What is the speed of dark?

Category: Physics
Published: June 20, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

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.

Topics: dark, darkness, light, shadow, speed of dark, speed of light, sun