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The Existence and the Nature
of Entanglements of Particles

When Erwin Schrödinger formulated the wave mechanics version of quantum physics in 1926 he did not specify what the wave function ψ represented. He thought its squared magnitude would represent something physical such as charge density. Max Born suggested that its squared magnitude represented spatial density of finding the particle near a particular location. Niels Bohr and his group in Copenhagen concurred and the notion that the wave function represents the intrinsic indeterminancy of the particle of the system came to be known as the Copenhagen Interpretation (CI).

Erwin Schrödinger disagreed with this interpretation, as did Albert Einstein. Both constructed gedanken (thought) experiments to discredit the logicality of the CI. Erwin Schrödinger's was one of a cat in a box with a mechanism that might or might not kill the cat. According to the CI until someone looked into box the cat would exist simultaneously in two states; one being alive and the other one being dead.

Einstein constructed more serious challenges to the CI. The most famous was published in an article by himself and two collaborators, Boris Podolsky and Nathan Rosen (EPR). In that article it was envisioned that two particles were created with the same wave function but opposite spins. The two particles were then separated. The spin of one particle was then determined. If, as according to the CI, the particle did not exist other than as wave function then the spin of the particle which was measured had to be communicated to the other particle so it could take the opposite spin. This would involve what the article referred to as "spooky action at a distance." The alternative to the CI was that the particles were material and each had their spins specified at their creation as apair and hence also at their separation.

Subsequently physicists envisioned an experiment in which the spin of one of the two particles was changed. If the particles carried the spins they had at separation then the change in the spin of one would not affect the spin of the other. On the other hand, if the two particles shared the same wave function, as in the CI, then the collapse of the wave function for one particle to a definite value of spin would induce the corresponding definite value of spin for the other particle. Erwin Schrödinger coined the term entanglement to denote what was postulated under the CI.

The issue was unresolved and probably thought by most physicists to be unresolvable for about three decades. Then in 1965 the Irish physicist, John Stewart Bell, formulated a test that in principle would resolve the issue. It was in the form of a theorem. If the alternative to the CI holds then a quantitative measurement would have to satisfy a particular inequality. The Bell test did not sit unimplemented for long. Within just a few years three groups designed experiments to test whether the assumptions of Bell's theorem had relevance for the physical world.

All of the subsequent tests of Bell's theorem utilized photons rather than the electrons in which EPR originally formulated the issue. There has been little or no concern about the possibility that photons differ so radically from electrons in this matter that the results from experiments involving photons have no bearing on the properties of electrons. Consider an application of the Uncertainty Principle to a photon. A photon travels at a precisely defined velocity and those generated by atomic transitions have a frequency also precisely defined. The momentum of a photon is hν/c, where h is Planck's constant, c is the speed of light and ν is the frequency associated with the photon. The uncertainty associated with a photon's momentum would appear to be near zero and thus according to the Uncertainty Principle the uncertainty associated with its location would seem to be extremely large. This situation is often interpreted as the saying the photon's location is spread over the entire universe. This is not really true because the variance of a probability distribution can be infinite without it being uniformly spread over the universe. For a probability distribution to have infinite variance it is only necessary for the probability density to go to zero more slowly than 1/x² where x is the deviation from the mean value for the distribution. But the Uncertainty Principle suggests that photons are more widely spread out than electrons and perhaps all photons could be considered to be entangled.

Using photons not only greatly simplifies the creation of a test apparatus; it simplifies the theoretical analysis. The effect of the alignment of the polarization detectors works out to be proportional to the cosine of the deviation between the angles of alignment of the two detectors.

The graph that shows the comparison of the CI versus the material particle theory is shown below.

It is astounding that the nature of the universe hinges on the relatively small difference between these two displays. The graph below shows the numerical difference between the two.

All but two of the first seven tests carried out found violations of the inequality of Bell's theorem or extensions of it. The astounding thing however is that none of the experimenters chose to validate their results by running independently generated photons through their apparatuses. There was always the possibility that the results for supposedly entangled photons was due to some peculiarity of the apparatuses. If independently generated photons gave the same results there could be two responses from physicists. One response would be that all photons are entangled. The second response would be that the phenomenon being measured is different from what was envisioned.

Here are the published experimental tests and the abstract of their results. In the abstracts there is a bit of jargon that has evolved on the topic. Local means no influence can propagate between two objects faster than the speed of light. Realism, realistic and reality refers to the continual material existence of particles, as opposed to the CI contention that particles do not have material existence except when they are subjected to measurement.

Although no actual experimental tests using independently generated photons appear in the literature there are a couple of articles by the same two physicists considering a theoretical analysis.

Some of the articles which question the entanglement theory and experimental results are:

Conclusions

The idea of entanglement seems to be generally confirmed. The missing piece is experimental results for independently generated particles. Pending that information one can only conclude that it is likely but not certain that entanglements of photons exists.

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