Given my profession, I am pleased that the recent movies Gravity, Interstellar, and The Martian have shown that outer space can be dramatically compelling even without aliens.
Interstellar is a movie about a black hole. There are also some human characters, but let me talk about my favorite character first.
Ordinarily, gravitational time dilation isn’t a big deal, even when orbiting a black hole. It’s only extreme very close to the horizon, and orbits usually become unstable a ways out where the effect is small. The exception would be a black hole spinning at very nearly the maximum allowed rate (i.e. very near the Kerr limit). Gargantua is such a black hole. Of course, having a planet orbiting this close in also requires that the tidal forces there be sufficiently small, which can only happen if the black hole is supermassive. Gargantua is a very massive, very high spin black hole. It’s surrounded by an opaque disk a little cooler than the sun emitting blackbody radiation. This is all confirmed in Kip Thorne’s book, where he provides an actual mass and spin. Overall, the science in the movie is good. You have to pretend to live in a world without Doppler shifts and gravitational redshifts, but the gravitational lensing is all correct. I’m happy to tolerate any imaginativeness about what happens inside a black hole, given that we don’t have (and will never have) any data on this, so I’ll just say that the resolution of the movie reminded me of Eddington’s joke about gravitational waves traveling at the speed of thought.
As reviewers noted, the emotional heart of the movie is the relationship between Cooper and his daughter Murph, whom he must abandon in order to save humanity. Some reviews complained about this relationship not being well drawn; maybe it’s just me having daughters, but I found the whole thing–their affection, her resentment, and his regret–very compelling, especially considering the amount of time the movie could afford to spend on it. In most movies, a chance to go into space and save humanity would feel glamorous and fun, while here it did feel to me like a sacrifice.
Anyway, at the end of the movie, Cooper is finally reunited with his daughter, only because of gravitational time dilation effects, she is now an old woman on the verge of death. He greets her on her deathbed, they speak for a short while, and then she sends him off. This is to spare him the sight of her death but also so she can spend her last moments with her own children and grandchildren. This dismissal feels incredibly jarring, given that this is the reunion we’ve been waiting the whole movie for, but it’s also profoundly right. When Murph was a girl, her father was the most important person in her life, and when he had to leave she was devastated. But she has long since grown up, made seminal contributions to physics, helped save the human race, and had her own children, grandchildren, and great-grandchildren. They have properly become more important to her.
My girls are and will always be the most important people in my life. Right now, I’m the most important man in the world to them, but in twenty years this will hopefully no longer be the case. If they call me once a week, they’ll be giving me as much attention as I give my own parents. Hopefully I will pass on my religion and culture to them, but I myself will rightfully diminish.
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