Einstein was a pretty smart guy. He was also an enigma. Einstein was very unhappy with the probabilistic nature of Quantum Mechanics (QM) – yet he effectively kick-started QM with his paper describing the photoelectric effect. However, this did not prevent Einstein from coming up with a number of “thought experiments” to show where he believed Quantum Mechanics was incorrect, or at best incomplete. One of these thought experiments is now known as the EPR paradox or the EPR problem after Einstein-Podolsky-Rosen. The 1935 paper of Einstein, Podolsky and Rosen “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” questions a fundamental aspect of the QM description of the Universe by considering some of its tiniest constituents.
In the EPR problem we consider two particles (usually photons) created at time t=0 separating from one another after having been emitted in opposite directions. Nothing unreal or unphysical about this, experimentally it is relatively simple to prepare two photons in this state. Conservation of momentum means that the two photons will go off on their separate paths in opposing polarisation states and it is the prediction of this polarisation that gave cause for concern to Einstein.
Suppose that at some later time t, we measure the polarisation state of particle A. Then at this same time we can predict what the polarisation state of particle (photon) B will be by two methods. One method uses “common-sense” and our knowledge of polarisation of photons. The second method uses a full Quantum Mechanical description. The very bad news is that these two approaches do not give the same answer to the question. Of course it gets even worse. We now need to perform the actual experiment to see which description agrees with what we actually observe.
Alain Aspect and co-workers actually carried out a series of experiments based on this particular problem. As you can imagine, setting up these experiments was extremely difficult in the first place. However, they managed to create the right experimental conditions, made the measurements, and the results of their experiments agreed with – Quantum Mechanics! Oh dear, good old common-sense fails again.
Well, that’s an end to it then, isn’t it? If Quantum Mechanics gives us such a good description of what’s going on in the real world, is there any more to discuss? Perhaps it’s just worth checking out one last thing. Let’s see where our common-sense approach came unstuck, and in finding out where the problem lies we might discover something new.
The common-sense approach to describing the EPR problem only has three basic assumptions (one or more of which must clearly be incorrect or incomplete).
1) The Reality principle. Regularity of phenomena is due to an underlying physical reality.
2) The Locality principle. Any influence of particle A on particle B must not propagate between them faster than the velocity of light.
3) The Induction principle. It is possible to reach valid conclusions for all systems of a particular type from a consistent set of observations on a large sample of systems of that type.
Which of the above assumptions would you throw out? The founders of Quantum Mechanics decided to throw out assumption 2! That would not have pleased Einstein at all. Even though a “signal” as such (that is information) does not need to pass between particles A and B, they are still “linked” – in other words, having once interacted the “memory” of that interaction is taken forward in time no matter what the separation of the particles is. Personally, I would not have dispatched assumption 2, thus creating a “non-local” theory of Quantum Mechanics, quite so quickly given the other two choices.
We now live in a post-Matrix Universe, and thanks to the Wachowski brothers we have been given an insight into what it might be like to pass our lives in a computer generated world. I can accept that the velocity of light in vacuo is an ultimate velocity dictated by the fine structure of the Universe, but I am very uncomfortable with tenuous connections (however unsignal-like) between particles separated by superluminal distances. Personally I would throw out common-sense assumption number one and accept that we might possibly live in a Matrix Universe. However, whichever common-sense assumptions you throw out it remains that we live in a Universe stranger than you can imagine!!!!