[THEME MUSIC] 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.