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Albert Einstein played a key role
in launching quantum mechanics
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through his theory of the
photoelectric effect
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but remained deeply bothered by its
philosophical implications.
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And though most of us still remember
him for deriving E=MC^2,
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his last great contribution to physics
was actually a 1935 paper,
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coauthored with his young colleagues
Boris Podolsky and Nathan Rosen.
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Regarded as an odd philosophical
footnote well into the 1980s,
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this EPR paper has recently become central
to a new understanding of quantum physics,
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with its description
of a strange phenomenon
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now known as entangled states.
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The paper begins by considering a
source that spits out pairs of particles,
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each with two measurable properties.
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Each of these measurements has
two possible results
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of equal probability.
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Let's say zero or one
for the first property,
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and A or B for the second.
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Once a measurement is performed,
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subsequent measurements of the same
property in the same particle
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will yield the same result.
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The strange implication of this scenario
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is not only that the state
of a single particle
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is indeterminate until it's measured,
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but that the measurement then
determines the state.
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What's more, the measurements
affect each other.
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If you measure a particle
as being in state 1,
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and follow it up with the second
type of measurement,
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you'll have a 50% chance of
getting either A or B,
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but if you then repeat
the first measurement,
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you'll have a a 50% chance of getting zero
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even though the particle had already
been measured at one.
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So switching the property being measured
scrambles the original result,
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allowing for a new, random value.
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Things get even stranger when you
look at both particles.
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Each of the particles will produce
random results,
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but if you compare the two,
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you will find that they are
always perfectly correlated.
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For example, if both particles
are measured at zero,
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the relationship will always hold.
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The states of the two are entangled.
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Measuring one will tell you the other
with absolute certainty.
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But this entanglement seems to defy
Einstein's famous theory of relativity
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because there is nothing to limit the
distance between particles.
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If you measure one in New York at noon,
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and the other in San Francisco
a nanosecond later,
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they still give exactly the same result.
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But if the measurement
does determine the value,
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then this would require one particle
sending some sort of signal to the other
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at 13,000,000 times the speed of light,
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which according to relativity,
is impossible.
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For this reason, Einstein dismissed
entanglement as "spuckafte ferwirklung,"
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or spooky action at a distance.
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He decided that quantum mechanics
must be incomplete,
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a mere approximation of a deeper reality
in which both particles
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have predetermined states that
are hidden from us.
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Supporters of orthodox quantum theory
lead by Niels Bohr
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maintained that quantum states
really are fundamentally indeterminate,
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and entanglement allows
the state of one particle
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to depend on that of its distant partner.
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For 30 years, physics remained
at an impasse,
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until John Bell figured out that the key
to testing the EPR argument
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was to look at cases involving different
measurements on the two particles.
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The local hidden variable theories
favored by Einstein, Podolsky and Rosen,
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strictly limited how often you could
get results like 1A or B0
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because the outcomes would have to be
defined in advanced.
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Bell showed that the purely
quantum approach,
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where the state is truly
indeterminate until measured,
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has different limits
and predicts mixed measurement results
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that are impossible in the
predetermined scenario.
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Once Bell had worked out how to test
the EPR argument,
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physicists went out and did it.
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Beginning with John Clauster in the 70s
and Alain Aspect in the early 80s,
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dozens of experiments have tested
the EPR prediction,
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and all have found the same thing:
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quantum mechanics is correct.
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The correlations between the indeterminate
states of entangled particles are real
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and cannot be explained by any
deeper variable.
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The EPR paper turned out to be wrong
but brilliantly so.
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By leading physicists to think deeply
about the foundations of quantum physics,
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it led to further elaboration
of the theory
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and helped launch research into
subjects like quantum information,
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now a thriving field with the potential to
develop computers of unparalleled power.
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Unfortunately, the randomness of
the measured results
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prevents science fiction scenarios,
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like using entangled particles
to send messages faster than light.
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So relativity is safe, for now.
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But the quantum universe is far stranger
than Einstein wanted to believe.