Grant: Each of these pieces of glass is what's called a "polarizing filter", which means

when a photon of light reaches the glass, it either passes through, or it doesn't.

And whether or not it passes through is effectively a measurement of whether that photon is polarized

in a given direction.

Henry:  Try this: Find yourself several sets of polarized sunglasses.

Look through one set of sunglasses at some light source, like a lamp, then hold a second

polarizing filter, between you and the light.

As you rotate that second filter, the lamp will look lighter and darker.

It should look darkest when the second filter is oriented 90 degrees off from the first.

What you're observing is that the photons with polarization that allows them to pass

through a filter along one axis have a much lower probability of passing through a second

filter along a perpendicular axis – in principle 0%.

Grant: Here's where things get quantum-ly bizarre.

All these filters do is remove light – they “filter” it out.

But if you take a third filter, orient it 45 degrees off from the first filter, and

put it between the two, the lamp will actually look brighter.

This is not the middle filter generating more light – somehow introducing another filter

actually lets more light through.

With perfect filters, if you keep adding more and more in between at in-between angles,

this trend continues – more light!

Henry:  This feels super weird.

But it's not just weird that more light comes through; when you dig in quantitatively

to exactly how much more comes through, the numbers don't just seem too high, they seem

impossibly high.

And when we tug at this thread, it leads to an experiment a little more sophisticated

than this sunglasses demo that forces us to question some very basic assumptions we have

about the way the universe works – like, that the results of experiments describe properties

of the thing you're experimenting on, and that cause and effect don't travel faster

than the speed of light.

Grant:  Where we're headed is Bell's theorem: one of the most thought-provoking discoveries

in modern physics.

To appreciate it, it's worth understanding a little of the math used to represent quantum

states, like the polarization of a photon.

We actually made a second video showing more of the details for how this works, which

you can find on 3blue1brown, but for now let's just hit the main points.

First, photons are waves in a thing called the electromagnetic field, and polarization

just means the direction in which that wave is wiggling.

Grant: Polarizing filters absorb this wiggling energy in one direction, so the wave coming

out the other side is wiggling purely in the direction perpendicular to the one where energy

absorption is happening.

But unlike a water or sound wave, photons are quantum objects, and as such they either

pass through a polarizer completely, or not at all, and this is apparently probabilistic,

like how we don't know whether or not Schrodinger's Cat will be alive or dead until we look in

the box.

Henry: For anyone uncomfortable with the nondeterminism of quantum mechanics, it's tempting to imagine

that a probabilistic event like this might have some deeper cause that we just don't

know yet.

That there is some “hidden variable” describing the photon's state that would

tell us with certainty whether it should pass through a given filter or not, and maybe that

variable is just too subtle for us to probe without deeper theories and better measuring

devices.

Or maybe it's somehow fundamentally unknowable, but still there.

Henry:  The possibility of such a hidden variable seems beyond the scope of experiment.

I mean, what measurements could possibly probe at a deeper explanation that might or

might not exist?

And yet, we can do just that.

Grant:...With sunglasses and polarization of light.

Grant: Let's lay down some numbers here.

When light passes through a polarizing filter oriented vertically, then comes to another

polarizing filter oriented the same way, experiments show that it's essentially guaranteed to

make it through the second filter.

If that second filter is tilted 90 degrees from the first, then each photon has a 0%

chance of passing through.

And at 45 degrees, there's a 50/50 chance.

Henry: What's more, these probabilities seem to only depend on the angle between the

two filters in question, and nothing else that happened to the photon before, including

potentially having passed through a different filter.

Grant: But the real numerical weirdness happens with filters oriented less than 45° apart.

For example, at 22.5 degrees, any photon which passes through the first filter has an 85%

chance of passing through the second filter.

To see where all these numbers come from, by the way, check out the second video.

Henry: What's strange about that last number is that you might expect it to be more like

halfway between 50% and 100% since 22.5° is halfway between 0° and 45° – but it's

significantly higher.

Henry: To see concretely how strange this is, let's look at a particular arrangement

of our three filters:  A, oriented vertically, B, oriented 22.5 degrees from vertical, and

C, oriented 45 degrees from vertical.

We're going to compare just how many photons get blocked when B isn't there with how

many get blocked when B is there.

When B is not there, half of those passing through A get blocked at C.  That is, filter

C makes the lamp look half as bright as it would with just filter A.

Henry: But once you insert B, like we said, 85% of those passing through A pass through

B, which means 15% are blocked at B.  And 15% of those that pass through B are blocked

at C. But how on earth does blocking 15% twice add up to the 50% blocked if B isn't there?

Well, it doesn't, which is why the lamp looks brighter when you insert filter B, but

it really makes you wonder how the universe is deciding which photons to let through and

which ones to block.

Grant: In fact, these numbers suggest that it's impossible for there to be some hidden

variable determining each photon's state with respect to each filter.

That is, if each one has some definite answers to the three questions “Would it pass through

A”, “Would it pass through B” and “Would it pass through C”, even before those measurements

are made.

Grant: We'll do a proof by contradiction, where we imagine 100 photons who do have some

hidden variable which, through whatever crazy underlying mechanism you might imagine, determines

their answers to these questions.

And let's say all of these will definitely pass through A, which I'll show by putting

all 100 inside this circle representing photons that pass through A.

Grant: To produce the results we see in experiments, about 85 of these photons would have to have

a hidden variable determining that they pass through B, so let's put 85 of these guys

in the intersection of A and B, leaving 15 in this crescent moon section representing

photons that pass A but not B. Similarly, among those 85 that would pass through B,

about 15% would get blocked by C, which is represented in this little section inside

the A and B circles, but outside the C circle.

So the actual number whose hidden variable has them passing through both A and B but

not C is certainly no more than 15.

Grant: But think of what Henry was just saying, what was weird was that when you remove filter

B, never asking the photons what they think about 22.5 degree angles, the number that

get blocked at C seems much too high.

So look back at our Venn diagram, what does it mean if a photon has some hidden variable

determining that it passes A but is blocked at C?

It means it's somewhere in this crescent moon region inside circle A and outside circle

C.

Grant: Now, experiments show that a full 50 of these 100 photons that pass through A should

get blocked at C, but if we take into account how these photons would behave with B there,

that seems impossible.

Either those photons would have passed through B, meaning they're somewhere in this region

we talked about of passing both A and B but getting blocked at C, which includes fewer

than 15 photons.

Or they would have been blocked by B, which puts them in a subset of this other crescent

moon region representing those passing A and getting blocked at B, which has 15 photons.

So the number passing A and getting blocked at C should be strictly smaller than 15 +

15...but at the same time it's supposed to be 50?

How does that work?

Grant: Remember, that number 50 is coming from the case where the photon is never measured

at B, and all we're doing is asking what would have happened if it was measured at

B, assuming that it has some definite state even when we don't make the measurement,

and that gives this numerical contradiction.

Grant: For comparison, think of any other, non-quantum questions you might ask.

Like, take a hundred people, and ask them if they like minutephysics, if they have a

beard, and if they wear glasses.

Well, obviously everyone likes minutephysics.

Then among those, take the number that don't have beards, plus the number who do have a

beard but not glasses.

That should greater than or equal to the number who don't have glasses.

I mean, one is a superset of the other.

But as absurdly reasonable as that is, some questions about quantum states seem to violate

this inequality, which contradicts the premise that these questions could have definite answers,

right?

Henry:  Well...Unfortunately, there's a hole in that argument.

Drawing those Venn diagrams assumes that the answer to each question is static and

unchanging.

But what if the act of passing through one filter changes how the photon will later interact

with other filters?

Then you could easily explain the results of the experiment, so we haven't proved

hidden variable theories are impossible; just that any hidden variable theory would have

to have the interaction of the particle with one filter affect the interaction of the particle

with other filters.

Henry:  We can, however, rig up an experiment where the interactions cannot affect each

other without faster than light communication, but where the same impossible numerical weirdness

persists.

The key is to make photons pass not through filters at different points in time, but at

different points in space at the same time.

And for this, you need entanglement.

Henry: For this video, what we'll mean when we say two photons are "entangled" is that

if you were to pass each one of them through filters oriented the same way, either both

pass through, or both get blocked.

That is, they behave the same way when measured along the same axis.

And this correlated behavior persists no matter how far away the photons and filters are from

each other, even if there's no way for one photon to influence the other.

Unless, somehow, it did so faster than the speed of light.

But that would be crazy.

Grant:  So now here's what you do for the entangled version of our photon-filter experiment.

Instead of sending one photon through multiple polarizing filters, you'll send entangled

pairs of photons to two far away locations, and simultaneously at each location, randomly