Hello, I was unable to find a similar thread, so I would like to ask about this myself. I have several textbooks on SR and GR at my disposal but none of them gave me the answer to my question. I remeber from undergraduate course that SR brakes down if we want to include gravitation. I feel that I need to properly understand why before I continue my study of relativity. I think it might somehow go against the relativity principle but I'd really like to see some conrete reasoning behind it. Could you shed some light on it or refer me to a book that treats this topic thoroughly? Thank you for your (space)time.