Interstellar, the scientific explanation, on the way we like

 Can one talk about science from science fiction? Of course we can. Today we are going to talk about a 2014 sci-fi film directed by Christopher Nolan: Interstellar. Not only because there is so much science in the film that is not mistreated, but also because the initial idea for the story was conceived by Kip Thorne, a Nobel laureate in physics two years after the film was released. The film is not perfect, in fact. There are inaccuracies, but they are attributable to artistic and cinematic choices. 

The wormhole in Interstellar

In one of the film's most important moments, a wormhole that appeared near Jupiter allows the crew of the starship Endurance to reach another galaxy in no time. At that juncture the astronaut Romilly explains very well to the main character what a wormhole is, using only a sheet of paper. If space had two dimensions we would have to draw a line to get from point "A" to point "B." By being able to fold the paper (and thus space) and create a passage (a hole) we could get from one point to the other in no time. If the mouth of the tunnel is a circle in two dimensions, it will become a sphere in three dimensions.

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Inside the wormhole we see distorted images of stars: the reason is that the space on the other side is curved precisely by the presence of the wormhole. The idea that there is a region of space called hyperspace that the spacecraft can pass through unscathed, on the other hand, is entirely hypothetical since no one has ever actually passed through a wormhole.

Let us turn to Gargantua, the supermassive black hole around which Miller and Mann's planets orbit. It has a mass 100 million times greater than that of the Sun, and its depiction is extremely realistic in the film. No one has ever seen a black hole so closely, but scientific simulations show something very similar to what we see in Interstellar. The team that produced Gargantua's graphics used equations provided by Thorne himself, but not only that. Another important thing to note is that the accretion disk around Gargantua contains little material-this can happen if such a large black hole does not engulf any stars for millions of years. So as you approach the black hole, you are not disintegrated, precisely because the disk is not extremely hot and does not emit large amounts of radiation. It is, however, hot and bright enough to make the planets orbiting it "habitable."

Miller's Planet

On Miller's planet, time flows faster than on those left on the Endurance. The time dilation referred to in the film is 7 Earth years for every hour spent on the planet. To achieve this effect there is a need for the planet to orbit near the event horizon, but at the same time not be swallowed by the black hole. Thorne imagined Gargantua rotating on itself at an enormous speed, the maximum speed allowed by general relativity. For those watching from a distance, Miller's planet makes an orbit around Gargantua in about two hours (at about half the speed of light). The orbit therefore takes place in a tenth of a second. Also, because of the enormous tidal effect, Miller's planet is deformed and always shows the same face to the black hole. Which is kind of what happens with the Moon, which always shows the same face to the Earth. There is a black hole involved in the movie, which is why the tides are a little more extreme.

The tesseract

We come to the most complicated part of the film. Toward the end Cooper decides to go inside Gargantua, both to save Brand (he would not make it to Edmunds' planet if he had to carry his weight) and for another reason. The interior of the black hole could have valuable information about quantum gravity, which would serve to complete the professor's equations, allowing humanity to conquer gravity and leave Earth. But by entering the black hole, shouldn't Cooper die?

If we limit ourselves to the description of a black hole that general relativity gives us, yes. Here, however, the film exploits entirely hypothetical theoretical ideas that allow Cooper to survive. In the fiction of the film there are then beings that can move between dimensions: they are the ones who save Cooper by creating a hypercube, or tesseract, which we also discussed in a previous article.

Cooper then finds himself observing his daughter's room. This is perhaps the most incomprehensible moment in the film, but let us try to analyze it. The beings who created the tesseract see time as if it were another physical dimension. So Cooper is faced with the same room seen at different times and realizes that he can move along the temporal dimension by leaping from one time to another. Beware, however: he is not physically in the same space as his daughter. Let's take a practical example.

The library in Interstellar

Let us imagine the usual sheet of paper (thus a two-dimensional universe), in which there is Murph on one side and Cooper on the other. According to the theoretical ideas on which this sequence is based, all the interactions and particles we know move in three-dimensional space, so they can also propagate through the two-dimensional faces of the hypercube and can reach Cooper. What can't happen is that these particles can propagate backwards in time, so Cooper can't physically go back to the past, but he understands that the only thing that can move between dimensions is gravity. That not only can it traverse hyperspace, but it can also do it back and forth in time. When Cooper realizes this, he realizes that the only way to communicate with his daughter is by creating perturbations. He does this by touching (not directly) the books. In this way he is able to communicate the data collected inside Gargantua to his daughter, who will then be able to solve the quantum gravity equations giving humanity a chance to save itself.