If you have 10 or 15 minutes to spare, go read this story. It's called Light of Other Days, and was written by the late science fiction author Bob Shaw.
As a parent, I was especially struck by this story. Don't worry if you don't like science fiction. There are no rocket ships or aliens, just slow glass. Slow glass is a glass that is so dense that light takes years to pass through it. Anyway, go read the story. Let me know what you think.
Hat tip to Thingamababy.
58 minutes ago