Black Holes, Wormholes and Christopher Nolan's Interstellar

There are few Hollywood sci-fi movies that have garnered as much press for their scientific rigor as Christopher Nolan’s Interstellar. In fact, a Google search for “the science of Interstellar” yields more than 300,000 results. It’s also the title of a book by Nolan’s primary scientific collaborator on the project, veteran Caltech theoretical physicist Kip Thorne, whose research into black holes, wormholes and time travel form the backbone of Interstellar’s narrative. But if Interstellar may accurately visualize many of Thorne’s theories—including a computer-generated image of a black hole based on real scientific data—let’s remember it’s still just a movie.

Sloan Science and Film spoke with fellow Caltech theoretical physicist Sean Carroll, author of From Eternity to Here: The Quest for the Ultimate Theory of Time, The Particle at the End of the Universe and a blog, titled Preposterous Universe, about the impossibility of worm holes, black-hole spaghettification, and the “crazy things” that can happen in quantum mechanics.

Sloan Science and Film: Let’s start with the basic premise of the film: The main characters search for another planet for humanity to survive on. How possible is this?

Sean Carroll: There’s no question that there are a lot of planets out there. We live in a galaxy with over a 100 billion stars, and we have reason to guess that many of these stars have planets. And we have data from telescopes, which seem to indicate a large fraction of these stars have planets surrounding them, in various shapes, sizes and conditions. So the chances are very high that some of those will have the right conditions to be Earth-like. But we have no real way of saying what the fraction of those that we might expect to be hospitable to life. It’s all very speculative. The possibility is there, but we have no way of telling.

SSF: The characters use a wormhole to travel through space—can you give us the basic definition of a wormhole?

SC: The best way to think of a wormhole is just as a shortcut through space-time. Einstein’s great contribution in general relativity was to say that space and time are curved, they have a dynamics of their own, we can bend them and stretch them and we experience that as gravity. Usually, if you’re in the solar system, and you’re being pulled by the earth or the sun, the gravitational pull and the stretching is mild, but once the doors open, you can imagine a tube connecting two distant regions of space-time; this definitely could be a way that space-time could be curved. That would be a wormhole. The wonderful thing about it is that the distance between two very far places in the universe could be short through a wormhole.

SSF: But this is theoretical, right? We haven’t proven the existence of wormholes.

SC: No, in fact, they probably don’t exist. What we have is a good theory. Einstein’s theory of general relativity gives us guidelines for what you would need to be true if you were to have wormholes. But there are a couple of problems: If you wanted to have a wormhole and you wanted to keep it open, you would need a negative amount of energy. Energy in large quantities is usually positive. There may be some small quantum mechanical fluctuations, which make it negative for a little bit, but basically, energies are positive.

You can imagine a microscopically small wormhole, which would be incredibly fascinating, but if you wanted a big wormhole, one that a spaceship could travel through, presumably, that would require an astronomically large amount of energy to create or keep open. Furthermore, we don’t know how to make a wormhole in the first place. If you tried to make one, it would probably collapse to make a black hole. So that would defeat the purpose. We can’t say, for sure, but the smart money is that wormholes don’t exist in nature.

SSF: What happens with time at the other end of a wormhole?

SC: Imagine a wormhole that’s big enough and smooth enough—this place where space-time is curved, and curved space-time is gravity, and gravity can crush you to death or stretch you and pull you to pieces. So if the wormhole is small, it’s impractical to travel through, so you need a very big wormhole.

This is where it gets very interesting. In relativity, there is no such thing as the same time. When two places in the universe are separated by a great distance, relativity says you need to give up on your ideas of simultaneity—that something is happening at the same time as over there. If you have a wormhole connecting these two places, there is no way to answer the question, do you come out at that same time? It depends on how you’re slicing the universe. What Kip Thorne helped invent is the idea that if you could manipulate wormholes in a sufficiently dramatic fashion, you could actually travel backward in time. Because space and time are all together in one four-dimensional space-time, if you take a shortcut from one place in space to another, then with a slight variation, you can take a shortcut from one time to another.

SSF: The other surprise in the film is a black hole. And a black hole is more provable than a wormhole, right?

SC: Yes, black holes almost certainly do exist. We have very good evidence for them in the real world. We have astrophysical data that says black holes are really out there. In the center of our galaxy, there may be a black hole that is a million times more the mass of the sun. But if you fall into one, you die. They’re not good for traveling through.

SSF: What is a black hole, exactly?

SC: A black hole is a place in the universe where the gravitational field has become so extreme that once you enter, you can never leave again. You will be pulled down to a point of infinite density—a singularity—and be crushed.

SSF: What would one experience inside a black hole?

SC: It depends on how big it is. The smaller a black hole, the more dramatic its effects are, because you get to the singularity faster. If it’s a very big black hole, you might not even notice you were inside, at first. There might be lots of time before you hit that singularity. The process you undergo along the way is called spaghettification. If you’re falling in head first, the gravitational pull on your head is bigger than on your feet, because your head is closer. So your head gets pulled away from your feet, so you get turned into a thin piece of spaghetti before you’re torn apart.

SSF: What specific areas of research are you focusing on?

SC: Many different things: Quantum gravity, how to reconcile quantum mechanics and gravity, which, in my mind, involves foundational questions in quantum mechanics, itself. What happens when you make a quantum measurement; what is the world really like based on quantum mechanics?

SSF: The world of quantum mechanics seems to open up things in whole new ways for science fiction. The rules go out the door, and we have more possibilities—like particles being in existence and simultaneously not being in existence.

SC: Yes, but filmmakers have to be especially careful here. Quantum mechanics is weird and counterintuitive and crazy things can happen, but there are definitely just as many rules there as in other arenas of science. The weirdness seems like a license to have fun, but you have to think hard about what the implications might be.

I think that a lot of moviemakers miss an opportunity not thinking like a scientist. Whether or not your movie obeys the laws of physics, it should obey some rules. If it doesn’t obey any rules at all, it’s just not interesting.