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:
Î 0──e-+e+
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
1/(2)1/2(up-down-+-down-up+)
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
It 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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