Hello Space Fans and welcome to another edition of Space Fan News.
In this episode, today I’m feeling froggy.
I’ve had a lot of rest from my holiday so I’m jumping in with both feet and we’re
going to take on the topic of quantum entanglement.
That feature of quantum mechanics that Einstein called a spooky action at a distance.
Well last month astronomers used very distant quasars to remove a possible bias in an experiment
designed to test the idea of quantum entanglement.
I told you I was feeling froggy, so let’s jump in.
Quantum mechanics is one of those things that most people don’t understand.
In fact the famous physicist Richard Feynmann said that, "If you think you understand quantum
mechanics, you don't understand quantum mechanics."
His test for whether we understood something about nature was to try to create a freshman
level lecture on the topic.
If he couldn’t do it, he’d say that we don’t understand it.
Quantum mechanics is one of those things that I’m not sure anyone really understands.
Not if they’re being honest anyway.
Still, the mathematics happen to explain a lot about the universe on the very tiny level,
so it sticks around.
One of the things that quantum mechanics says is that the universe’s smallest particles
have a limited set of innate properties, they can only be in just so many states and what
those states are is a mystery to us humans until we measure them.
When you do the math, one of the things that comes out is that if two particles have the
same set of properties, they are said to be entangled and their joint properties can be
described by the same exact math.
In fact they are so entangled you can’t tell them apart, they are the same particle.
And it gets weirder.
If you take those two entangled particles and move them to the opposite ends of the
universe and measure them again, they maintain this connection.
One particle on one side of the universe is perfectly connected to the other particle
at the other end of the universe.
This means that you can learn all about one particle by measuring the other one.
You’ll be able to infer all of its properties by studying the either one.
This is a very strange idea and intuitively goes against a lot of what we think we know
about how information is exchanged in the universe.
So when it comes to entanglement, one of two things is happening: either the particles
somehow communicate faster than the speed of light, which Einstein’s has shown to
be impossible, or there was something hidden, some information hiding from humans that we
weren’t accessing that ensured particles took on these properties no matter where they
were.
Scientists for a long time have thought there was no way to really test this idea, because
one of the annoying things about observing very small things, the act of observing them
influences the particle itself.
We need to get out of the process if we were going to learn anything about quantum mechanics.
But that’s easier said than done and it doesn’t stop stubborn physicists in the
slightest.
They keep trying to measure entanglement without being part of the process.
The usual experiment that is done is they pick some properties of particles that can
be measured, usually photons fired through a laser into two very distant detectors, and
try to measure both entangled photons at the same time.
Now as I said before, quantum mechanical particles can have a limited set of very defined properties,
and one of those properties that each particle can entangle themselves with is polarization.
Polarized light is a simple thing to imagine: it is a photon or beam of photons all wiggling
up and down in the same direction.
You can make polarized light by taking light from the Sun (which is mostly unpolarized-photons
are wiggling in all different directions) and reflecting it off of a surface.
A good example of that is water.
Any fisherman worth their salt doesn’t hit the water without a good pair of polarized
sunglasses because the light reflected off the surface of the water is all wiggling in
the same direction and you can cut down on the glare produced by that reflected light
by wearing polarized lenses that won’t let that light through.
Then, you can see deeper into the water and see those fish.
Or so they tell me, I don’t catch much fish, try as I might.
Anyway, back to entanglement.
Scientists can create entangled particles that have the same exact polarization.
Then, they send those two particles out to two different detectors very, very far away
setup to look at particles from two different angles.
If the particles make it through one angle they are entangled.
If the make it through the other one, they are unentangled.
So each detector sits and waits ready to look at both angles and which one they look at
is changed for each event.
If everything lines up, and two identical blips appear at the same time at the right
angle on each of the two very far away from each other detectors, then boom, you’ve
seen the entangled particles.
Sigh.
Well maybe you did.
Quantum mechanics is a statistical science so you need to see a certain threshold of
those particles or else you’re still not sure.
You could just be seeing random events.
So you need a threshold, and I don’t know what it is, but it doesn’t really matter
for the purposes of this episode because I’m about to introduce the astronomy part of this
whole thing.
Just remember that a certain number of these blips should occur for them to be entangled
otherwise they aren’t entangled.
Now on to astronomy.
Remember what I said before about the observer always getting in the way of directly measuring
quantum properties, like entanglement?
Well what if the apparatus used to measure these polarized particles, somehow influences
the measurement?
How can we get rid of that?
The first thing they do is randomly switch the detector between the two measurement angles.
But then, because scientists are a pain in the butt, you gotta ask, um, what if the random-number
generator determining the measurement angle isn’t really random; what if what we see
as randomness has actually been predetermined by the laws of physics that brought humans
to this point?
Right, so… and this is really cool.
Astronomers used as their random number generator the light from very distant quasars.
Here’s the setup:
They shined laser light into a special crystal that generated pairs of entangled photons,
which the scientists repeatedly sent through the open air toward two telescopes in the
Canary Islands.
While the photons were on their way, in mid-flight, the detectors in each observatory would decide
which polarization angle to look at by referencing light that had travelled from the distant
reaches of the universe from these quasars.
Two telescopes on La Palma in the Canary Islands: the Telescopio Nazionale Galileo, pointing
at quasars on one side of the sky that emitted their light 7.78 billion and 3.22 billion
years ago, and the William Herschel Telescope, pointing to a light source that emitted light
12.21 billion years ago.
If each telescope observed light that was slightly bluer than a reference color, its
corresponding detector would measure the light’s polarization in one setting.
If the light was slightly redder, then the detector would use the other setting.
Since the light that was influencing the measurement was billions of light years away, any hidden
force that could have influenced this measurement of both particles would have needed to happen
billions of years ago to somehow influence the way scientists measured these particles
here on Earth today.
So, how did they do?
In a test of 30,000 pairs of particles, they observed way over the threshold necessary
to prove Einstein wrong, there wasn’t some hidden information that humans didn’t have
access to.
The polarizations correlated too closely to be explained by one of these local hidden
variable theories.
It looks like Quantum entanglement is real and is embedded in the particles in a way
we’re still working out.
Whew!
That was a hard one and I hope I did the subject justice.
If you’re still scratching your head after this episode, don’t feel bad, I am too,
but I thought this was a great example of trying to use information from the distant
universe to influence experiments here on Earth.
I want to thank all of the Deep Astronomy Patreon Patrons for your continued support
of my humble channel, you are all rock stars to me.
Thanks to all of you for watching, and as always keep looking up!