and Other Classic Thought Experiments

A very intersting book on this and other great thought experiments, A great and interesting read. Take a look.


Testing Phase

Experiment with slit A narrowed, and slit B wide open. Should the two particle show equal scatter in their momenta? If they do not, Popper says, the Copenhagen interpretation is wrong. If they do, it indicates entanglement acts over distance.
Experiment with slit A narrowed, and slit B wide open. Should the two particle show equal scatter in their momenta? If they do not, Popper says, the Copenhagen interpretation is wrong. If they do, it indicates entanglement acts over distance.

Superb analysis of Popper's work

Karl Popper: Critical Appraisals
Karl Popper: Critical Appraisals

The best analysis of Popper's work and life. Great book well worth a read.


We now make slit A very small and slit B very wide, see image on right. We now perform the experiment as before. According to the EPR argument, we have measured position y for both particles (the one passing through A and the one passing through B), and not just for particle passing through slit A. 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 due to the entangled nature of the photons. We can do this, argues Popper, even though slit B is wide open.

Our measurements measure the y position of particle 2. Since it is, according to the Copenhagen interpretation, our measurement collapses the wavefunction of the entire system and because momentum of entangled pairs must be equal but opposite - we should expect the momentum of particle 2 scatters as much as that of particle 1, even though the slit A is much narrower than the widely opened slit at B.


If the Copenhagen interpretation is correct, then any increase in the precision in the measurement of the spread along the y axis will cause the spread in the wide slit to match that of the narrow slit, thus proving that the particles are dependent upon each other, because they must have the same momentum along the y-axis.

Popper believed that the test would decide against the Copenhagen interpretation, and this, he argued, would undermine 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.

This experiment was actually performed in 1999 and the results it produced where that instead of the spread increasing on the wider slit the spread actually decreased on the narrow slit. However there experiment was not perfect and the results just caused a heated debate.

However we know that this experiment was built on a apparent flaw due to the no-communication theorem which states that you cannot send information faster than the speed of light, which means that if you did this experiment over a massive distance and when the person at the one end say at mars and the other on earth, the person on earth doesn’t know what state the slit on mars is and the particle is released and the spread is measured on earth, this means he instantly knows the state on mars which is impossible because if this was the case you would be able to send “useful” information instantaneously over distance, this can’t happen.


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      dipless 6 years ago from Manchester

      @Clavicus I totally agree even though the man was not classically trained in Physics he obviously understood the principles to come up with the experiments proposed. I agree it is controversial and the ad hom attacks on him were totally out of line, thanks for commenting.

      @Yoron the whole theory behind the experiment was based on a floored assumption.

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      Yoron 6 years ago

      Some stupid Q.

      It seems to me that this hypothesis assume that a entangled particle must behave the 'exact same' no matter what it meets?


      In a positional system as 'SpaceTime' where we can define those particles as localized at different locations in the 'same instant' (even though those 'instants' won't correlate with each other from a observer) we shouldn't define their causality, after the measurement, to be the 'exact same'. The only thing we might expect, as it seems to me, is that there should be a equivalence, or symmetry, between them.

      So as long as the 'momentum' is the same, although expressed differently depending on locality, the prerequisites for a entanglement should be fulfilled as it seems to me?

      But Popper was a great theorist, so maybe I'm missing something here?

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      clavius 6 years ago

      it is a quite challenging experiment designed to test the standard interpretation & which has on that account occupied serious physicists (like those mentioned by Barnard S, but other as well) for some time. BTW that Popper thought the outcome would be other than what SEEMS to have been obtained (the matter is still quite controversial) does not justify Bernard's the ad hom that Popper, "being just a philosopher", "understood science" worse than most scientists.

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      dipless 8 years ago from Manchester

      AS I said in the hub, we know that the experiment was designed on a flawed assupmtion, however I felt that his interpretation warented conversation, as to how it fitted into the grand scheme of things. Thank you for your contribution

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      Bernard S. 8 years ago

      I disagree with Paraglider. Popper, being just a philosopher but not himself a scientist, he understands science WORSE than most scientists.

      Popper’s conclusion that either Copenhagen interpretation or Uncertainty Principle is wrong, is itself incorrect, insofar as it only adds to the confusion already started with EPR “paradox”. EPR has been factually proven wrong in the sense the QM wave function is complete. There is no hidden variable, and hence, no paradox. Correct is: Copenhagen and Uncertainty Principle are BOTH correct. But Popper and EPR paradox are both wrong.

      Not Copenhagen, but Popper is wrong! The passage through slit A causes the wave function 1 to collapse to ?y1=slitwidth A, thus resulting in ?py1=h/?y1 by virtue of Uncertainty Principle, and, by virtue of QE also ?y2=?y1 measured at slit B (wide open). However, this happens ONLY if one measures Q-entangled particles by coincidence measurement (see Shih & Kim experiment in Foundations of Physics, 29 (12), 1999, pp.1849-1861; see also interpretation by T. Qureshi in American Journal of Physics, Vol. 73, No. 6. (2005), pp. 541-544).

      Note, NOT ?py2= h/?y2 at slit B, because there is no position restriction of y2 by a physical slit B. Without Sih/Kim’s coincidence technique (not restricted to QE) a normal diffraction commensurate with actual/physical slit B will be obtained.

      Coincidence measurement needs coincidence signal be sent from one slit location to the other, which can only occur with speed smaller than c. Thus, Special Relativity is NOT violated.

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      Dave McClure 8 years ago from Kyle, Scotland

      Very nicely tackled. Popper was rare in understanding science almost better than many of the scientists. He deserves to be better known.

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      john 8 years ago

      A very good explination of a complex topic. Thanks.