The Science of Interstellar

Official trailer for Interstellar.

Interstellar is Christopher’s Nolan latest film, and one of the biggest science fiction films of the last few years. Before I say anything, be aware this article contains spoilers, so if you haven’t seen Interstellar, go see it now! It is one of those films you’ll really want to see on the big screen. (Summary for those who haven’t seen it: there are a few moments where the film takes artistic liberties, but overall it respects the laws of physics even better than most films, even those about the space race!) Now, Hollywood isn’t known for paying attention to scientific accuracy (it is often joked that the biggest drawback of studying physics is seeing all those glaring issues), so it may surprise some that Nolan has actually consulted a professional physicist for his film. And not just any physicist, but Kip Thorne, one of the world’s leading experts on Einstein’s theory of general relativity. In general the film never fails to demonstrate its love of science, with the main character wanting his son to study sciences, and the film even remembers the importance of genetic diversity. So all in all, a very exciting film for science and space nerds like me! I’ll especially focus on the physics of the film, going from the more practical to the more theoretical.

One of the main themes of Interstellar is gravity, and indeed it is one of very few films to get artificial gravity right. Generating gravity on a ship is simply impossible, as an absolutely gigantic amount of mass is required. Interstellar does it correctly by making the ship rotate, which creates a centrifugal force, and so the occupants can walk around as if they were on the surface. This is why the station is ring-shaped, so that the occupants feel a similar force across the ship. Another detail the film remembers are g-forces, which you feel when accelerating, for example when taking off in an aeroplane. At several points, when the ship turns, the seats orient themselves so that the pilots are better able to withstand the strong g-forces.

The journey to Saturn takes about two years. This is a lot faster than the most fuel-efficient journey, which would last about five and a half years, but still within the realms of plausibility. The gravitational slingshot around Mars helps in reducing that too: gravitational slingshots are real manoeuvres, and real life spacecrafts perform them regularly. Rosetta, (which, if you haven’t been following the news, recently landed on a comet) performed several of those, including around the Earth and Mars. The idea behind gravitational slingshots (also called gravity assists) is to use the gravity of a planet to push the spacecraft even faster than it moved previously. The idea is the same as throwing a ball at a driving truck, which will make the ball go faster, though I suggest you do not try this. Quite naturally, with our current more limited technology, gravitational slingshots are more about saving fuel than time.

The docking scenes are quite possibly the most exciting scenes in Interstellar, and the film essentially nails the physics of it. The Endurance has its port located on its axis of rotation, so that the port moves as little as possible. The spacecraft first moves to align its docking port with the rotating station’s port, and once they are well aligned, you start spinning the craft so that you don’t damage the port with friction. However, in the second (and more exciting) docking, the crew were very lucky that the station started spinning the right way. The spinning was also so fast it would have been very tough for the crew in the Ranger ship to withstand the g-forces of about 6–10 times that on the Earth, but it is feasible for trained pilots to do so. In addition to that, the explosion will have shifted the centre of mass and hence the position of the axis of rotation, so again they were lucky that it balanced out anyway. To add more to the difficulty of this scene, the Endurance was rotating so fast that the edges of the ship felt a g-force equivalent to a hundred g. With some very good engineering, it is plausible that the ship won’t have broken apart under its own centrifugal forces (especially when you consider it was made to traverse a wormhole), but a significant part of the equipment will be seriously damaged, in addition to those in the explosion.

Another detail Interstellar gets right, well most of the time, is the position of engines being fired. If they’re not symmetric, the spacecraft will start spinning – just push a box or book on an edge to observe this. Non-physicists often won’t pay attention to those details, but it’s refreshing to see a film that pays attention to that. However, the film does break from that just after the famous docking scene as they use engines from the ship underneath the station.

The film makes heavy use of Einstein’s theory of general relativity, the theory of gravity that works best with our current understanding of the universe. The concept is that heavy objects will distort space, a bit like a ball on a trampoline would. Parts of the theory also says we cannot travel faster than light (despite over half of sci-fi doing so anyway), and to make sure this is always the case, this also has to relate space with time, effectively making time a fourth dimension. This might seem strange, but it makes a lot of sense: we almost always say that something happened at a particular time and a particular place. Physicists call those ‘events’, just like in the everyday meaning of the word! The interesting consequence is that heavy objects distorting what we call ‘space-time’ will therefore make time go slower in strong gravity. So The Doctor was right in saying that time is a ball of “wibbley-wobbley-timey-wimey stuff”.

The crew are able to move very far from the Earth thanks to a wormhole. The wormhole is this very strange object that effectively acts as a portal between different points in space. While this sounds like something straight out of science fiction, general relativity does predict that they can exist, and if they do they would be spheres rather than flat portals, just as the film says. However, wormholes cannot form naturally, and they would probably require objects with negative mass to be stabilised, and such objects have not been found.

Of course, Interstellar’s piece de resistance was Gargantua, the super massive black hole. Gargantua’s visual portrayal is in fact the most accurate portrayal of a black hole that has ever been made: two scientific papers are going to be published, one for the computer science community, and another for the astrophysics community. The black hole has a lot of material around it in the form of a disk accumulating by gravity, not too unlike Saturn’s rings. And no, unlike what a lot of sci-fi will tell you, black holes are not “space sinks” that absorb everything around them! Now, this flat disk doesn’t appear flat visually, because according to Einstein’s theories, a strong gravitational field will bend the path that light takes. This is why the flat disk appeared as a halo around the black hole. The appearance as a halo is something astrophysicists have suspected for a long time, but never had the confirmation for this – using the necessary computing power is very expensive. What the film doesn’t portray is how very hot it is near a black hole, at about 100,000 degrees – I can (just about) understand how the Endurance survives 100 g, but at those temperatures it will vaporise rather quickly!

Miller’s planet, the water planet near the black hole, is subject to extremely large time dilation due to gravity, just as I mentioned earlier. Let’s ignore the temperature problem I just mentioned for now. A planet will take hundreds of millions of years to form, and with the current age of the universe, Miller’s planet could not have formed near the black hole. It is however possible that the planet formed further away, and then was caught by the black hole’s gravity. Another important characteristic of Miller is its massive waves. There seems to be disagreement on whether those are plausible: some argue that the massive gravity of the black hole could power those waves through tides, like the Moon does. But others claim that having a surface completely covered in water would stabilise waves, and the planet’s surface gravity, about 1.3 times higher than on Earth, would flatten the waves. And though this isn’t breaking the laws of physics, the crew do break scientific common sense by not sending surveying probes on the planets before sending human crew, which would have saved them time and fuel. They most likely could have also determined that the surface was mostly covered in water by using a very advanced and groundbreaking piece of technology, known as their eyes.

A curious effect of black holes is that called ‘spaghettification’. Basically, as you approach a black hole, the tidal forces become extremely significant, and since your feet are closer to the black hole than your head, your feet are pulled much more. This effect is mostly noticeable with small black holes. In the case of supermassive black holes like Gargantua, the tidal forces are small enough that an astronaut could enter the black hole without feeling stretched, and even enter the event horizon – the point of no return – without noticing it. That being said, general relativity tells us nothing on what happens inside the event horizon, so no one can comment on exactly how accurate the film’s portrayal of it is, though it almost certainly won’t allow someone to manipulate gravity.

Having said that, for now let’s ignore how Cooper got into the tesseract: this term is basically the equivalent of a cube, but in 4 dimensions. Basically, we can say that, although we can choose our position in space, we can’t do so with our position in time, as time always flows whatever we do. Adding another dimension changes that, by turning our dimension of time into a dimension of space, and so Cooper is free to travel along time as he wishes. In fact, Professor Brand tries to combine quantum physics and general relativity, and some of our best theories attempting to do that have as many as eleven dimensions!

In conclusion, Interstellar is a fairly solid film in its scientific foundations. While I would not call it a ‘hard sci-fi’ film as it does take a few liberties, it is a strong piece of evidence that the laws of physics don’t have to make a story boring, and it very refreshing to see Hollywood try to appeal to science lovers alongside a more general public. Faster-than-light travel is almost overdone and wormholes provide a more realistic, yet just as exciting, interstellar means of travelling – possibly even more so!

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