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Hey.
I'm Dianna, and you're
watching Physics Girl.
I want to talk to you about
whether the laws of physics
are possibly wrong.
Like so wrong that say you made
a quantum computer to encrypt
your million dollar
self-driving A.I. butler,
but because we got the
laws of physics wrong,
it could be hacked like that.
Chances are the
laws are not wrong,
because we've tested
this stuff a lot.
But it's kind of the point
of science to be skeptical.
Sometimes you have
to revise laws.
Like when quantum mechanics
was first discovered.
Classical mechanics couldn't
fully explain light.
To this day,
physicists are testing
accepted laws of physics.
And this video is about
a brand new experiment
with breaking results
testing quantum behaviors
using lasers and telescopes
and stars 600 light years away,
and the Austrian National Bank.
 My name is David Kaiser.
I teach physics and the
history of science here at MIT.
 Dave Kaiser and his colleagues
have conducted an experiment
to test whether quantum
entanglement is real.
But first, we've got to get
on the same page about quantum
mechanics.
Quantum mechanics
is a physical theory
that describes how the tiniest
things in the universe act.
And some of their
behaviors are weird.
 You know, we really,
we've had quantum mechanics
in one form or another now
for 90 years, not quite
a century of quantum mechanics.
It has never failed us.
It has led us to predictions,
albeit probabilistic ones
in most instances, that
match observations sometimes
to extraordinary accuracy.
Sometimes we can match
to 13 decimal places.
That's amazing.
And that's by far the most
accurate scientific theory ever
developed by people and tested.
But it also has these
very strange sort
of inbuilt features that still
make some people uncomfortable.
 In other words, Dave agrees.
Quantum mechanics
is strange stuff.
Like quantum entanglement
and Schrodinger's cat.
Schrodinger's cat was
a thought experiment
devised by Erwin
Schrodinger when
he thought that quantum
mechanics had become just too
strange.
The math of quantum implies
something called superposition,
where a quantum particle
can be in two or more states
at the same time when
you're not observing it.
Take for example spin, which
is a property of particles.
If you don't know what it is,
you're probably the only one.
Just kidding.
So if you had an
electron in a box,
quantum theory says it can
be both spin up and spin down
at the same time.
Schrodinger thought
superposition was weird
because it's unintuitive
for something
to be two different
things at the same time.
It'd be like a cat in a closed
box being both dead and alive
at the same time.
But when you open the
box and observe the cat,
it instantly is just one state.
It's dead!
Or it's alive.
And this is real life stuff.
And it gets weirder with
quantum entanglement.
Imagine two kittens,
each in a box,
who happened to behave
like quantum particles.
Their two possible states
are asleep and awake.
Each kitten has
a 50% probability
of being awake and 50%
probability it's asleep.
They're in that
weird state of both.
And they're not just any quantum
kittens, they are entangled.
Which basically means that
their fates are connected.
Determine the state of one, and
you will instantly determine
the state of the other.
Mess with one, and you
will affect the other one
instantaneously.
So for example, you could
have two entangled kittens
wherein one has to have the
opposite state of the other.
So both kittens start
in a superposition
of awake and asleep.
You open one box, observe
kitten, find it asleep.
So the other one must be awake.
You could separate these
kittens to opposite sides
of the planet, and you'd
still get the same results
instantaneously.
Do you see why this is weird?
It's almost like the
kittens or quantum particles
communicated with each
other instantaneously
and telepathically.
This was enough to
freak Einstein out.
You may have even heard of his
over-quoted description of it,
"spooky action at a distance."
Quantum mechanics says
that entanglement is real,
even if it's weird.
And we've seen its effects
in many, many experiments.
But there's still a slim
chance it's just an illusion.
Like this trick.
It's funny, because
for a split second
the kid actually thinks
their head is full of coins.
So when it comes
to entanglement,
maybe the physicist
are the kids who
haven't figured out yet that
the hand brought the coin there.
Like maybe it just looks
like entangled particles can
communicate instantaneously,
but it's just some trick
of the universe pulling it off.
Dave Kaiser is checking
for a mystery hand
creating the illusion
of entanglement.
He's looking for
some unknown thing.
And he knows it's a
little far fetched.
 So I, it turns out, think
there's a lot of good reasons
to still believe and
have great confidence
in quantum mechanics.
And I had that
assumption going in.
 He may as well be
looking for a unicorn.
He's pretty sure
unicorns don't exist,
but he hasn't logically
dis-proven it yet.
Which for a physicist,
means that it's possible.
Unlikely, but possible.
So David and
experimentalists colleagues
came up with an idea
for an experiment
to test for this
invisible hand fooling us.
 And so we were able to
put the team together.
So by 2015, yeah, 2015 or
so we had things in place.
 So how do you
look for something
when you don't know
what you're looking for?
You do what you YouTube
is doing to you right now,
you collect statistics
in a clever way conceived
by physicist Jon Stewart Bell.
 Just over 50
years ago in 1964,
John Bell, a really great,
very creative physicist,
published an article
that was really
not very much noticed or
talked about in its day,
but now has come to be
this really incredibly
interesting thing.
And his argument was that one
could actually put to the test,
put to real experimental
quantitative test
a debate that had already
been raging for 30 years
before that.
 Bell figured out that if
measurements on close kitten
were independent
of faraway kitten,
then the results of tests
on the pairs of kittens
could only line up so often.
They'd obey an upper limit
known as Bell's inequality.
But if the kittens were
genuinely entangled,
then measurements on
the pairs of kittens
should line up more often
than Bell's limit would allow.
Quantum theory
predicts their behavior
should be more strongly
correlated with each other.
People have taken
lots of statistics
on entangled photons,
and everything so far
supports that
entanglement is real.
But physicists argue that there
is a slim yet real possibility
that these experiments
are flawed.
In come Dave and his colleagues.
They sent pairs of entangled
photons in two directions.
 So we set up in
the city of Vienna.
And we shot lasers in
the night sky of Vienna.
First of all, let's just
pause on that for a second.
DIANNA: Oh, that's amazing.
 It's amazing.
So we set up a very powerful
laser in Anton's laboratory
near the roof.
And that shot these entangled
particles, really specially
prepared laser light, half a
mile in this direction, a mile
in that direction.
 One got sent to a
telescope at a university,
and the other to
another detector
at the Austrian National Bank.
These detectors were
designed to make
a measurement like checking
whether the kitten is awake
or asleep.
But in this case, determining
the polarization state
of the photon.
The thing is, the
measurement depends on how
the detector is oriented.
And you have to set that
before your measurement, which
is the crux of the
experimental test here.
In the past, people have
worried that somehow, something
could have biased the
detector settings.
Either one detector was
transmitting information
to the other, or
the person operating
was somehow unconsciously
biased, or who knows why.
But Dave and his team came
up with this crazy idea
to make sure that their
detectors were not biased,
because human
influenced things are
notoriously bad at
generating random numbers.
So instead of using conventional
random number generators
to set the detectors,
they decided
to use giant burning
balls of plasma.
Yeah, stars.
Each detector had an
accompanying telescope
pointing at a different star
in our galaxy, closest of which
was 600 light years away.
And as is photon from the
star hit the telescope,
the detector would
reorient itself
depending on the random
color of the star light.
So cool.
They were using starlight as
a tool in their experiment.
The detectors were
randomly resetting
after the entangled
particles were admitted,
but before they arrived
at the detectors.
So the measurements
performed on each particle
couldn't have been
known at the time
the particles were created.
They counted them up and
look for correlations
between the measurements,
and they found them.
They found exactly
what quantum mechanics
said that they would find.
So in the end, the only
way that entanglement
could have been faked
is if some thing
had interfered over 600 years
ago to bias the detectors.
 Because 600 light years,
as I said, you know,
that's before there was a
Gutenberg printing press.
Or as I'm fond of saying, it's
when Joan of Arc was so young,
her friends called her Joanie.
I mean, that's really,
that's a long time ago.
 Yeah, like the knights in
the Crusades or the Mayans.
And they would have had
to been at that star
600 light years away.
Anyway, it seems
pretty convincing
that entanglement is real.
But Dave thinks he's
just narrowed the odds
of finding his unicorn.
He eventually wants
to use the oldest
light in the universe, maybe
even the cosmic microwave
background to set his detector.
That way we can make
sure that no bias
snuck into the experiment
since the big bang.
So part of this is just
that like quantum mechanics
came along and it was weird.
 Hmm-mm.
DIANNA: And it made
people uncomfortable.
 Yep.
DIANNA: And this is like, let's
make sure we're validating it.
Let's make sure that
it is true through all
these different
experimental means.
 Right.
DIANNA: And checking
any ways there could
be some weirdness sneaking
into the experiments.
 That's right.
I don't know anyone
in the field who says,
I think this is a likely,
a plausible alternative
to quantum mechanics.
And I don't.
I mean, I don't think
this is like really
how I think the world's
going to work if we could
zoom in and watch it unfold.
But there's still some, I
think, interesting motivations
to keep trying.
People around the
world, as you know,
are working hard to build
actual industries now,
at the basis for
which are things
like quantum entanglement.
The whole spheres of
quantum computing,
of quantum encryption,
of quantum teleportation,
of everything else people
dreamed up for "Star Trek"
many years ago.
All these ideas depend at their
core on quantum entanglement
being real in the
world, not just
some trick that
somehow we got duped
by in a handful of experiments.
 So no cracks in the
foundation of quantum mechanics.
Which is great,
because that means
I can still use it to
explain how lasers work.
But not now, I have some--
I got some cats to go play with.
But thank you for watching this
video, and happy physicsing.
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