Q & A: Are Shadows Matter?

Q:
- Anonymous
A:
To really understand what a shadow is, we really have to know what a shadow is not. When we look around, we see objects because light is bouncing off of them and that light is hitting our eyes. When something blocks the light that would shine on an object, there is a shadow. This is because a certain area has no light hitting it, and so no light can be scattered off of it for us to see.
Matter is usually defined as something that has mass and takes up space. So a shadow is not matter, since it has no mass and takes up no space. Instead, a shadow is a lack of light on a certain area.

math dan

Here's another way to look at it. I'm not sure what gets called "matter" and what doesn't, but I am sure that if it can travel faster than the speed of light it shouldn't get called "matter".  Take a little light bulb and very quickly pass your finger above it. On a screen far enough away, the shadow pattern moves faster than the speed of light. /mw

(published on 10/22/2007)

Follow-Up #1: superluminal transmission of shadows?

Q:
I quote your reply as below :- 'Take a little light bulb and very quickly pass your finger above it. On a screen far enough away, the shadow pattern moves faster than the speed of light.' My question is - Is it possible to send information faster than speed of light by using these shadow patterns which move faster than speed of light? That is by encrypting information in shadow patterns
- Indrajit Kuri
New Delhi, India
A:
That's a nice question. The answer is no.

The information flow from your finger out to the shadow occurs at the speed of light. Although the shadow pattern is moving across a screen faster than the speed of light, it's not transmitting information. Here's what I mean by that. Say you stood in the shadow and did something physical to alter the shadow near you, maybe shining a flashlight on it. That change in the pattern does not propagate with the pattern. So no information gets transmitted faster than c.

Mike W.

(published on 07/13/2011)