How To Go Faster Than Light Speed
- [Joe] Thank you to FOREO for supporting PBS.
- [Speaker] Three, two one.
(reactor clicks)
- Whoa!
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 radioactivity itself,
and there's not some blue light bulb down there
to make it look cool.
- [Speaker] Three, two one.
(reactor clicks)
- Whoa!
That was amazing.
I should probably explain what happened 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.
That'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.
(lively music)
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 we think of as the speed of light, c,
but, more accurately,
this is just the speed light travels in a vacuum.
It's true, nothing can go faster than that,
but light doesn't always travel at that speed.
Any time 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, or even air,
it's not the fastest possible thing anymore.
The reasons why this happens 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?
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 like 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 particles like a pinball machine
before popping out the other end.
If that were true, light could still travel at c,
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 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 light's 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 of these fields tugging on each other
essentially makes it harder
for light's electric and magnetic fields
to generate each other,
compared to if they were 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.
When those water molecules relax
back to whatever orientation they were
before the charged particle passed by,
they give off a pulse of light.
If the charged particle is moving slower than
whatever light speed is in that material,
we can't really see that ripple of light.
It just radiates outward and, pft, dissipates.
Kind of like the ripples spreading around a swan
that's drifting slowly across a lake, all lah-dee-dah,
but now imagine the swan hits the turbo
and starts screaming through the water
faster than the ripples can expand.
The ripples all get bunched up along the leading edge.
This is a shock wave.
We can see the same thing in 3D with sound.
If a jet or a bullet travels faster than
the speed that 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 shock wave,
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,
rain down on Earth.
Something like a million cosmic rays
pass 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 off 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,
this strange phenomenon that happens
when things move faster than the speed of light,
is literally shedding light
on the invisible realms of the universe.
That's pretty cool.
Stay curious.
(lively music)
Ah, I'm glowing with thanks
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See you in the next video.
And thank you to FOREO for supporting PBS.
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There you go.
Literally shedding light. (PA speaker blares)
Which feels pretty good. That's dumb.
Start this off with me glowing blue, with like a happy face.
So excited.
