Let's say we have a cloud of dust which is a lightyear across and someone shoots a beam of light from point A to B , why it is not possible for an observer far far away to see the light while it travels through the cloud at the speed of light?
Answer
Sometimes we do, and the phenomenon is called a light echo.
What you're looking at there is NOT moving gas. It's an "echo" exactly as you describe.
The problem is that you need a pulse of light. If you have a constant stream of light, the "light echos" will be exactly like what you see in fog on earth.
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