The EPR Paradox
Now
that you at least understand the uncertainty principle and interpretation of quantum
mechanics, it’s time to see how physicists eliminated the realist and
agnostic positions. Einstein in
particular was troubled by the orthodox interpretation of quantum mechanics,
and he along with two other realists, Podolsky and Rosen, wrote a paper that
was supposed to prove (relying on theory only) that the realist position is the
only possible position.
Consider
a decay of a particle with zero angular momentum into two other particles, for
example, the decay of the neutral pi meson into an electron and a
positron. The electron and positron both
MUST have spin, and by conservation of angular momentum, they must have
opposite spins. For example, if the
electron has spin up, the position must have spin down, and vice versa. Quantum mechanics doesn’t tell you which spin
combination you will observe, but it (with the classical physics it doesn’t
destroy) does tell you they will be correlated, and on average you’ll get each
combination 50% of the time. This has
been observed literally countless times, and has never, ever, ever been
observed to have been violated.
Now
assume that the pi meson decays and the two particles fly away from each
other. If someone at 20 m measures the
spin of the electron to be up, then someone at 20 m in the other direction had
better measure that the spin of the positron is down. To the realist, this isn’t a problem- the
electron has spin up since the moment it was created, and the positron has spin
down since the moment it was created, even if you don’t know why, and all you
did was observe them. However, for
someone in the orthodox position, this seems to be problematic. They believed that your measurement of the
electron at 20 m PRODUCED the spin up, and somehow or other also PRODUCED a
spin down in the positron, 40 m away!
Furthermore, what if you wait until the particles go 3 light-years away,
and then measure the electron, then have a friend measure the positron a second
later. In this case, the information
from your observation of the electron, to tell the positron to be spin up, must
travel faster than the speed of light to get there before your friend measures
it! Surely this is preposterous, because
one of the principles of physics, famously enshrined in relativity, is the
principle of locality- nothing with any influence can travel faster than the
speed of light. Short of rejecting locality,
then, one must reject the orthodox interpretation of quantum mechanics, and in
doing so one must embrace what Einstein famously called “spooky
action-at-a-distance”.
So
this is the EPR Paradox: either one must reject the orthodox interpretation of
quantum mechanics, or one must reject locality.
Unfortunately, as we now know, things break down in quantum mechanics,
and locality is one of them, but at the time this seemed like a decisive
victory for the realists… at least until Bell’s Theorem.
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