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What Is The Speed of Dark?
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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
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