The EPR paradox; mystery of science

                                     THE EPR PARADOX
In 1935, einstein,podolsky and rosen published the famous epr paradox which was desined to prove that the realist position is the only sustainable one , here I will describe a simplified version of epr paradox introduced by david bohm, consider the decay of the neutral pi meson into an electron and a positron:
Assuming the pion was at rest, the electron and positron fly off in opposite direction, now the pion has spin zero so conservation of angular momentum requires that the electron and positron are in the singlet configuration
If the electron is found to have spin, the positron must have spin down and vice versa, quantum mechanics cannot tell you which combination you will get in any particular pion decay  but it does say that the measurements will be correlated, and you’ll get each combination half the time (on average). Now suppose we let electron and positron fly way off- 10 meters in a practical experiment in space or in principal 10 light years and then you measure the spin of the electron say you get spin up , immediately you know that someone 20 meteers away or 20 light years away will get spin down , if that person examines the positron .
To the realist there is nothing surprising in this the electron really had spin up (and the positron spin down) from the moment they were created
The-EPR-paradox-space-scienceIt is quantum mechanics that did’nt know about this but the orthodox view holds that neither particle had either spin up  or spin down until act of measurement intervened. Your measuement of the electron collapsed the wave function and instanteously produced the spin of positron 20 meters or 20 light years away, einstein podolsky and rosen considered such “spooky” action-at-a-distance,they concluded that the orthodox position is untenable;the electron and positron must have had well defined spins all along whether quantum mechanics can calculate them or not.
The fundamental assumption on which the EPR argument rests is that no influence can propogate faster than the speed of the light, we call this principle of locality. You might be tempted to propose that the collapse of the wave function is not instantaneous but “travels” at some finite velocity. However this will lead to violations of angular momentum conservation,for if we measured the spin of the positron before the news of the collapse had reached it , there would be a 50-50 probablity of finding both particles with spin up, whatever one might think of such a theory in the abstract, the experiment are unequivocal no such violations occur the (anti)correlation of the spins is perfect . evidently the collapse of wave function – whatever its ontological status is instantaneous

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