Einstein, Podolsky and Rosen Paradox (EPR Paradox)
Einstein attempted to demonstrate incompleteness of the standard quantum mechanical description of physical reality in a famous paper written with B. Podolsky and N. Rosen, where a thought experiment is described with persuasive reasoning.
- Let us consider a particle at rest. Let us say this particle is inherently unstable, and disintegrates into two particles. (In reality, there exist particles of this kind. An example is pi-zero, which decays into two photons and has a lifetime of a tiny fraction of a second.)
- The fundamental laws of conservation, which are valid for all systems, guarantee that momentum and angular momentum must be conserved. That is, the sum total of these quantities for the two particles remains the same before and after the disintegration.
- Hence we can say that the momentum and angular momentum of the two resulting particles must be equal and opposite, since before disintegration they were equal to zero.
- Therefore, measurement of the momentum of one particle can be used to deduce the value of the momentum of the partner. Alternatively one can deduce the position of the target particle with certainty and unlimited accuracy by making a position measurement on its partner.
- This trick would bypass the uncertainty principle and show that the quantum theory is incomplete.
The statement that measurements done on one part of the separated system should not affect the other part of the system has been known as the reality condition.
The EPR paper was a powerful challenge to the Copenhagen group. Bohr refuted EPR by reiterating his philosophy: what is important is the whole set of conditions under which the measurement is made. He said that in the EPR scenario, the two particles form an irreducible quantum system with one wave function which incorporates both particles. Although no direct signal can travel between them, still one cannot ignore the influence of measurements on one or the other.
Einstein could never agree with this idea, calling it 'ghostly action at a distance'. But a physicist named John Bell proved in 1964 that Einstein's reality condition necessarily implies a relation among the results of a series of measurements. Numerous experiments have since been carried out to test Einstein's reality condition as well as Bell's theorem, and the results have proved that Einstein was wrong, and quantum physics was right.
Nevertheless, Bohr's Copenhagen interpretation was (and still is) not the only explanation for quantum phenomena, and even today scientists do not seem to agree on any one interpretation.
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