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 and 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 hear on earth cast a shadow on to 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 shadowy 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 things 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 penumbra

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 kilometers long

that's how far away you 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

for 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 colors

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

light after the sunsets 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 that 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 thats 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 believes 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 constructed interference

but crests colliding with troughs cancel out

destructive interference if these waves or 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 to

seriously Wheeler is an extreme example

and was the inspiration for the dunning-kruger effect

novacist 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 over whelming 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 10 more

eisenstein 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 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 white

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 Levinbook 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 Stewart 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