How does darkness travel?

Recently, I got a question on Patreon asking what the speed of darkness is. It is a great question because it forces us to think deeper about what ‘light’ and ‘dark’ actually means. At the same time, it requires us to put our physics knowledge into context of everyday life – this part is what I love the most about questions like this one. It is simple on the surface and tricky underneath.

Let us attack this peculiar question by first understanding what darkness is, before we move on to figuring out how it travels. IF it travels.

Let’s go.

Darkness Is Nothing But Absence Of Light

The first thing we need to do, is to get straight what darkness actually is.

In our everyday conversations, light and dark are used as each other’s opposite. A room can be bright and full of light, or it can be dark and full of darkness. However, this opposite’ness of light and dark doesn’t work in physics. Here’s why:

Light is a particle. The particle’s name is photon and it’s basically just a small electromagnetic wave. It does not have a mass or a charge (you don’t get electrocuted or bumped into if you get hit by a sun-ray). We can detect light with our eyes when we see colors, and if we see enough colors at the same time, we experience it as white light. There is also light that we can’t see with our eyes, for instance UV rays or X-rays, but the principle is the same. They are just small packages of energy in the form of a massless wave traveling through air, glass or what ever transparent material we have. In vacuum, light travels at 300,000 km/second or 186,000 miles/second, and when it goes through materials such as glass it travels a bit slower.

The opposite of light -if we stay in physics lingo- would be perhaps something like a heavy particle with a charge. It would be something that is somehow traceable, measurable and possible to describe somewhere in the standard model of particle physics. Darkness is none of those things!

Darkness is just the absence of light, and this means that darkness is an expression to state that no photons are present. I guess in some ways, it is similar to how a dry surface is just a surface that does not contain water (or any other form of liquid). Dry is not a specific material, but all liquids are. We can measure how much a liter of fluid a weighs, but we can’t ask how much ‘dry’ weighs.

You can’t add ‘dry’ to a surface, you can only remove ‘wet’ from it.

And now you might see where this is going. Light is a particle, a photon, and darkness is nothing but a description of the absence of photons. Darkness is not a particle, it’s not a material that we can chase around in a laboratory.

You can’t add darkness to a room, you can only remove light from it.

Sigh. Now what?

Darkness Doesn’t Move

Well, we know now that light is a particle and darkness is simply the absence of light. From this follows a few things we can be sure of:

  1. When we remove light, darkness will dominate.
  2. When we add light, darkness disappears.
  3. Light (or photon) travels at 300,000 km/second or 186,000 miles/second.

If we were to switch off all (ALL!) light in the solar system, it would not get dark instantaneously. It would take some time for light to travel away from the sun and planets and all our gadgets here on Earth. In empty space, it would take a split second for the photons to have flown away, but it takes more than 100,000 years for photons to travel from the center to the surface of the Sun before they are released into empty space and can travel freely.

It would take MANY years, but the speed at which darkness will dominate our solar system is entirely determined by how fast photons travel away from us, i.e. the speed of light.

The answer to our question is simple and perhaps a bit boring: If you have a lit area and you remove all the light, darkness will propagate (or rather: be revealed) at the speed of light.

Photons determine the speed at which darkness can propagate.

There we have it. Simple and tricky.

Can Darkness Exist Without Light?

…but… what if light was never present in the first place? What if we have a room (or an entire universe, even) that never contained any photons at any point in time?

In that case, probably no one would even consider darkness as a description because it does not have a counterpart. Not in physics, not in everyday life. That universe would simply just…be.

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