• Question: What is the speed of a shadow? Is it the speed of light, or half the speed because we have to see it? Or because it's nothing, just the absence of light is it faster than light?

    Asked by jakejones to Emma, Jimmy, Janet, Niall, Simon on 18 Mar 2013.
    • Photo: Niall Crawford

      Niall Crawford answered on 18 Mar 2013:

      You’re right, shadow is just an absence of light, and so it doesn’t move, so all you are seeing is the light moving around it, with it being light free. This means that the position of a shadow is dictated by the light around it, and therefore must look like it moves as fast as the light.

    • Photo: James Holloway

      James Holloway answered on 18 Mar 2013:

      Never heard this question before. You can actually make shadows appear to move faster than the speed of light.

      Imagine a vast set of blinds going all the way around the sun and they are all open. You could close a set of the blinds at one side of the sun and then the next and the next until you’ve gone all the way around. The shadow cast by the blinds would zip across earth, and would appear to do so faster than the speed of light if you closed the blinds quick enough.

      But you’re not actually breaking the speed of light at all because, as Niall said, shadow is the absence of light. Nothing is really moving.

Comments