Update (Nov. 15): A third of my confusions addressed by reading Kip Thorne’s book! Details at the bottom of this post.
On Saturday Dana and I saw Interstellar, the sci-fi blockbuster co-produced by the famous theoretical physicist Kip Thorne (who told me about his work on this movie when I met him eight years ago). We had the rare privilege of seeing the movie on the same day that we got to hang out with a real astronaut, Dan Barry, who flew three shuttle missions and did four spacewalks in the 1990s. (As the end result of a project that Dan’s roboticist daughter, Jenny Barry, did for my graduate course on quantum complexity theory, I’m now the coauthor with both Barrys on a paper in Physical Review A, about uncomputability in quantum partially-observable Markov decision processes.)
Before talking about the movie, let me say a little about the astronaut. Besides being an inspirational example of someone who’s achieved more dreams in life than most of us—seeing the curvature of the earth while floating in orbit around it, appearing on Survivor, and publishing a Phys. Rev. A paper—Dan is also a passionate advocate of humanity’s colonizing other worlds. When I asked him whether there was any future for humans in space, he answered firmly that the only future for humans was in space, and then proceeded to tell me about the technical viability of getting humans to Mars with limited radiation exposure, the abundant water there, the romantic appeal that would inspire people to sign up for the one-way trip, and the extinction risk for any species confined to a single planet. Hearing all this from someone who’d actually been to space gave Interstellar, with its theme of humans needing to leave Earth to survive (and its subsidiary theme of the death of NASA’s manned space program meaning the death of humanity), a special vividness for me. Granted, I remain skeptical about several points: the feasibility of a human colony on Mars in the foreseeable future (a self-sufficient human colony on Antarctica, or under the ocean, strike me as plenty hard enough for the next few centuries); whether a space colony, even if feasible, cracks the list of the top twenty things we ought to be doing to mitigate the risk of human extinction; and whether there’s anything more to be learned, at this point in history, by sending humans to space that couldn’t be learned a hundred times more cheaply by sending robots. On the other hand, if there is a case for continuing to send humans to space, then I’d say it’s certainly the case that Dan Barry makes.
OK, but enough about the real-life space traveler: what did I think about the movie? Interstellar is a work of staggering ambition, grappling with some of the grandest themes of which sci-fi is capable: the deterioration of the earth’s climate; the future of life in the universe; the emotional consequences of extreme relativistic time dilation; whether “our” survival would be ensured by hatching human embryos in a faraway world, while sacrificing almost all the humans currently alive; to what extent humans can place the good of the species above family and self; the malleability of space and time; the paradoxes of time travel. It’s also an imperfect movie, one with many “dangling wormholes” and unbalanced parentheses that are still generating compile-time errors in my brain. And it’s full of stilted dialogue that made me giggle—particularly when the characters discussed jumping into a black hole to retrieve its “quantum data.” Also, despite Kip Thorne’s involvement, I didn’t find the movie’s science spectacularly plausible or coherent (more about that below). On the other hand, if you just wanted a movie that scrupulously obeyed the laws of physics, rather than intelligently probing their implications and limits, you could watch any romantic comedy. So sure, Interstellar might make you cringe, but if you like science fiction at all, then it will also make you ponder, stare awestruck, and argue with friends for days afterward—and enough of the latter to make it more than worth your while. Just one tip: if you’re prone to headaches, do not sit near the front of the theater, especially if you’re seeing it in IMAX.
In the rest of this post, I’m going to list the questions about Interstellar that I still don’t understand the answers to (yes, the ones still not answered by the Interstellar FAQ). No doubt some of these are answered by Thorne’s book The Science of Interstellar, which I’ve ordered (it hasn’t arrived yet), but since my confusions are more about plot than science, I’m guessing that others are not.
SPOILER ALERT: My questions give away basically the entire plot—so if you’re planning to see the movie, please don’t read any further. After you’ve seen it, though, come back and see if you can help with any of my questions.
1. What’s causing the blight, and the poisoning of the earth’s atmosphere? The movie is never clear about this. Is it a freak occurrence, or is it human-caused climate change? If the latter, then wouldn’t it be worth some effort to try to reverse the damage and salvage the earth, rather than escaping through a wormhole to another galaxy?
2. What’s with the drone? Who sent it? Why are Cooper and Murph able to control it with their laptop? Most important of all, what does it have to do with the rest of the movie?
3. If NASA wanted Cooper that badly—if he was the best pilot they’d ever had and NASA knew it—then why couldn’t they just call him up? Why did they have to wait for beings from the fifth dimension to send a coded message to his daughter revealing their coordinates? Once he did show up, did they just kind of decide opportunistically that it would be a good idea to recruit him?
4. What was with Cooper’s crash in his previous NASA career? If he was their best pilot, how and why did the crash happen? If this was such a defining, traumatic incident in his life, why is it never brought up for the rest of the movie?
5. How is NASA funded in this dystopian future? If official ideology holds that the Apollo missions were faked, and that growing crops is the only thing that matters, then why have the craven politicians been secretly funneling what must be trillions of dollars to a shadow-NASA, over a period of fifty years?
6. Why couldn’t NASA have reconnoitered the planets using robots—especially since this is a future where very impressive robots exist? Yes, yes, I know, Matt Damon explains in the movie that humans remain more versatile than robots, because of their “survival instinct.” But the crew arrives at the planets missing extremely basic information about them, like whether they’re inhospitable to human life because of freezing temperatures or mile-high tidal waves. This is information that robotic probes, even of the sort we have today, could have easily provided.
7. Why are the people who scouted out the 12 planets so limited in the data they can send back? If they can send anything, then why not data that would make Cooper’s mission completely redundant (excepting, of course, the case of the lying Dr. Mann)? Does the wormhole limit their transmissions to 1 bit per decade or something?
8. Rather than wasting precious decades waiting for Cooper’s mission to return, while (presumably) billions of people die of starvation on a fading earth, wouldn’t it make more sense for NASA to start colonizing the planets now? They could simply start trial colonies on all the planets, even if they think most of the colonies will fail. Yes, this plan involves sacrificing individuals for the greater good of humanity, but NASA is already doing that anyway, with its slower, riskier, stupider reconnaissance plan. The point becomes even stronger when we remember that, in Professor Brand’s mind, the only feasible plan is “Plan B” (the one involving the frozen human embryos). Frozen embryos are (relatively) cheap: why not just spray them all over the place? And why wait for “Plan A” to fail before starting that?
9. The movie involves a planet, Miller, that’s so close to the black hole Gargantua, that every hour spent there corresponds to seven years on earth. There was an amusing exchange on Slate, where Phil Plait made the commonsense point that a planet that deep in a black hole’s gravity well would presumably get ripped apart by tidal forces. Plait later had to issue an apology, since, in conceiving this movie, Kip Thorne had made sure that Gargantua was a rapidly rotating black hole—and it turns out that the physics of rotating black holes are sufficiently different from those of non-rotating ones to allow such a planet in principle. Alas, this clever explanation still leaves me unsatisfied. Physicists, please help: even if such a planet existed, wouldn’t safely landing a spacecraft on it, and getting it out again, require a staggering amount of energy—well beyond what the humans shown in the movie can produce? (If they could produce that much acceleration and deceleration, then why couldn’t they have traveled from Earth to Saturn in days rather than years?) If one could land on Miller and then get off of it using the relatively conventional spacecraft shown in the movie, then the amusing thought suggests itself that one could get factor-of-60,000 computational speedups, “free of charge,” by simply leaving one’s computer in space while one spent some time on the planet. (And indeed, something like that happens in the movie: after Cooper and Anne Hathaway return from Miller, Romilly—the character who stayed behind—has had 23 years to think about physics.)
10. Why does Cooper decide to go into the black hole? Surely he could jettison enough weight to escape the black hole’s gravity by sending his capsule into the hole, while he himself shared Anne Hathaway’s capsule?
11. Speaking of which, does Cooper go into the black hole? I.e., is the “tesseract” something he encounters before or after he crosses the event horizon? (Or maybe it should be thought of as at the event horizon—like a friendlier version of the AMPS firewall?)
12. Why is Cooper able to send messages back in time—but only by jostling books around, moving the hands of a watch, and creating patterns of dust in one particular room of one particular house? (Does this have something to do with love and gravity being the only two forces in the universe that transcend space and time?)
13. Why does Cooper desperately send the message “STAY” to his former self? By this point in the movie, isn’t it clear that staying on Earth means the death of all humans, including Murph? If Cooper thought that a message could get through at all, then why not a message like: “go, and go directly to Edmunds’ planet, since that’s the best one”? Also, given that Cooper now exists outside of time, why does he feel such desperate urgency? Doesn’t he get, like, infinitely many chances?
14. Why is Cooper only able to send “quantum data” that saves the world to the older Murph—the one who lives when (presumably) billions of people are already dying of starvation? Why can’t he send the “quantum data” back to the 10-year-old Murph, for example? Even if she can’t yet understand it, surely she could hand it over to Professor Brand. And even if this plan would be unlikely to succeed: again, Cooper now exists outside of time. So can’t he just keep going back to the 10-year-old Murph, rattling those books over and over until the message gets through?
15. What exactly is the “quantum data” needed for, anyway? I gather it has something to do with building a propulsion system that can get the entire human population out of the earth’s gravity well at a reasonable cost? (Incidentally, what about all the animals? If the writers of the Old Testament noticed that issue, surely the writers of Interstellar could.)
16. How does Cooper ever make it out of the black hole? (Maybe it was explained and I missed it: once he entered the black hole, things got extremely confusing.) Do the fifth-dimensional beings create a new copy of Cooper outside the black hole? Do they postselect on a branch of the wavefunction where he never entered the black hole in the first place? Does Murph use the “quantum data” to get him out?
17. At his tearful reunion with the elderly Murph, why is Cooper totally uninterested in meeting his grandchildren and great-grandchildren, who are in the same room? And why are they uninterested in meeting him? I mean, seeing Murph again has been Cooper’s overriding motivation during his journey across the universe, and has repeatedly been weighed against the survival of the entire human race, including Murph herself. But seeing Murph’s kids—his grandkids—isn’t even worth five minutes?
18. Speaking of which, when did Murph ever find time to get married and have kids? Since she’s such a major character, why don’t we learn anything about this?
19. Also, why is Murph an old woman by the time Cooper gets back? Yes, Cooper lost a few decades because of the time dilation on Miller’s planet. I guess he lost the additional decades while entering and leaving Gargantua? If the five-dimensional beings were able to use their time-travel / causality-warping powers to get Cooper out of the black hole, couldn’t they have re-synced his clock with Murph’s while they were at it?
20. Why does Cooper need to steal a spaceship to get to Anne Hathaway’s planet? Isn’t Murph, like, the one in charge? Can’t she order that a spaceship be provided for Cooper?
21. Astute readers will note that I haven’t yet said anything about the movie’s central paradox, the one that dwarfs all the others. Namely, if humans were going to go extinct without a “wormhole assist” from the humans of the far future, then how were there any humans in the far future to provide the wormhole assist? And conversely, if the humans of the far future find themselves already existing, then why do they go to the trouble to put the wormhole in their past (which now seems superfluous, except maybe for tidying up the story of their own origins)? The reason I didn’t ask about this is that I realize it’s supposed to be paradoxical; we’re supposed to feel vertigo thinking about it. (And also, it’s not entirely unrelated to how PSPACE-complete problems get solved with polynomial resources, in my and John Watrous’s paper on computation with closed timelike curves.) My problem is a different one: if the fifth-dimensional, far-future humans have the power to mold their own past to make sure everything turned out OK, then what they actually do seems pathetic compared to what they could do. For example, why don’t they send a coded message to the 21st-century humans (similar to the coded messages that Cooper sends to Murph), telling them how to avoid the blight that destroys their crops? Or just telling them that Edmunds’ planet is the right one to colonize? Like the God of theodicy arguments, do the future humans want to use their superpowers only to give us a little boost here and there, while still leaving us a character-forming struggle? Even if this reticence means that billions of innocent people—ones who had nothing to do with the character-forming struggle—will die horrible deaths? If so, then I don’t understand these supposedly transcendently-evolved humans any better than I understand the theodical God.
Anyway, rather than ending on that note of cosmic pessimism, I guess I could rejoice that we’re living through what must be the single biggest month in the history of nerd cinema—what with a sci-fi film co-produced by a great theoretical physicist, a Stephen Hawking biopic, and the Alan Turing movie coming out in a few weeks. I haven’t yet seen the latter two. But it looks like the time might be ripe to pitch my own decades-old film ideas, like “Radical: The Story of Évariste Galois.”
Update (Nov. 15): I just finished reading Kip Thorne’s interesting book The Science of Interstellar. I’d say that it addresses (doesn’t always clear up, but at least addresses) 7 of my 21 confusions: 1, 4, 9, 10, 11, 15, and 19. Briefly:
1. Thorne correctly notes that the movie is vague about what’s causing the blight and the change to the earth’s atmosphere, but he discusses a bunch of possibilities, which are more in the “freak disaster” than the “manmade” category.
4. Cooper’s crash was supposed to have been caused by a gravitational anomaly, as the bulk beings of the far future were figuring out how to communicate with 21st-century humans. It was another foreshadowing of those bulk beings.
9. Thorne notices the problem of the astronomical amount of energy needed to safely land on Miller’s planet and then get off of it—given that this planet is deep inside the gravity well of the black hole Gargantua, and orbiting Gargantua at a large fraction of the speed of light. Thorne offers a solution that can only be called creative: namely, while nothing about this was said in the movie (since Christopher Nolan thought it would confuse people), it turns out that the crew accelerated to relativistic speed and then decelerated using a gravitational slingshot around a second, intermediate-mass black hole, which just happened to be in the vicinity of Gargantua at precisely the right times for this. Thorne again appeals to slingshots around unmentioned but strategically-placed intermediate-mass black holes several more times in the book, to explain other implausible accelerations and decelerations that I hadn’t even noticed.
10. Thorne acknowledges that Cooper didn’t really need to jump into Gargantua in order to jettison the mass of his body (which is trivial compared to the mass of the spacecraft). Cooper’s real reason for jumping, he says, was the desperate hope that he could somehow find the quantum data there needed to save the humans on Earth, and then somehow get it out of the black hole and back to the humans. (This being a movie, it of course turns out that Cooper was right.)
11. Yes, Cooper encounters the tesseract while inside the black hole. Indeed, he hits it while flying into a singularity that’s behind the event horizon, but that isn’t the black hole’s “main” singularity—it’s a different, milder singularity.
15. While this wasn’t made clear in the movie, the purpose of the quantum data was indeed to learn how to manipulate the gravitational anomalies in order to decrease Newton’s constant G in the vicinity of the earth—destroying the earth but also allowing all the humans to escape its gravity with the rocket fuel that’s available. (Again, nothing said about the poor animals.)
19. Yes, Cooper lost the additional decades while entering Gargantua. (Furthermore, while Thorne doesn’t discuss this, I guess he must have lost them only when he was still with Anne Hathaway, not after he separates from her. For otherwise, Anne Hathaway would also be an old woman by the time Cooper reaches her on Edmunds’ planet, contrary to what’s shown in the movie.)