Slow Glass – glass that is so dense that you see the past through it

Light takes ages to get through it so you see what happened seconds, minutes or years ago – a late mirror. Could be used to see yourself walk into a room, see where you lost stuff, or even bend light round a black hole to see how the universe started. Not sure how possible that is though. The idea came from this guy’s book: https://en.wikipedia.org/wiki/Light_of_Other_Days