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This episode is supported by 23andMe.
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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