Hey Crazies.
Experience with physics has taught us that the universe is inherently local.
Each point in space can only affect its immediate surroundings,
so influence at a distance must propagate through the intervening space.
And that process is always limited by the speed of light.
Or is it?
Quantum entanglement might have something to say about this.
Before we get into this, I should mention that this video is part of a big collab for World Quantum Day.
But more on that later. Hmm. Where do I begin?
Everyone loves a good thought experiment.
Imagine there's a high energy photon just minding its own business, traveling along.
Then suddenly an atomic nucleus jumps out in front of it.
The nerve of some nuclei.
Anyway, the interaction transforms the photon into an electron positron pair.
So it collided with the nucleus and broke apart? Well, no, at least not in the way you're probably thinking.
This isn't like two objects being smashed together or something.
Particles never really touch.
Plus, it's not like this photon is made of an electron and a positron.
They're not inside there.
The photon just ceases to exist, and some of its properties are given to the new particles.
Of course, energy must be conserved.
Conservation of energy shall not be violated!
Linear and angular momentum are also conserved and the atomic nucleus helps with that.
Yes, this particle pair, this electron and positron, are antiparticles, but they're so much more than that.
They're entangled particles.
Their properties are linked. We know they must be opposite.
If the electron is oriented up, the positron must be oriented down.
If the electron is oriented down, the positron must be oriented up.
They share a single state.
It doesn't make sense to talk about them separately.
But this is quantum mechanics. So those two states are not the only ones available.
Say we don't know how either of them is oriented.
The property is in what we call a superposition.
Both particles are showing an essence of upness and downness at the same time.
Because, you know, quantum particles can do that.
They're still entangled, though, so their states are still opposite.
It's just unclear which of the base states we'll find when we measure them.
If you want more details, I have a video on superposition you can check out later.
You know, in case you need better nonsense.
Available at the Science Asylum store.
But there's really only one more thing you need to understand right now.
Superpositions are not measurable.
We can only create devices that we intuitively understand,
and measured particles will always be in states that those devices measure.
This random particle's orientation may be in a superposition,
but that all changes when it encounters one of our devices.
If the device measures up and down,
we'll find that the particle is either oriented up or down -
not in a superposition.
If the device measures left and right,
we'll find the particle is oriented left or right.
Not in a superposition.
The interaction with the device changes the particle.
Or at least it changes what we can know about the particle.
And before you ask, this has nothing to do with the person taking the measurement.
Quantum mechanics is not magic.
It's about the information that's available, not the information we have.
So now that we have entangled particles in a superposition, let's separate them.
And I mean separate them - like, a lot.
We'll put them so far apart that it would even take light
a noticeable amount of time to travel between them.
The earth-moon distance ought to do nicely.
That's a minimum one way trip of 1.3 seconds.
Next, we break the universe.
I measure the electron here on earth and find that it's oriented up.
In that case, I immediately know that the positron on the moon is oriented down.
If Astronaut Clone measures his positron, it must be oriented down.
The superposition has collapsed into this state.
How does that break the universe? How did the positron know to be oriented down?
But it was in that state the whole time, wasn't it? We just didn't know, right?
Well, that's certainly what a whole group of physicists thought back in the day.
To the timeline!
Quantum mechanics was born in the 1920s.
Almost a century ago.
The De Broglie wave hypothesis, Schrodinger's equation,
the Heisenberg uncertainty principle, all published in the 1920s
and all suggesting that the universe is inherently probabilistic.
It was a philosophy championed by Niels Bohr.
But then in 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a joint paper
where they outlined a thought experiment a lot like the one we just described.
This paper was designed to settle a debate.
Do superpositions physically exist?
Can a particle really be in a state that shows an essence of upness and downness?
If that's true, then my measured electron had to communicate with the positron on the moon instantaneously.
Just in case Astronaut Clone decided to measure his positron at the same time.
It didn't have the luxury of waiting the 1.3 seconds it takes light to get there.
Then again, maybe quantum mechanics is just a statement of our ignorance.
Maybe there are hidden variables at play.
The particles could have definite states the whole time.
We just didn't know what they were.
This thought experiment, now known as the EPR paradox, doesn't truly answer this question.
It certainly shows how counterintuitive superpositions are, but counterintuitive doesn't mean wrong.
Einstein was involved in the invention of modern relativity.
He should have known better.
Anyway, the debate wouldn't be settled until 1964 by John Stuart Bell,
who proposed a revised experiment.
It turns out that everyone was so focused on measuring up and down and left and right
that no one thought to measure along a diagonal.
For intelligent beings, humans can get some serious tunnel vision.
Anyway, let's redesign our detectors a bit.
Each of the two detectors is capable of taking a measurement along three different directions.
It doesn't matter which three directions, as long as there are three different directions
and the detectors are designed identically.
For simplicity, we'll make them all 120 degrees apart.
Which of the three directions is being tested will be selected at random for each particle
and the two detectors are changed independently of each other.
Next, let's label the outward orientation to be positive for each of the three directions,
which makes all the inward orientations negative by default.
Then we do this experiment a crap ton of times.
The question is: how often do these particles have opposite signs?
If the two detectors happen to be measuring the same direction, we'll get what we expect:
two entangled particles with opposite orientations, opposite signs.
But something interesting happens when the two detectors aren't measuring the same direction.
If they're not, then there's no requirement the signs be opposite.
They could be the same.
How often we expect those cases to have opposite signs depends on our philosophy.
So this experiment can test if there are hidden variables.
Assuming the superposition isn't physical and the particles had a definite state the entire time,
there are only six possible outcomes.
So all we have to do is count.
If the detectors measure the same direction,
the particles always have opposite orientations as required by the conservation of angular momentum.
If the detectors measure along different directions,
the particles only have opposite signs one third of the time.
That's a total probability of 5/9 or 55.6%.
If quantum mechanics only represents our ignorance,
then entangled particles will have opposite signs in this experiment 55.6% of the time.
But that's not what we get when we do the experiment.
When we go through and do this thousands of times, we get exactly 50%.
Entangled particles have opposite signs in this experiment exactly 50% of the time.
Isn't that close enough? Nope. Not this time.
Oh, no, what does quantum mechanics predict? It predicts exactly 50%.
As we saw earlier, the quantum entanglement looks like this.
We can also write it like this.
Or better yet, like this since "a" measures up and down.
In quantum mechanics, probabilities are complex squares,
so the probability of getting opposite signs when measuring along different directions would be written like this.
And here is where an author would say something like "The details are an exercise left for the reader."
But that's so not my style.
After a ton of work, which you can see right here,
we get exactly one half or 50%.
50% does not equal 55.6%.
Quantum mechanics reigns supreme.
But ... What does this all mean? It means classical physics is a goner.
Just put a fork in it.
But what this means for quantum mechanics is a bit more nuanced.
Locality is the idea that each point in space can only affect its immediate surroundings,
so any distant influence must travel through the intervening points in space.
Does Bell's experiment mean that entangled particles can interact faster than light? Yes, it does.
Our classical understanding of locality is done.
It can't possibly be true.
Does that mean superpositions are a real physical thing?
Not necessarily.
It's possible that entangled particles are just connected by a wormhole.
It's possible that quantum wave functions don't actually describe the particles,
but just describes a new force - some quantum force - that affects what the particles do.
It's possible that all outcomes of every circumstance exist in a quantum multiverse,
and we just don't know what universe we're in until the experiment is done.
What we know for sure is that Bell's experiment shows us that we need to relax our expectations.
Thanks for liking and sharing this video.
A special thanks goes out to my Patreon patrons and YouTube members like Fabio Manzini,
who's been pledging at the Asylum Orderly level for over two years.
Thanks, Fabio!
Don't forget to subscribe if you'd like to keep up with us.
And until next time, remember: It's okay to be a little crazy.
So crazy story.
I was trying to figure out what my April video was going to be, and then all of a sudden I get this Twitter DM from Kyle Hill.
Yes, that Kyle Hill, the one running The Facility.
He tells me that he's been looped into the White House's participation in World Quantum Day.
Yes, that White House. Home of the US president.
As per the website, World Quantum Day is an international community-driven event
intended to spark interest and generate enthusiasm for quantum mechanics.
So then Kyle says he wants to organize this big YouTube content drop.
Quantum content or "quantent," as we like to call it.
Of course I say I'm in because I'm just excited to be invited to a thing.
But lots of other creators are dropping stuff today too.
Check out hashtag World Quantum Day on all the platforms to see more stuff.
Many of you pointed out that it makes sense to think of the speed of light as infinite,
since light experiences no time as it moves from one place to another.
That's true!
While we see light traveling through space and time, the light itself doesn't experience either one.
It's almost as if it teleported. Anyway, thanks for watching!