9
votes
6answers
5k views

Why can't light escape from a classical black hole?

Photons do not have (rest) mass (that's why they can move at speed of "light"). So, my question is how the gravity of classical$^1$ black hole can stop light from escaping? -- $^1$ We ignore ...
22
votes
4answers
1k views

Why does the sky change color?

Why the sky is blue during the day, red during sunrise/set and black during the night?