Name: What Is The Speed of Dark?
Uploaded: 2014-09-27T13:20:58.000Z
Duration: 13 min 32 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

but there's another fear that's more chilling

light travels at the fastest speed possible for a physical object

darkness is erased when light appears and returns

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

that can move faster than light speed for instance

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

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

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

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

of the universe but that doesn't mean two shadows

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

that's how far away you have to be from the earth for it to no longer have a

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

night is just the Earth's shadow falling on you

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

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

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

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

cut to the chase because we are looking for fast darkness