I thought Interstellar was one of the worst movies I’ve ever seen—in fact about an hour in, I’d taken my wrist watch off, started smelling it, and playing with it as if it was a bird. To me—and I read, write and edit work every day—it felt like the Nolans threw millions of dollars at a half-baked short story they wrote over the weekend. But Michael Park, lecturer at Stanford University and holder of a Ph.D in theoretical particle physics from Rutgers University, saw the movie’s merits—and then some. Park, whose research has focused mostly on developing strategies for searching for new laws of physics at the Large Hadron Collider but who now is studying cosmology, is, um, really smart. He and I talked about the gravity, Einstein and the blight.

– Jake Uitti

JU: Okay… I saw the movie and laughed all the way through… maybe because I didn’t get it, or maybe because it didn’t tell the story it wanted to properly. Let’s start at the beginning… why were they trying to get off earth, and what was their plan?

MP: The reason they needed to get off Earth was because of a vaguely described scourge they called the “blight”. The only specific things I remember them saying about the blight, was that it breathed nitrogen. So I would take a stab and say that the blight is some kind of microbial life form unlike any we have seen before on Earth. Maybe it was some virus-like organism that was lying dormant in a comet that hit Earth at some point. When it reawakened, it found a delicious way to reproduce and spread by voraciously consuming certain crops. I got the impression that the effects of the blight were progressive in some way. So maybe the blight was naturally equipped to reproduce by consuming all fruits and vegetables, and when those ran out maybe it evolved the ability to consume wheat. All we know for sure is that corn is blight-resistant for untold reasons. Maybe blight hates starch. Plants and their roots play a big role in keeping dirt cohesive, so the systematic destruction of large swaths of plant life could lead to crazy dust storms like we saw in the movie.

It’s worth noting that I think the blight might also be a metaphor for global warming. It seems to me that the effect of the blight on human society shares similarities with some of the speculative effects that global warming might have. Namely, the systematic destruction of habitable and resource-nurturing territories of Earth.

Regarding their plan: apparently about a decade before the events in the movie, scientists discovered strange “gravitational anomalies” on Earth. Basically just instances of gravity acting weird. This gave them a theoretical handle with which to really make progress in their understanding of things like worm holes and black holes. Then out of nowhere, a worm hole appeared near the planet Saturn, giving them a view of many potentially habitable planets that were previously prohibitively far away. This convinced them beyond any reasonable doubt that there was some benevolent intelligent life form trying to give them away to survive the destruction of Earth. So their plan was to send teams of scientists to all the potentially habitable planets in view, in order to collect data about whether or not they were really truly habitable. Then send missions to those planets to populate them with humans.

JU: So they needed to get off Earth and inhabit a new planet. To do this, they sent four astronauts, two silly-looking robots, and a space ship to check out three options to move humanity. They went through a black hole, saw Saturn (in all of its cardboard cut out-looking glory) and debated whether to visit Matt Damon on a planet whose time passages were different than that of Earth. Also they could receive messages from Earth but couldn’t seem to send any back. TELL ME WHY I SHOULDN’T THINK ALL OF THIS IS RIDICULOUS! PLEASE!

MP: By the time Cooper got to NASA, there had already been an ongoing full scale exploration mission to 12 potentially habitable planets accessible through the worm hole. The “Lazarus” missions. Scientists had been sent out decades earlier and were waiting on those planets collecting data. When Cooper got there, it was clear that they were desperately short on resources and trained pilots. They mentioned that many of the 12 potentially habitable planets that had been explored would receive no retrieval teams and the scientists would be left there to die. So Cooper, a former trained NASA pilot falls into their lap, and they convince him to join one of the retrieval teams. Also because there are mysterious signs that the intelligent beings are pushing for him to be the one that goes.

So the object that they went through near Saturn was not a black hole, it was a worm hole. A worm hole is a short-cut bridge from one part of the universe to another. A black hole is an object with such a massive gravitational pull that even light cannot escape. The reason time passes differently on the water planet than it does far from the planet, is because the planet is so close to a black hole. The other thing about gravity is that gravity can stretch the fabric of space and time. Clocks near the Earth run more slowly than clocks on satellites in orbit, which is why GPS systems were kilometers off until people realized that they needed to correct for this effect.

JU: What do you think the movie really nailed, in terms of the laws of physics and the difficulty of the mission?

MP: Everything. Given what we know about the laws of physics, how gravity stretches spacetime, and what role extra-dimensions might play in the laws of nature, I thought it was a totally well motivated and plausible presentation of how such a scenario might play out. Most of the questions you could ask about the seemingly absurd aspects of the movie, like how come the 5th dimensional beings can only talk to us using gravity? and why would he use Morse code on a watch to communicate with his daughter? All have surprisingly legitimate physics arguments that single them out as particularly plausible scenarios.

JU: Which leads me to my next flurry of questions: Why was gravity an essential theme? How did it bridge the gap between the third, fourth and fifth dimensions? Why was Cooper behind a bookcase? How did he tap the book covers for Morse code? Who is the “they”? What the fudge is going on???

MP: So to understand why gravity was such an important theme, we have to have a brief discussion about what gravity is exactly. Einstein’s theory of general relativity teaches us that the effect of gravity can be thought of as being due to the stretching of space and time. One way to see this is to notice that gravity’s main job in life is to curve the trajectories of objects. Baseballs, cannonballs, comets, planets, all follow curved trajectories as they travel through space due to gravity. Einstein’s theory says that these trajectories curve because massive bodies (like the Earth or the Sun) have the effect of stretching space and time in such a way that a baseball “thinks” it’s traveling in a straight line, but travels instead along a curved trajectory from our perspective because the space and time around it are curved. The fact that gravity can be described in this way is related to a very special property of gravity. It affects everything in the same way. Contrast this to the electric force. With the electric force we can have positively charged particles, like protons, and negatively charged particles, like electrons. But also protons and electrons can bind to each other and create atoms that are electrically neutral, meaning the electric force does not affect them at all.

Gravity is different. Gravity affects everything, and there is no such thing as “gravitationally neutral”. Everything lives in space and time, gravity is the stretching of space and time, so gravity affects everything. This explains another very peculiar aspect of gravity that might be surprising. It is a billion billion billion billion times weaker than the electric force. This seems counterintuitive because we see gravity affecting us in a seemingly strong way in day-to-day life. But remember, most objects in life are electrically neutral. Gravity is only more apparent to us because nothing is gravitationally neutral, but it is really a billion billion billion billion times weaker.

If you study the equations for the gravitational force and the electric force for long enough, you realize that trying to fit them into a sensible picture together is really difficult because of this huge disparity in their strengths. The mystery of why gravity is so weak compared to other forces is one description of the central outstanding problem in theoretical physics – the so called “hierarchy problem”. One general class of solutions to this “hierarchy problem” is to suppose that there exist extra dimensions of space. In this picture, our 3-dimensional space is embedded into these extra dimensions, in much the same way a 2-dimensional sheet of paper is embedded into our 3-dimensional world. If this is true, it’s possible that the reason gravity is so weak, is because gravity alone can “leak out” into the extra dimensions thus diluting its strength when we observe it. So this idea of extra-dimensions that only gravity can access, is one of the most well-studied and plausible scenarios in physics.

So in Interstellar, they muse about beings that live in 4-dimesional space (or 5-dimensional spacetime). For this we are told explicitly to suspend our logic and remain agnostic about their nature, motivations, and origins. As Anne Hathaway’s character says, we can’t understand how they live in their higher dimensional environment “to them the past could be a valley that they might go play in”. We may have no way of comprehending anything about them, as our system of logic and rationale might be just some 3-dimensional relic of a bigger higher dimensional concept. What we do know, is that they can only communicate to us by disturbing gravity. Which, remember, is equivalent to having the ability to stretch space and time.

So why does Cooper get stuck behind a bookcase after he falls into the black hole? In the movie it is explained that these beings basically artificially constructed a human-friendly operating system for the higher dimensions. Specifically to allow Cooper to access the past and transmit information crucial to human survival. This operating system seems to have a fixed point in space, some region behind the bookcase in Murphy’s room. There seem to be webs of string separating Cooper from the bookcase, and it looks like each of these strings can be tugged to stretch space or time in a particular way. After struggling to communicate with Murphy, he finds the string that stretches time near the watch, and communicates with her in Morse code through the hands of the watch.

JU: Did you buy, as a viewer, the watch use?

MP: I know he chose it because it was the only way to communicate with Murph through gravity (which remember is the stretching of space or time, so watches and meter sticks are the most obvious candidates) and also let her know that it was him doing it (because the watch shared special meaning for them). Man, elegant physics is woven so beautifully into the story. Love it.

JU: If you were to change one thing about the movie to make it more clear to people like me, what would you change?

MP: I wouldn’t change anything. It’s just clear enough to generate awesome conversations like this one. People all over America now are wondering why time slows down near a black hole, and they’re realizing that physics is not a dry subject. I get to talk about why it might make sense to have an extra dimension that only gravity can access, and people don’t tune me out. It’s pretty sweet. Also whether you like or dislike the movie, you will walk out with a more intuitive grasp of the weird things relativity says about our universe.