Compare Jean-Pierre Luminet's https://arxiv.org/abs/1804.03909 https://arxiv.org/abs/1804.03909 which discusses Interstellar starting at the bottom of the third

Compare Jean-Pierre Luminet's https://arxiv.org/abs/1804.03909 https://arxiv.org/abs/1804.03909 which discusses Interstellar starting at the bottom of the third page of the pdf.