What is the speed of darkness?

Most of us already know that darkness is the absence of light, and that light travels at the fastest speed possible for a physical object. So, what does this mean? In short, it means that, the moment that light leaves, darkness returns. In this respect, darkness has the same speed as light. However, in some instances, darkness actually moves faster than light. That might sound like the tagline of a grim and gritty movie that's trying way too hard, but it also happens to be true.

Darkness travels at the speed of light. More accurately, darkness does not exist by itself as a unique physical entity, but is simply the absence of light. Any time you block out most of the light – for instance, by cupping your hands together – you get darkness. In the context of talking about speeds, darkness is what you get after the light stops coming, and therefore travels at the speed of light. For instance, consider that you are in distant space, far from all light sources such as the sun, and you have a light bulb on the nose of your space ship. The light from the light bulb is spreading out in all directions through space at the speed of light. If you briefly turn off your light bulb and then turn it back on, there is light traveling out in all directions from before you dimmed the bulb, and behind it there is light traveling in all directions from after you dimmed the bulb. But between the two spheres of light there is no light, because no light was created when the blub was briefly off. And no light means darkness. So there is a band of darkness in between the two spheres of light. Since both spheres of light are expanding outwards in all directions at the speed of light, the band of darkness between them must also be traveling at the speed of light. You can think of darkness as what you get right after the last bit of light arrives. Since the last bit of light travels at the speed of light, the state right after must also travel at the speed of light.

If you turn on a flashlight, the shadow region created by an object reaches a distant wall at the exact same moment that the light reaches the wall, indicating that darkness travels at the speed of light. Public Domain Image, source: Christopher S. Baird. If the sun suddenly disappeared, it would stop shining light on the earth and the earth would go dark. But it takes 8 minutes and 19 seconds for the light from the sun to reach earth. The last bit of light given off by the sun right before it disappeared would take 8 minutes and 19 seconds to reach us, and the darkness that comes right after the last bit of light would also take 8 minutes and 19 seconds to reach us. We on earth would not see the sun disappear from the sky until 8 minutes and 19 seconds after it had vanished.

Shadow Racing

This is a little hard to wrap your head around, but shadows can move faster than the speed of light, even though nothing can move faster than the speed of light. In a second, we'll explain how exactly that's possible without breaking the most fundamental law of physics. But first, this thought experiment might make things clearer.

Imagine you have a light that's powerful enough to reach the planet Jupiter. Imagine also that it casts that beam in a cone that's broad enough to cover the entire diameter of the planet. When you pass your finger over the lens, the shadow will cross the entire diameter of the planet — a distance of 86,881 miles (139,821 kilometers). The speed of light is 186,000 miles per second (299,338 kilometers per second). So if it takes you less than half a second to move your hand that distance, then that shadow will have 'broken' the speed of light.

Shining a Light on the Subject

So, remember how we said that nothing can move faster than the speed of light? Well, that's the key. Shadows aren't anything. Shadows are the absence of something — specifically, photons, or particles of light. Since there's nothing that's actually traveling the distance, the only thing that's 'moving' is an area where photons aren't. There's no information that's being transmitted faster than light, only a blockage of information. That means your interplanetary shadow-puppet show doesn't break any physical laws — only the hearts of your interplanetary audience.

In order for something to have speed, it must be able to move relative to other bodies. Since darkness doesn’t actually exist, it cannot move and therefore cannot have a speed. However, since the level of illumination of a given area cannot change at a different rate than the speed of light, you could also equate it as having the speed of light, though less correctly.

I’ll use an example to make my point. Lets say you are in a room which is completely sealed off from the outside world, and nothing can enter it (particularly light). In this room you have a perfect light bulb. It has no warm up or cool down time, when you flip the switch, it instantly starts or stops producing a steady stream of light. In addition to this, let’s say that the walls do not reflect light, but instead completely absorb it. For convenience, let’s make the room round and have the light bulb at the center. Most of this can’t actually be done, but this is still a helpful thought experiment.

Now lets say we suddenly turn off the light bulb. The light will travel at a predictable speed from the bulb, and as it does, it will leave behind it an area without any light at all (meaning the light in the room is receding at the speed of light. Note that the darkness isn’t advancing, but the light is receding. As I’ve said earlier, darkness isn’t actually a thing; therefore, it cannot have a speed.

However the lack of light in our hypothetical room is spreading at the speed of light. Colloquially, this is a minor point. It is however a very important distinction. Darkness doesn’t exist, and therefore cannot spread or move, but light (which obviously does exist) can. And in doing so it can also leave an absence of light, and this absence will grow or shrink at the speed of light.

Since darkness isn’t a tangible thing, it is hard to properly quantify its characteristics. It doesn’t have a speed, but light will always advance or recede at the universal speed limit 186,000 miles per second, or 299,792,458 meters per second

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Written by Ms. Pratiksha Thapli

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