File:Shadow propagation.gif

The shadow actually propagates at the speed of light. But the shadow's projection may appear to reach an infinitely large speed, even faster than the speed of light.

The image shows a light source being blocked by an object, with each frame representing 1 year. The left side shows a 3d perspective, while the right side shows an overhead view of the shadow's movement. The shadow's movement down the middle is at the speed of light, therefore the shadow (absence of light) propagates upwards a light year every frame. When the shadow is finally projected upon the back surface, the shadow's projected width is larger than 1 light year, making the shadow appear to have moved across the the surface faster than the speed of light. However, the shadow's true movement is not actually along the back surface, it is outward from the interference object. Therefore, only the shadow's projection can travel faster than the speed of light.

This diagram should only be used for helping understand shadow propagation, and does not realistically portray other physics.

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