Ask the Experts: Dr. Edward Rhoads on "Gravity" and "Interstellar"

Ask the Experts is an ongoing feature on where we invite faculty to discuss what is going on in their respective field of work. This week we're featuring Dr. Edward Rhoads and his take on the scientific review of "Gravity" vs. "Interstellar" debate. Dr. Rhoads is a lecturer in science at IUPUI and the author of "A Space Odd-yssey."

Writers often want us to suspend disbelief and give them artistic license. However, far too often when it comes to science in movies the writing gets the science so incorrect that they end up with inartistic license. Usually this is the fault of the writer for doing such a poor job creating a coherent plot that they have to break the laws of physics in order to get the script to work. This is different from writers who bend physics for an ooh or aah shot that has very little to do with the plot and therefore earns the moniker of “artistic license.”

The movies "Gravity" and "Interstellar" are opposites when it comes to how well the science was portrayed in the movie. "Gravity," a stereotypical point-A-to-B-to-C movie, also has the usual quality control of science: none. Interstellar, on the other hand, gets some science wrong but at least gets most of it close enough to allow it the “artistic license” stamp.

The entire plot of “Gravity” is based on broken science. The characters have to get from one space station to the next using minor thrusters and have to do so within 90 minutes to avoid the swarm of space debris. The problem with this premise is that many times they are using their space suits to do this. The writer found the travel time by dividing their distance by their orbital velocity. The problem with that is the space stations they are going to are also in orbit. Since most human-made objects orbit in about the same direction, you cannot use orbital speed; you need the velocity relative to the objects. In this case, that would be a few hundred miles per hour at best and it would take days to get there. Not only would the swarm of debris reach the characters, but the characters would run out of oxygen long before they arrive at their destination.

The swarm scenes are also unrealistic. They have three problems. The first is relative velocity. If the swarm is in orbit and you are in orbit, the velocity isn’t quite as fast. The second issue is most of the time they seem to be headed straight toward the Earth and not in orbit. The final problem has to do with momentum, which is conserved. Objects seem to be pushed at very high velocities perpendicular to whichever direction the swarm is coming from. This simply would not happen, as this does not conserve momentum in that direction.

Perhaps the worst science in the movie is the scene where George Clooney’s character is holding on for dear life to the tether line. It is a standard cliffhanger scene with one big problem: there is no force to do that when you are in orbit. While it is true you still have gravity in space, the gravity is canceled out by the Centrifugal Force due to the orbit. So, there is simply no force to pull Clooney away from Sandra Bullock’s character. All Bullock’s character would need to do to reunite with Clooney’s is give a small tug on the tether and he would come back. So, it became a very cliché cliffhanger scene that had no cliff to actually hang from. This is the very essence of inartistic license.

“Interstellar,” while it did get some science wrong, was a lot better scientifically than “Gravity”  The first science we encounter in the movie is the wormhole. Wormholes are mini-black holes that warp space. Science fiction always uses them incorrectly; in real life they would not be safe and would rip apart anything that approaches them. However, they do correctly show that it would bend light around it.

The largest errors in “Interstellar” all occur at the first planet they encounter. The black hole near the planet is correct. The largest discovered black holes reside in the centers of enormous galaxies and can have masses in the tens of billions of solar masses (our sun is 1 solar mass). In terms of size, they would be the size of a solar system. Oddly enough, the density of a black hole is inversely proportional to its mass. Therefore, this black hole would have a density close to the density of water and it would be safe to approach it. Black holes a few times the mass of our sun would be the size of a city and would tear you apart as you approached. However, everything else about the first planet was incorrect. The time dilation, which does happen, was way off. To get that amount of time dilation you would need to be about three miles from the event horizon of the black hole. Since the atmosphere is thicker than that, you were more than three miles away. If you use Einstein’s laws of general relativity you would get a time dilation of a few years instead of a few decades in this case.

Another error for the first planet was the light from the star. The size of the time dilation would also give you an equal-sized shift in the light from the star that is further from the black hole and therefore time dilated less. The light would be shifted to the x-ray or gamma rays and be absorbed by the upper atmosphere. The surface would be very dark to the human eye.

Allowing artistic license for the planet with the frozen atmosphere, we move on to the final scenes where we are taken inside the black hole. They got the sequence of entering the black hole they got correct. From the astronaut’s perspective the entrance would seem normal. It is the rest of the universe that would seem to change. Inside the black hole, the laws of physics in our four-dimensional universe break down. It is very possible that the dimensions inside and outside the black hole are independent, sort of like the inside of the Tardis in the TV show Doctor Who. This includes the dimension of time itself. The strings represent a branch of physics that used to be called String Theory, which has evolved to Membrane Theory or M Theory for short (which, I should add, is misnamed since it is not actually a Theory but a model; to be a theory it needs to be thoroughly tested, which currently is not possible). While it may be true that 100 years from now this scene might seem a bit silly, for now, due to the hypothetical nature of interdimensionality, we can give this scene an interesting “artistic license” stamp.

So, we have two movies. One messes up all the science and has the science integral to the plot. With real science, “Gravity” would not exist as a movie. So, Gravity is given an “inartistic license” stamp. “Interstellar,” on the other hand, gets some science correct and some incorrect. However, most of its incorrect science is not integral to the plot; and the movie would still exist with correct science, it would just change a little.  Therefore, “Interstellar” is the scientifically superior movie of the two and gets an “artistic license” stamp.

For more from Dr. Rhoads, you can check out his YouTube channel or his eBook "A Space Odd-yssey."