The real physics of Interstellar

Interstellar is, by and large, a run of the mill space epic, with a vast budget, stunning props, a laboured plot, some questionable dialogue and an overwhelming soundtrack, which seems to be based on the premise that the “fortes” of Saint-Saens’s organ symphony should be redefined as “pianissimo.” The film’s most novel feature is that it has made a serious attempt to render the big science authentically. To this end, the distinguished astrophysicist, Kip Thorne, was brought in to advise on the “gravity equation.” Travel through space and time is de rigueur for the genre, but the makers of Interstellar have had a crash course in relativity, gravity, the warping of space and time. And they have absorbed the lessons well.

Nonetheless, there are questionable episodes throughout the film. For example, when Cooper prepares to leave on the mission, he attempts to allay his daughter’s fears by explaining how time will run at different rates for the two of them. He sets her watch to the same time as his and promises that when he returns they can compare the two again. He will have travelled in a spacecraft “at nearly the speed of light, perhaps” and will experience intense gravitational fields, all of which will make time “almost stand still” for him. All of the physics is correct, and nicely explained. However, the psychology is all over the place. Cooper, bubbling with excitement, tells Murph: “When I come back we might be the same age!” And when this fails to win her over, he assures her: “I’m coming back, even if I am 20 years younger than you when I return!” The inference that she might be dead and he merely middle-aged hangs there, unstated.

The spacecraft duly takes off with a crew comprising Cooper as pilot, Professor Brand’s daughter, Dr Brand, played by Anne Hathaway, and a third member who is a “particle physicist.” I confess it wasn’t clear to me why a member of my own scientific specialism was critical to the venture, unless it was to work on the gravity equation. Medical staff and a radiation scientist might have been more useful. One area of science the film overlooks is a relatively mundane one, but one that will probably undermine future space exploration in fact, if not in fiction: the effects of chronic radiation in outer space.

The particle physicist taps the metal innards of the capsule and observes anxiously that “there are only millimetres of aluminium and nothing outside.” There’s no air outside, certainly, but a lot of cosmic rays and nasty radiation, which can be lethal unless astronauts are protected.

On earth, we are protected by a blanket of air and by the planet’s magnetic field. These protect us from the potentially lethal effects of cosmic rays and solar emanations. In space, radiation is present at levels which, although not immediately lethal, become so when humans are exposed to them over an extended period of time. A voyage to Mars is probably at the limit of what human beings could bear, though even here women of child-bearing age should consider their options carefully lest any foetus—present or planned—be genetically affected. Older astronauts, with a lower life expectancy than their youthful counterparts, have less to concern them: the effects of radiation are statistical—if there is a 50:50 chance of damage occurring within the next 50 years, then a 50-year-old man has less to be concerned about than a teenager.

The cast of Interstellar seem to have been chosen more for their youth and glamour than for their likely chances of getting radiation sickness, or worse. Unless Anne Hathaway’s lip-gloss, which remains miraculously unblemished throughout the crew’s adventures in outer space, is some novel means to absorb radiation, the odds are that the entire crew will be dead or dying by the time they reach Saturn, let alone survive their encounter with a wormhole.

According to Einstein’s general relativity theory, wormholes can exist. They arise when gravitational forces become extreme, so much so that they would disrupt the atoms of everything that enters them, including those from which astronauts and spaceships are composed. So the idea of a human entering a wormhole and emerging unscathed on the other side is, literally, mind-blowing. Also, science suggests that wormholes are unstable and collapse when something enters them. Therefore, the very act of entering a wormhole destroys it: there is a “cosmic logic” principle at work here whereby contradictions, such as going back in time through a wormhole to kill one’s grandparents, cannot happen.

Having emerged from the wormhole in another galaxy, the crew arrive at some potentially habitable planet, near Gargantua, a supermassive black hole, the largest type of black hole. The gravitational field is such that time runs more slowly. This is nicely alluded to when Cooper urges the crew not to “waste time,” because “seven years [on earth time] go by every hour here. Let’s make it count.” Seven years for an hour—that must be some huge gravitational field, so it’s remarkable that the astronauts can survive, let alone move about. For a planet that near to a supermassive black hole, perhaps hundreds of millions of times bigger than the sun, you might expect that tidal forces—the difference in strength of gravity between one side of a body and another—would rip the planet apart. But if Gargantua is spinning, then the “gravity equation” is difficult to solve, and it’s possible that the astronauts and the planet could survive. As this is still a matter of debate among physicists in the real world, it amounts, for the time being at least, to a “get out of jail” card for science fiction.

There were moments during the film, when I was reminded of the cinema of yesteryear, when “with one bound Dick Barton was free.” The ability to liberate the hero from an impossible situation by appealing to magic is the stuff of fiction. And, for all its hype about scientific authenticity, Interstellar is fiction. And for that reason we should probably ignore other inconsistencies, such as the suspension of the second law of thermodynamics: when we enter Professor Brand’s secret laboratory, 23 years after the crew departed, the gravity equation is still there, pristine on the board. And when Cooper finally returns, in early middle age, his daughter is by now nearly 100 years old, and is lying on her deathbed in hospital. Technological progress must have ceased, because the medical equipment and monitors of the 22nd century seem to be identical to those of 2014.

Coming out of the film, I thought again about Rosetta, and science fact. There’s plenty of spectacle in Interstellar, but I can look up at the stars to see that. As David Parker, head of UK Space, put it when the Rosetta mission successfully landed on its comet: “Hollywood is good, but Rosetta is better.”