Einstein was pretty unhappy with the way Quantum Mechanics was developing – which was a bit odd seeing as he came up with the concept of the photon and an explanation of the photoelectric effect – but I digress.
In trying to show those poor wayward Quantum Scientists where they were going wrong, Einstein came up with a number of “thought experiments” which tested and probed Quantum Mechanics to its limits. One such thought experiment involved the simultaneous emission of two oppositely polarised photons from a source (something which can be achieved, and which has actually been practically carried out in ground-breaking experimental work by Aspect et al) and then measuring the polarisation state of each photon when separated by a distance greater than that which would allow “communication” between the two photons during the polarisation measurement. In other words the polarisation state of each photon was measured in a time shorter than that which would allow a photon to travel between the two polarised photons being measured.
For many years I didn’t think this was much of an experiment. For conservation reasons the two photons will be emitted in opposite polarisation states from the source, so that if at some large separation distance I measure the polarisation state of photon A and then I measure the polarisation state of photon B it is hardly surprising that I find the polarisation states are separated by 90 degrees. This is in fact true and correct and shows that both common-sense and Quantum Mechanics agree for this special case. Now what I cannot simply explain is that if the polarisation measurements are made for angles other than 90 degrees (and in fact 45 degrees where again common-sense and Quantum Mechanics agree) we find a discrepancy between the common-sense expected result of the polarisation measurement and the Quantum Mechanical result. How very odd!! As mentioned above, this experiment has been carried out practically by Alain Aspect and his team, and the experimental results agreed with …………………………Quantum Mechanics. How extremely odd!!!!!
Now this is not the first time that a Quantum Mechanical result has gone against “common-sense” but the repercussions of this are a little more far-reaching than in some of the other cases. Einstein, together with Podolsky and Rosen (hence EPR) came up with this thought experiment to show an inconsistency in the Quantum Mechanical theory that required the theory to be “non-local” that is it allowed photon A to know what polarisation state photon B was in at any separation distance, even if that distance was greater than a photon could travel during the measurement time. Einstein having created the Special Theory of Relativity would have been extremely unhappy with this possibility existing within another theory (Quantum Mechanics) – and this was the whole idea behind the EPR thought experiment – to show that the current ideas of Quantum Theory were “incomplete” as they required QM to be a non-local theory.
If you go into great detail regarding this experiment and where it “goes wrong” as far as common-sense is concerned, there are just three basic principles, one (or more) of which must be violated. These principles are:
1) The Reality principle.
2) The Induction principle.
3) The Locality principle.
Reality – regularity of phenomena is due to an underlying physical reality.
Induction – it is possible to reach conclusions valid for all systems of a given type from a consistent set of observations on a large sample of systems of that type.
Locality – if two systems have for a time been in dynamical isolation from each other, then a measurement on the first system can produce no real change in the second.
Now it is the Locality principle that was being “probed” by Einstein’s thought experiment – so it seems extremely perverse to me that out of the three possible principles that could be at fault – this is the one chosen by the Quantum Mechanical theorists to be the “joker”.
Surely in this post-Matrix age where the possibility exists that we are all part of a computer simulation, it is the Reality principle that needs to be called into question – and that the EPR Paradox is actually an extremely testing experiment into the very reality of our Universe – not simply a statement about the light-like separation of particles that had once interacted.