ELI5, does DARK have a speed at which it travels, in the same way that LIGHT does?

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Does dark travel at the same speed that it takes for light to disappear or move on? Or does it have its own metrics entirely?

In: Physics

7 Answers

Anonymous 0 Comments

“dark” is the absence of light. so it moves at lightspeed if you want to give it a speed, but in general it doesnt make much sense to define the speed of “nothing”.

Anonymous 0 Comments

Light is a wave (enough of the time for this to work). So imagine you have a long piece of rope attached to the wall near the floor and pulled out perpendicular. You can give that rope a snap and send a pulse down it. If The speed of the pulse represents the the speed of light, then what you asking is what speed does the rope return to flat. So yeah, I think as far as we can talk about the speed of the returning absence of something, in this case light, the speed of dark is the speed of light.

Anonymous 0 Comments

Adding to the other comments, there is a nice video on youtube about this exact question by vsauce. As far as I remember shadow’s can move across a surface faster than light, but not in a way that it would be able to transfer information.

Anonymous 0 Comments

I think the ‘speed of dark’ is probably the amount of time it takes to go from light to dark after moving light across a surface. So wouldn’t it kind of be the same?

Anonymous 0 Comments

From the incomparably hilarious Terry Pratchett: “Light thinks it travels faster than anything but it is wrong – no matter how fast light travels it finds darkness has always got there first and is waiting for it”.

Anonymous 0 Comments

Talking about a ‘speed of dark’ is similar to talking about how fast the spaces between cars on a freeway are moving.

Anonymous 0 Comments

There isn’t technically a “speed of dark” at all. Darkness is the *absence* of light, it is not in an of itself a thing, therefore it can not have a speed. The word “dark” describes the concept of not having light, but it doesn’t refer to any actual physical phenomenon, it’s just simpler to say “it’s very dark outside” than it is to say “it’s very not light outside.”

That being said, if you turn off a light there is a very small delay between when the light goes out and when your eyes see the change. It’s basically imperceptible, but it can be measured.

As to the second part of the question: the speed is the same as the speed of light.

Here is an example…

What is the speed of an open road? We can measure the speed of cars, but what is the speed of the space between the cars? Well technically that space isn’t anything, the road isn’t moving and the space between the cars is just filled with air that isn’t traveling along with them for the most part. But there is a space, it’s just not a physical thing, it’s a concept. And as the cars move, the concept of that space moves as well. And what speed does it move at? The same speed as the cars.

Think of it another way, if darkness is just “not light” then what we are really measuring isn’t the darkness, we are measuring the speed of the *last light* that was emitted. As soon as that light passes, it leaves darkness in its wake. So the darkness “spreads” at the same speed that the light recedes and vice versa.