Christopher Nolan's new film, Interstellar, is a near-future tale of astronauts departing a dying Earth to travel to Saturn, then through a wormhole to another galaxy, all in search of somewhere else humanity could call home.
It’s a gorgeous, ambitious work, with outstanding performances from a star-studded cast augmented by high-fidelity visual effects and a soul-stirring score. Watching it in a sumptuous 70-millimeter format on a super-sized IMAX screen, you’ll feel like you’re right there with the crew as they clamber around hostile alien planets and make daring orbital maneuvers. And, when they are forced to confront the personal sacrifices they’ve made to go on their relativistic journey, you may momentarily find a speck of dust in your eye. In Nolan’s film, love truly conquers all—even the deadly gravitational field of a supermassive black hole and the yawning gulfs between the stars. I recommend you see it.
If you watch movies for what they do to your mind rather than to your heart, though, the film may leave you less than starry-eyed. Despite being heavily promoted as hewing close to reality—Caltech physicist Kip Thorne wrote the first version of the story, and served as a consultant and producer on the film—some of the science in Interstellar is laughably wrong. Less lamented but just as damning, some parts of the story having nothing to do with science lack the internal self-consistency to even be wrong.
Much has already been written about the film’s scientific faults, pointing out fundamental problems with the astrophysics, planetary science and orbital mechanics that underpin key plot points. Just as much ink has been spilled (or pixels burned) saying that such details shouldn’t get in the way of a good story, that this movie wasn’t made for the edification of scientists but for the entertainment of the general public. I will save you from boredom (and spoilers) by not summarizing those arguments here. Instead, I’ll simply say what, to me at least, is the film’s biggest flaw, which it shares with essentially every other space opera ever to grace celluloid: It paradoxically presents interstellar travel as both ridiculously easy yet impossibly hard.
This paradox stems from the film’s central plot device: a wormhole, a tunnel through spacetime that, if traversed, could allow for essentially instantaneous travel between far-distant points. Albert Einstein and his collaborator Nathan Rosen first popularized the idea in 1935, but wormholes have since found their greatest exposure in science fiction, since they in theory offer a way to shuttle fragile, corporeal characters around the cosmos at superluminal speeds. Place one end at, say, Saturn’s orbit, and another in the heart of the Andromeda Galaxy, and a chisel-cheeked hero like Matthew McConaughey can make an intergalactic crossing in a few moments that takes light itself—the fastest thing there is—a round trip of more than 5 million years. That’s the easy, appealing, compelling part.
The hard part is that outside of decidedly speculative equations there is no evidence whatsoever that wormholes actually exist, let alone that we could ever manipulate or traverse them if they did. Based on the theoretical work of Thorne and others, making a wormhole stable enough to use would require the existence and manipulation of another entirely hypothetical thing for which we have very little actual evidence: “exotic” matter—that is, matter that possesses negative mass and energy. So, to get to the stars, all we have to do is just rely on not one but two distinct entities, both of which we do not know how to create or manipulate and which in fact may be only mathematical mirages, like the impossible staircases in an M. C. Escher lithograph. All other schemes for faster-than-light travel—warp drives, hyperspace jumps, and the like—also have similar reality-challenging requirements.
Suffice to say that the task of traveling faster than light is in all likelihood even harder than making an interstellar voyage within the firm constraints of well-understood physical laws. If you put your faith in wormholes and warp drives to take us to the stars, you might as well rely on the intercessions of gods, ghosts, and demons as well—they'll probably help just as much, which is not at all. Wormholes and other faster-than-light travel schemes won't take us to the stars anytime soon, if ever.
Of course, wormholes seem so appealing because standard, old-fashioned interstellar travel is no picnic. Here's a thought experiment I first heard from the radio astronomer Frank Drake, illustrating just how difficult an “easy” interstellar crossing can be: Imagine if we found a habitable planet orbiting a star some 10 light-years away, and that we chose to send an expedition to it, in a spacecraft the size and mass of a 737 jetliner traveling at 10 percent the speed of light. A much greater speed would risk the destruction of your spacecraft mid-flight, for if it struck an errant dust grain the energy released would rival that of a small nuclear explosion. At that velocity, the expedition would take a century to reach its destination.
It’s straightforward to calculate how much kinetic energy this voyage would require, a simple matter of plugging in the numbers for mass and acceleration to derive the required force. (If it were to decelerate to enter orbit in the target system, its travel time and energy needs would be even greater—let’s assume a flyby for simplicity’s sake.) As easy as the calculation is, its answer is daunting: To accelerate our starship to 10 percent light speed calls for the energetic equivalent of roughly two centuries of electricity production in today’s U.S.. Even so, plausible designs exist for ships that could potentially reach such extreme speeds, using concepts like nuclear pulse propulsion or laser-powered lightsails; no exotic physics are required.
There are, of course, other paths to the stars less dependent on velocity. The sun is surrounded by the Oort cloud, a sparse, trillion-strong swarm of comets that starts well past the orbit of Pluto and extends out perhaps halfway to Alpha Centauri, our nearest neighboring star system. Comets are rich in ice and other resources that could be processed into potable water, rocket fuel, and raw materials for horticulture—fueling stations for any interplanetary civilization on the move. One could imagine people and machines in the far future traveling into the final frontier by comet-hopping, moving from oasis to oasis in that ocean of night, taking generations but eventually arriving at some new world under an alien sun.
If you’re willing to spend tens or hundreds of thousands of years making your crossing, you can even embark on an interstellar voyage right now. All it would take is at most a few hundred million dollars to put a small capsule on top of a rocket capable of lofting the payload to heliocentric escape velocity. It would then join Earth’s other interstellar emissaries—Pioneer 10 and 11, Voyager 1 and 2, and the New Horizons mission bound for Pluto—on a slow journey to the stars. Such a timespan seems poorly suited for humans, but would perhaps be within reach for extremely well-engineered machines.
The challenges that confront us in these scenarios are difficult, but still firmly within the realm of reality, having more to do with social organization, resource allocation, economic incentives and long-term planning than with the fringes of theoretical physics. They suggest that, for a civilization at most only slightly more advanced than ours, interstellar voyages might only be as difficult as building the Great Pyramids was for the ancient Egyptians.
None of this means we shouldn’t try to push the boundaries of what’s possible, seeking ways around the hard facts of any future beyond our solar system. But it does mean we can and should do better than fooling ourselves with quasi-magical wishful thinking about comfortable and convenient interstellar travel. Maybe there is a magic bullet out there waiting to catapult us to the stars, nascent in the margins of a textbook or the back corner of a lab or at the outskirts of Saturn. Or maybe interstellar travel inevitably involves a long, boring slog. Acknowledging that might not make the best plot for a Hollywood sci-fi blockbuster, but it would bring us closer to those far-off places where, deep down, many of us wish we could someday go.