The Tweet: “In some instances, darkness moves faster than light.”

It is possible for darkness to move faster than light. In this Friday long read we explain how this can happen and why something moving faster than the light barrier still fits with our understanding of the universe. But first, what is the speed of darkness?

“Strictly speaking dark cannot have a speed,” says Pete Edwards of Durham University. “It does not move or travel in any way. However, if we think of dark as the absence of light, dark is chased away by light and so it disappears at the same speed as light arrives. In this sense the speed of dark is equivalent to the speed of light.”

Edwards explains that every physical thing in the universe should comply with Einstein’s theory of special relativity; nothing can exceed the speed of light. What makes light special? It doesn’t have a rest mass – a mass when it isn’t moving.

“As the speed of an object… is increased the mass of the object also increases. For everyday speeds the increase in mass is insignificant, but as the speed of light is approached the mass of the object starts to increase very rapidly toward infinity. Consequently the force required to increase its speed still further also increases rapidly as we creep ever more slowly towards the speed of light. Theoretically we would require an infinitely large force to reach the speed of light. Therefore, it is impossible to accelerate any object with [a rest mass] to light speed. The speed of light really is the ultimate limit for all the ordinary matter in the universe.”

But darkness has no mass and isn’t like an ordinary object, it’s simply the absence of light. So perhaps the rules that apply to physical objects can be modified in the case of darkness. As Martin Stevens, of the YouTube channel Vsauce explains: “Light travels at the fastest speed possible for a physical object. The speed of dark is the speed of light, but there are other types of darkness that 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 but in the same amount of time. Make a shadow large enough and it could move across a surface faster than light,” he says, giving an example of how you could do this yourself: “If you here on Earth cast a shadow onto the moon” with your finger, that went from one side of the moon, point A, to the other, point B, your finger would travel only a few centimetres in a fraction of a second but your shadow would travel thousands of kilometres in the same amount of time. “Do it right and you will easily produce 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 can travel from you to that somewhere else.” The shadow we are casting on the moon is transferring no information from point A to point B. Sure darkness is travelling from point A to point B faster than light could travel from point A to point B, but darkness isn’t travelling from point A to point B. It is travelling between you and point A and point B, concludes Stevens.

“The darkness you are causing only changes shape when newly unblocked light fills the previous gap. That’s all a shadow is; a gap. So in a way a shadow doesn’t travel at all. it’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.”

So darkness is the absence of light, and it is possible to create an illusion, by casting shadows, of darkness travelling faster than the speed of light. This still complies with everything we know about physical objects, because darkness isn’t a physical object it’s just an abstract phenomenon to which we have given a name. So in a sense, darkness can travel faster than light, but it isn’t a physical thing. No physical thing can travel faster than light according to Einstein’s theory of special relativity.

Below, Stevens explains the speed of dark in full, giving more examples of how dark can move faster than light and it’s well worth a watch:

Image: David Melchor Diaz/ Flickr

This post was first published on The Untweetable Truth (13/02/2014)