Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [160]
Alas, Bohr's response to EPR was less than clear. Years later, in 1949, he admitted to a certain 'inefficiency of expression' when he re-read his paper. Bohr tried to clarify that the 'essential ambiguity' he had alluded to in his EPR rejoinder lay in referring to 'physical attributes of objects when dealing with phenomena where no sharp distinction can be made between the behaviour of the objects themselves and their interaction with the measuring instruments'.34
Bohr did not object to EPR predicting the results of possible measurements of particle B based on knowledge acquired by measuring particle A. Once the momentum of particle A is measured, it is possible to predict accurately the result of a similar measurement of the momentum of particle B as outlined by EPR. However, Bohr argued that that does not mean that momentum is an independent element of B's reality. Only when an 'actual' momentum measurement is carried out on B can it be said to possess momentum. A particle's momentum becomes 'real' only when it interacts with a device designed to measure its momentum. A particle does not exist in some unknown but 'real' state prior to an act of measurement. In the absence of such a measurement to determine either the position or the momentum of a particle, Bohr argued that it was meaningless to assert that it actually possessed either.
For Bohr, the role of the measuring apparatus was pivotal in defining EPR's elements of reality. Thus, once a physicist sets up the equipment to measure the exact position of particle A, from which the position of particle B can be calculated with certainty, it excludes the possibility of measuring the momentum of A and hence deducing the momentum of B.
If, as Bohr conceded to EPR, there is no direct physical disturbance of particle B, then its 'elements of physical reality', he argued, must be defined by the nature of the measuring device and the measurement made on A.
For EPR, if the momentum of B is an element of reality, then a momentum measurement on particle A cannot affect B. It merely allows the calculation of the momentum that particle B has independently of any measurement. EPR's reality criterion assumes that if particles A and B exert no physical force on each other, then whatever happens to one cannot 'disturb' the other. However, according to Bohr, since A and B had once interacted before travelling apart, they were forever entwined as parts of a single system and could not be treated individually as two separate particles. Hence, subjecting A to a momentum measurement was practically the same as performing a direct measurement on B, leading instantly to it having a well-defined momentum.
Bohr agreed that there was no 'mechanical' disturbance of particle B due to an observation of particle A. Like EPR, he too excluded the possibility of any physical force, a push or pull, acting at a distance. However, if the reality of the position or momentum of particle B is determined by measurements performed on particle A, then there appears to be some instantaneous 'influence' at a distance. This violates locality, that what happens to A cannot instantaneously affect B, and separability, that A and B exist independently of each other. Both concepts lay at the heart of the EPR argument and Einstein's view of an observer-independent reality. However, Bohr maintained that a measurement of particle A somehow instantaneously 'influences' particle B.35 He did not expand on the nature of this mysterious 'influence on the very conditions which define the possible types of predictions regarding the further behaviour of the system'.36 Bohr concluded that since 'these conditions constitute an inherent element of the description of any phenomenon to which the term "physical reality" can be properly attached, we see that the argumentation of the mentioned authors does not justify their conclusion that quantum-mechanical description is essentially incomplete'.37
Einstein mocked Bohr's 'voodoo forces' and 'spooky interactions'. 'It seems hard to look into the cards of the Almighty', he wrote