Quantum physics - Popper's experiment

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

• The Double Split Experiment – Read section on Copenhagen interpretation
• Bells Inequality Experiment – Section on Quantum Entanglement

Karl Popper was a 20^th 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!

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.

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.

• Quantum Physics - Central point for this series
• Quantum Physics - The Double Slit Experiment
• Quantum Physics - schrodinger's cat
• Quantum Physics - Bell's Inequality Experiment
• Quantum Physics - Quantum Suicide