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Hey, Vsauce. Michael here.

Nyctophobia is the fear of the dark.

But there's another fear that's more chilling.

It's the fear that darkness will go away.

Optophobia, the fear of opening your eyes.

Light travels at the fastest speed possible for a physical object.

Darkness is erased when light appears, and returns

when light leaves. The speed of dark

is the speed of light but there are other types of darkness

that can move faster than light speed. For instance,

a shadow.

Across a distance, a shadow can become much larger than the object creating it,

but still mimic its source, moving in the same way for the same amount of time.

So when a shadow is bigger than the object casting it, it moves a greater distance

when the object moves but in the same amount of time. Make a shadow large enough

and it can travel across the surface faster than light.

If you, here on Earth, cast a shadow onto the Moon,

not an easy thing to do, that pointed from, say, point A

on the moon's surface, and then you moved your finger so that the shadow moved

to point B, your finger would only move a few centimetres

in a fraction of a second. But the shadow it cast on the Moon

would move thousands of kilometers in the same amount of time.

Do it right and you're easily producing a shadow

that breaks the light barrier.

But nothing's wrong here.

The rule is that information can't travel faster than light.

You can't cause something to happen somewhere else

faster than light could travel from you to that somewhere else.

And our superluminal shadow is transferring

no information from point A to point B.

Sure, point B is being cast into darkness

sooner than a light speed message from A could warn him it's coming

but darkness isn't traveling from point A to point B.

It's traveling from you to point A and point B

at the speed of light. What we tend to call a shadow

is really just a cross-section of a three-dimensional region.

The darkness you are causing only changes shape

when newly unblocked light fills

the previous gap. That's all shadow is:

a gap. So, in a way, a shadow doesn't

travel at all. That's an illusion caused by us thinking

that a shadow is a physical thing, when in reality

a shadow is just the lack of physical things -

photons - which chug along at speed limit

of the universe. But that doesn't mean two shadows

can't kiss. Or, at least,

look like they are. Watch as Guy brings two shadows

near each other. Right before they actually make contact,

the shadows seem to magically bulge toward one another,

in a sort of smooch of darkness. What's going on is the

shadows blister effect and it has to do with the anatomy of a shadow.

The region where an object completely blocks a light source is called

the umbra. It's the darkest part of the shadow in the most prototypical

part of the shadow. Where only a portion of the light source is

blocked, we find the fainter penumbra. But as two or more penumbras

approach and overlap, the combined amount of light they block can be enough to

produce a perceivable difference,

the shadow blister. The Earth has a big

umbra, it's 1.4 million kilometres long.

That's how far away you'd have to be from the Earth for it to no longer have a

large enough apparent diameter

to block out all of the sun. Here, on the surface of Earth, we are nowhere near that

far away, which is why night is so umbral.

Night is just the Earth's shadow falling on you.

A you eclipse.

Sunsets are cool, they're beautiful to look at, but look the other way

and you can see the lumbering shadow of our planet.

Our atmosphere scatters shorter wavelengths of light

more than longer wavelengths, which makes the sky

appear blue. But in Earth's shadow there's less light to scatter and the sky

appears darker. During twilight you can see

the demarcation. While driving east from Denver to Kansas City

I got a particularly great view of it.

This is Earth's approaching night-making shadow.

The beautiful pink band above it? That's the belt of Venus.

It's caused by the sky reflecting the the colours

of the sunset behind us. You've probably noticed that

right after the sun sets, and disappears from view,

there's still light in the sky, scattered from the

no longer visible sun. This is what we call twilight

and there are many different stages of twilight.

If the sun is less than six degrees below the horizon it's technically

civil twilight. You can still do plenty of stuff outdoors without the need for artificial lights.

Down to 12 degrees below the horizon we have

nautical twilight: artificial lights are more or less necessary but the sky

still scatters enough light to be bright enough for ships at sea

to navigate by seeing a contrast of the horizon between dark sea

and faintly lit sky. Down to 18 degrees

an astronomical twilight is occurring. It looks like

night but the sky can still get darker. Until

astronomical twilight ends not all night time astronomical observations can be made.

Below 18 degrees is technically,

honestly, night. If you live at greater than 48.5 degrees north or south latitude,

during the summer the sun never goes more than 18 degrees below the horizon.

It's never technically night. Places like London only reach astronomical twilight at the most

during these months. So, if you live in one of these areas,

and you want to avoid doing something during the summer, just tell people

you'll do it tonight. You'll buy yourself a few weeks.

But that's slow darkness. Let's

cut to the chase because we are looking for fast darkness.

When scissor blades snip, the intersection point between both blades moves faster

than the blades themselves. Think of it this way:

if you had a pair of scissors with blades that were a light year long

and it took one second to close them, the intersection point would've traveled an entire

light year, in not a year but...

a second. No laws are being broken here because such a snip

would be physically impossible. As I've mentioned before,

rigid objects don't move instantaneously all over

when a push force is applied to them.

Instead, that force moves via electromagnetic forces, from one atom

to the next, and so on down the line. A compression wave

that travels at the speed of sound through the material.

But what if we ignored that problem by allowing the blades to simply be separately

already in motion? Well, their point at intersection

can still travel faster than light, because it's not a physical thing.

It's just a geometric point and it carries

no more information than you could already gather

by witnessing the approaching blades.

But don't count out that geometric point of intersection just yet. It's the key to

another type of darkness that can move faster than light.

When waves collide their crests can fuse into larger crests,

their troughs into larger troughs. This is constructive interference.

But crests colliding with troughs cancel out.

Destructive interference if these waves are light,

the result is darkness. And, in certain circumstances,

darkness created this way can travel like the intersection between two lines -

faster than light. Imagine these concentric circles as waves of light.

The lines are wave crests and the gaps in between

are troughs. When they meet the points where they intersect

flee up and down faster than the waves travel,

especially in the middle, which, in the case of light waves, makes them

faster than light. The superluminal speeds of these dark patches can be seen really clearly

if we make the wave crests of one source black

as well as the background. Overlapping regions where red

peeks through represent destructive interference -

darkness. And you can see how, especially in the middle,

this darkness races up and down faster than the waves.

In 1995 a man named McArthur Wheeler

robbed a bank in Pittsburgh. He was caught

because his only disguise was lemon juice.

He covered his face with it. He knew that lemon juice could be used as an invisible ink

when writing on paper, revealed by heating, and he knew so little about

why that worked and he knew so little about how cameras worked

that he assumed, with extreme confidence, that lemon juice

could make him invisible too.

Seriously. Wheeler is an extreme example

and was the inspiration for the Dunning-Kruger effect

Novices, people unskilled in particular disciplines

will often overestimate their knowledge and abilities

in said disciplines because they don't even know

how little they know, how much more there is to learn.

On the flip side, experts in particular field will often

underestimate their knowledge, have less confidence in their abilities,

or think that everyone else has the same level of knowledge that they do.

What drives the Dunning-Kruger effect

is the fact that often the more you learn about something the more you realise

just how rich and complex and overwhelming and full of as of yet unanswered questions it really is.

George Bernard Shaw once famously toasted Albert Einstein by saying

"Science is always wrong.

It never solves a problem without creating ten more."

Einstein didn't exactly disagree. He used

geometry to illustrate how ignorance grows faster than knowledge, saying

"as our circle of knowledge expands so does the circumference

of darkness surrounding it. Learning

studying, shedding light on a field of inquiry also reveals just how

in the dark we continue to be. How many shadowy things there are left

for us to illuminate. The diameter of light

never exceeds the shadowy circumference."

But what's the speed of that kind dark? The speed of the growth

of the number of things we know we are in the dark about.

What's the speed of ignorance?

If we define ignorance as the difference between questions we know to ask,

and answers we have, the field of agnotology,

the study of ignorance, suggests that the amount of

things we know we are in the dark about is growing faster

than the amount the things we have shed light on.

Is it a coincidence that the phrase "in the dark" originated during of all ages

the age of enlightenment? When Leeuwenhoek put a scraping

from his tooth under a collection of magnifying lenses he built,

he saw, for the first time in human history,

little moving creatures... microorganisms.

He called them 'Animalcules'. The discovery shed light on why

food spoiled life didn't spontaneously come from old meat,

it was already there, we just couldn't see it.

But the discovery also showed us that we were in the dark

about an entirely new realm of biology.

As Philippe Bourdeau has poetically put it, "enlightenment leads to benightedness

science entails nescience".

What's really cool about the expanding size of our nescience circumference

is what Stuart Firestein, the Chair of Biological Sciences at Columbia University, has said about it,

"it is there that science begins,

where the facts run out, just beyond them."

He says, "it is a mistake to bob around in the circle of facts,

instead of riding the wave to the great expanse

lying outside the circle."

If science is a road trip, facts are the photos

we take along the way, the fuel that drives it forward

is ignorance. Facts... more like

fax. Part of the past, not the way forward.

When it comes to understanding our world,

knowing why is obsolesce by asking why.

Knowing facts makes you bright, but the

equally quick, sometimes quicker, and most rewarding prize

is the dark.

And admitting that you don't know everything but that you would like to know some of it.

And as always,

thanks for watching.