Kind: captions
Language: en
in the 1930s Albert Einstein was upset
with quantum mechanics he proposed the
thought experiment where according to
the theory an event at one point in the
universe could instantaneously affect
another event arbitrarily far away he
called this spooky action at a distance
because he thought it was absurd it
seemed to imply faster-than-light
communication something his theory of
relativity ruled out but nowadays we can
do this experiment and what we find is
indeed spooky but in order to understand
it we must first understand spin all
fundamental particles have a property
called spin no they're not actually
spinning but the analogy is appropriate
they have angular momentum and they have
an orientation in space now we can
measure the spin of a particle but we
have to choose the direction in which to
measure it and this measurement can have
only one of two outcomes either the
particle spin is aligned with the
direction of measurement which we'll
call spin up or it is opposite the
measurement which we'll call spin down
now what happens if the particles spin
is vertical but we measure it spin
horizontally well then it has a 50%
chance of being spin up and a 50% chance
of being spin down and after the
measurement the particle maintains this
spin so measuring it spin actually
changes the spin of the particle what if
we measure spin at an angle 60 degrees
from the vertical well now since the
spin of the particle is more aligned to
this measurement it will be spin up
three-quarters of the time and spin down
1/4 of the time the probability depends
on the square of the cosine of half the
angle now an experiment like the one I'm
Stein proposed can be performed using
two of these particles but they must be
prepared in a particular way for example
formed spontaneously
out of energy
now since the total angular momentum of
the universe must stay constant you know
that if one particle is measured to have
spin up the other measured in the same
direction must have spin down I should
point out it's only if the two particles
are measured in the same direction that
their spins must be opposite now here's
where things start to get a little weird
you might imagine that each particle is
created with a definite well-defined
spin but that won't work and here's why
imagine their spins were vertical and
opposite now if they're both measured in
the horizontal direction each one has a
50/50 chance of being spin up so there's
actually a 50% chance that both
measurements will yield the same spin
outcome and this would violate the law
of conservation of angular momentum
according to quantum mechanics these
particles don't have a well-defined spin
at all they are entangled which means
their spin is simply opposite that of
the other particle so when one particle
is measured and it's been determined you
immediately know what the same
measurement of the other particle will
be this has been rigorously and
repeatedly tested experimentally it
doesn't matter at which angle the
detectors are set or how far apart they
are they always measure opposite spins
now just stop for a minute and think
about how crazy this is both particles
have undefined spins and then you
measure one and immediately you know the
spin of the other particle which could
be light-years away it's as though the
choice of the first measurement has
influenced the result of the second
faster than the speed of light which is
indeed how some theorists interpret the
result but not Einstein Einstein was
really bothered by this he preferred an
alternate explanation that all along the
particles contained hidden information
about which spin they would have if
measured in any direction it's just that
we didn't know this information until we
measured them now since that information
was within the particles from the moment
they formed at the same point in space
no signal would ever have to travel
between the two particles faster than
light now for a time scientists accepted
this view that there were just some
things about the particles we couldn't
know before we measured them but then
Along Came John
Bell with a way to test this idea this
experiment can determine whether the
particles contain hidden information all
along or not and this is how it works
there are two spin detectors each
capable of measuring spin in one of
three directions these measurement
directions will be selected randomly and
independent of each other
now pairs of entangled particles will be
sent to the two detectors and we record
whether the measured spins are the same
both up or both down or different we'll
repeat this procedure over and over
randomly varying those measurement
directions to find the percentage of the
time the two detectors give different
results and this is the key because that
percentage depends on whether the
particles contain hidden information all
along or if they don't now to see why
this is the case let's calculate the
expected frequency of different readings
if the particles do contain hidden
information now you can think of this
hidden information like a secret plan
the particles agreed to and the only
criterion that plan must satisfy is that
if the particles are ever measured in
the same direction they must give
opposite spins so for example one plan
could be that one particle will give
spin up for every measurement direction
and it's pair would give spin down for
every measurement direction or another
plan plan two could be that one particle
could give spin up for the first
Direction spin down for the second
direction and spin up for the third
direction whereas its partner would give
spin down for the first direction spin
up for the second direction and spin
down for the third direction all other
plans are mathematically equivalent so
we can work out the expected frequency
of different results using these two
plans here I'm visually representing the
particles by their plans their hidden
information with plan one the results
will obviously be different a hundred
percent of the time it doesn't matter
which measurement directions are
selected but it does for particles using
the second plan for example if both
detectors measure in the first direction
particle a gives spin up while particle
B gives spin down the results are
different but if instead detector be
measured in the second direction the
result would be spin up so the spins are
the same we can continue doing this for
all the possible measurement
combinations and what we find is the
results are different five out of nine
so using the second plan the results
should be different five ninths of the
time and using the first plan the
results should be different a hundred
percent of the time so overall if the
particles contain hidden information you
should see different results more than
five ninths of the time so what do we
actually see an experiment well the
results are different only 50% of the
time it doesn't work so the experiment
rules out the idea that all along these
particles contain hidden information
about which spin they will give in the
different directions so how does quantum
mechanics account for this result well
let's imagine detector a measure spin in
the first direction and the result is
spin up now immediately you know that
the other particle is spin down if
measured in the first direction which
would happen randomly one-third of the
time however if particle B is measured
in one of the other two directions it
makes an angle of 60 degrees with these
measurement directions and recall from
the beginning of this video the
resulting measurement should be spin up
three quarters of the time since these
measurement directions will be randomly
selected two-thirds of the time particle
B will give spin up 2/3 times 3/4 equals
half of the time so both detectors
should give the same results half of the
time and different results half of the
time which is exactly what we see in the
experiment so quantum mechanics works
but there is debate over how to
interpret these results some physicists
see them as evidence that there is no
hidden information in quantum particles
and it only makes sense to talk about
spins once they've been measured whereas
other physicists believe that entangled
particles can signal each other faster
than light to update their hidden
information when one is measured so does
this mean that we can use entangled
particles to communicate faster than
light
well everyone agrees that we can't and
that is because the results that you
find it either detector are random it
doesn't matter which measurement
direction you select or what's happening
at the other detector
there's a 50-50 probability of obtaining
spin-up or spin-down only if these
observers later met up and compared
notebooks would they realize that when
they selected the same direction
they always got opposite spins both sets
of data would be random just the
opposite random from the other observer
that is indeed spooky but it doesn't
allow for the communication the sending
of information from one point to another
faster than light so it doesn't violate
the theory of relativity and that at the
very least would make Einstein happy
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