You might want to consider that a "shadow"can only occur when there is some form of occlusion of light. A shadow is not necessarily the absence of light, since shadows have gradations (pure dark would imply the complete absence of any photons). Both the words "shadow" and "darkness" are not scientific descriptions without scientific modifiers.
No it is within the guide line, cause that is one of those equal and opposite things which are in Einstein's laws. The answer is they are the same speed, cause a shadow is the absence of light, and light is the absence of shadow. Most would used the word darkness for this but it refers to the same thing. However, I could see how a mod would see it as a question with no answer and remove it.