- 3, 2, 1.

- Oh, Hey smart people Joe here.

This is an active nuclear reactor, and down there are uranium fuel rods undergoing fission reactions.

The blue glow that you see is not from the radio activity itself.

If there's not some blue light bulb down there gonna make it look cool.

- 3, 2, 1.

- Oh, That was amazing.

I should probably explain what happens though.

You are watching what happens when matter travels faster than the speed of light.

Yeah, I'm serious.

I can already hear you typing your comments okay.

You may have heard that nothing can travel faster than the speed of light.

It's the fastest speed there is, right?

Not exactly.

Things can travel faster than the speed of light right here on Earth.

And when that happens, it looks like this.

That glow is the echo of matter moving faster than light speed.

It's true.

This bizarre phenomenon is real, and this is how it happens without breaking any laws of nature.

(fantasy music) Now, to move something faster than the speed of light there's one trick that you need to do first you have to slow light down.

This is the number that we think of as the speed of light C, but more accurately, this is just the speed.

The light travels in a vacuum, and it's true.

Nothing can go faster than that but light doesn't always travel at that speed.

Anytime light travels through something transparent, it slows down.

Now, if you think about it, technically whatever speed light is going is the speed of light.

But when that light passes through something like water, glass, even air it's not the fastest possible thing anymore.

The reasons why this happened are astonishingly strange and they require you to think about light a bit differently than you might be used to.

So why does light slow down when passing through stuff and just going on intuition, you might think, okay when a moving thing cruising along hits a denser material it's gonna get bogged down.

There's more resistance, right?

As if you're sledding down an icy hill and you hit a patch of mud, you're gonna lose some speed.

But this isn't what happens to light even though light slows down in a medium light glass it comes out the other end and instantaneously it's moving just as fast as it came in.

So that analogy with the sled doesn't quite work.

Okay, maybe we can imagine photons of light passing through a medium, bouncing off of particles like a pinball machine before popping out the other end.

If that were true, light could still travel at sea the fastest speed there is but it would be taking a longer path from A to B, which would take longer.

But if that were what was happening a beam of light entering glass or water would get scattered in all different directions and that isn't what it actually does.

So our pinball model can't be right either.

To understand what's actually happening here we need to understand how electromagnetic waves like light travel across space.

You can think of a light wave as being made of both an electric field and a magnetic field.

Both are oscillating or wiggling in different directions.

These fields are directly related to each other.

A changing electric field produces a changing magnetic field which produces a changing electric field and so on.

This relationship between electric fields and magnetic fields is a fundamental property of the universe.

Now, as these two oscillating fields move across space that is what is known as an electromagnetic wave.

The speed that this wave moves across space is determined by how well those wiggling electric and magnetic fields can create each other in a vacuum.

There's nothing to get in the way of this feedback loop.

So the electromagnetic wave we call light moves as fast as the universe will let it the ultimate speed limit C but if that light wave passes through a medium like water the lights, electric and magnetic fields jostle the atoms and molecules of the water to create their own electric and magnetic fields, and well, that creates a mess.

All these fields tugging on each other essentially makes it harder for lights, electric and magnetic fields to generate each other compared to if they're in a vacuum with nothing in the way.

What we observe as a result of all of this is that whenever light passes through a transparent medium, it slows down.

Light travels at this slower light speed the whole way through the medium but because it doesn't get permanently altered by all of those other fields it interacts with as soon as the light wave gets to the other end it shoots back up to its original speed.

If your brain is hurting a tiny bit right now that is a completely normal reaction.

This stuff is really weird.

Now, how much the light slows down depends on the material.

In air, light travels just a smidge below the speed of light in a vacuum, but in water the speed of light is a full 25% slower and we can make particles travel faster than that Which is what is happening in a fission reactor.

Down below, uranium is getting split apart and releasing a bunch of heat, radiation and high speed particles like negatively charged electrons and their positively charged counterparts known as positrons.

Now, as those charged particles move through water whether they're going fast or slow, they pull on the water molecules so that the charges kind of align.

It's like if a celebrity walks through a crowd and everyone turns to look for a moment all the bodies are aligned.

Then after they pass by, everyone turns back to whatever random direction they were facing.

You know, when those water molecules relax back to whatever orientation they were before the charge particle passed by they give off a pulse of light.

If the charge particle's moving slower than whatever light speed is in that material we can't really see that ripple of light just radiates outward and dissipates.

Kind of like the ripples spreading around a swan that's drifting slowly across a lake, all ladi da.

But now imagine the swan hits the turbo and starts screaming through the water faster than the ripples can expand.

The ripples get all bunched up along the leading edge.

This is a shockwave.

We can see the same thing in 3D with sound.

If a jet or a bullet travels faster than the speed than the sound waves can travel then those sound waves bunch up and create a shock wave that we hear as a sonic boom in the pool around the reactor.

Something similar is going on with light in that reactor electrons and positrons can shoot out from those fission reactions faster than the speed of light in water, and as those particles tug on the water molecules, the ripples of light given off are moving slower than the particle is, so they pile up along that front edge a shockwave, just like a sonic boom except you see it instead of hear it a photonic boom maybe that's what that blue light is.

The first person to see this that we know of anyway was Marie Curie.

But it wasn't until the 1930s that the Soviet physicist Pavel Cherenkov finally explained why it happens which is why today we know this blue glow as Cherenkov radiation.

Now here in this nuclear reactor, Cherenkov radiation is mostly just a fun side effect, but Cherenkov's discovery actually won the Nobel Prize in 1958, and one reason that it was so important was because it opened a whole new window to the universe.

Every day high energy particles like neutrinos and cosmic rays launched long ago by distant supernovas, stars and black holes rained down on earth.

Something like a million cosmic rays passed through your body every night while you sleep, and trillions of neutrinos are flying through me and you every second Astronomers have been wanting to know where all these high energy particles come from for a long time.

Unfortunately, it's not that easy to study something that's not only invisible, but that whizzes by or through you at nearly the speed of light.

But luckily, those high energy particles can give of Cherenkov radiation, so that gives us a way to see them Cherenkov detectors are big, high tech thingies full of water as high energy particles shoot into them traveling faster than light can travel.

In that medium, they create a cone of Cherenkov radiation, like a wake that we can detect.

In other words, Cherenkov radiation.

The strange phenomenon that happens when things move faster than the speed of light is literally shedding light on invisible realms of the universe.

That's pretty cool.

Stay curious.

There you go.

Literally shedding light Which feels pretty good.

It's dumb.

Start this off with me glowing blue like a happy face.

So excited.