SIAM News Blog

How Interstellar Was Born

By James Case

The Science of Interstellar. By Kip Thorne, W.W. Norton, New York, 2014, 336 pages, $24.95.

The idea for the recent blockbuster movie Interstellar was hatched over dinner in October 2005 by astrophysicist Kip Thorne and his former girlfriend Lynda Obst, by then an A-list Hollywood producer. Wishing to make a science fiction movie about both the end of (human) life on Earth and space travel through wormholes—a concept pioneered by Thorne in the 1960s—Obst wondered if he would care to collaborate on such a project. Would he! Before long he resigned the Feynman Professorship (emeritus) at the California Institute of Technology (Caltech) to devote more time to the project.

Within months of their dinner conversation, Obst had persuaded Stephen Spielberg to sign on as director and had scheduled a brainstorming session with Spielberg, Spielberg’s father, Thorne, and 13 of his Caltech colleagues to discuss scientific content. Thorne proposed two guidelines:

  1. Nothing in the film would violate firmly established laws of physics or firmly established knowledge of the universe.
  2. Speculations (however wild) about ill-understood physical laws and the nature of the universe would spring from real science – ideas that at least a few respectable scientists regard as possible.

The resulting movie was directed by Christopher Nolan, rather than Spielberg, who cited more pressing commitments and left the project. It opened to mixed reviews in November 2014, with an all-star cast that included Matthew McConaughey, Anne Hathaway, Michael Caine, and Jessica Chastain. Ancillary products include the book under review, a novelization of the film by Greg Keyes, an official script notable for the cryptic “sound bites” in which onscreen characters attempt to explain complicated science to the theatre audience, and a downloadable version of the film itself.

Thorne’s book explains the science underlying the movie. Chapters I, II, and IV–VI describe current knowledge of the cosmos, with an emphasis on black holes, wormholes, singularities, gravitational anomalies, and the fifth dimension, while Chapter III describes a sequence of events on Earth that at some future point could force mankind to seek a home elsewhere in space.

Briefly, global warming could generate giant dust storms (larger than the ones that menaced even East Coast cities during the Dust Bowl of the 1930s), paving the way for a “universal blight” that would strike one weakened crop after another until, as is revealed in the movie’s opening scene, corn (maize) alone remains to feed humanity. Soon after that, as one of the characters in the film predicts, that crop too, would seem certain to fail.

In reality, the world has never known such a blight. Although some, like the one responsible for the Irish Potato famine of the 1840s, have devastated specific plant species, none has displayed an ability to progress from one species to another. Far-fetched as the idea may be, agronomists seem unable to deny that so aggressive a blight is at least possible.

Figure 1. Black holes and wormholes extending out of the brane into and through the bulk. Figure 4.5 in The Science of Interstellar.
The rest of the story seems even more far-fetched. For a manned mission to reach a potentially habitable planet, the writers postulate a wormhole connecting a location near Saturn to one in a distant galaxy, put in place by an unknown race of benefactors known only as “they.” These more advanced beings appear to reside in the higher dimensional space wherein our four-dimensional space–time is embedded and to communicate among themselves by means of gravity waves. In the language of cosmology, our own space–time is known as the brane (short for membrane), while the higher dimensional space in which it is embedded is called the bulk (see Figure 1). Superstring theorists suggest that the weak, strong, and electro-magnetic forces of nature operate only in the brane, while gravity operates throughout the bulk. They also suggest that the bulk has many more than five dimensions but, for his expository purposes, Thorne elects to ignore that aspect of their speculations.  

For ease of visualization, Thorne makes frequent use of diagrams depicting a two-dimensional brane embedded in a three-dimensional bulk. One of his more fanciful renditions, prepared by artist friend Lia Halloran, appears below. In it, black holes appear as (roughly conical) singularities of curvature in the brane, while wormholes resemble singularities joined together in pairs, vertex to vertex, connecting one sheet of the brane with another. These wormhole-forming pairs, it is now known, are unstable objects that (unless held open by outside forces) soon degenerate into separate black holes.  

Whereas ordinary celestial objects such as stars and planets are but shallow depressions in the brane, sufficiently massive stars may (as they exhaust their nuclear fuel) implode into singularities of curvature in space–time. Indeed it was shown (by J. Robert Oppenheimer in 1939) that, if the implosion is exactly spherical, the imploding object must (i) create a spherical black hole around itself, (ii) create a curvature singularity at the hole’s center, and (iii) subsequently get swallowed up into the singularity, leaving behind no matter at all. The resulting black hole is made entirely of warped space-time.

Figure 2. The stars in Gargantua’s galaxy as seen around the black hole’s shadow, depicted in Figure 3.3 in the book. The black hole bends the light rays coming from each star, “gravitationally lensing” the galaxy.
In his quest for a general theory of relativity, Einstein became convinced that everything likes to live where it will age most slowly, and gravity pulls it there. Accordingly, his field equations imply that time has to slow down in the presence of massive objects (i.e., space-time warps), and Thorne describes some of the experimental evidence confirming this aspect of the general theory. When director Nolan asked Thorne if time on a certain planet could be slowed to the extent that a single hour on its surface could correspond to seven years on Earth, Thorne initially doubted that it could. However, after playing around with the field equations, Thorne discovered such slowing to be possible in the environs of a black hole spinning at 99.8% of the limiting rate of spin imposed by the fact that nothing can exceed the speed of light. In time, Thorne was able to develop a specialized set of field equations that enabled the project’s technically sophisticated special effects team to produce a picture of a massive and rapidly spinning black hole silhouetted against a star-studded patch of night sky (see Figure 2).

This, according to Thorne, is new science! It shows with great precision how “gravity lensing” would distort any picture one might take of the firmament beyond such an object. Indeed, Thorne is currently writing a technical paper describing the new science discovered during the making of Interstellar.

Thorne cofounded the Laser Interferometer Gravitational Wave Observatory (LIGO) project in 1983 along with Rainer Weiss at MIT and Ronald Drever at Caltech. The project he envisioned and spent two decades bringing to fruition is now an international collaboration of 900 scientists in 17 nations. With gravitational wave detectors in Hanford, Washington and Livingston, Louisiana (in the U.S.), a third location planned in India, and similar detectors under construction in Italy and Japan, it will form a worldwide network to explore the universe using gravitational waves. In the book, Thorne explains in particular how the movie’s NASA personnel might have used LIGO measurements to detect the presence of a wormhole near Saturn. 

As interesting as these recent discoveries are, they fall well short of explaining how the hero of the story, a NASA-trained space pilot/engineer named Cooper—played onscreen by Matthew McConaughey—manages to survive his fall into the black hole Gargantua and return through the wormhole in a tesseract (a four-dimensional hypercube) provided by “them” in time to witness his daughter’s death on a space station orbiting Saturn. It does even less to explain how Cooper manages to send data recorded inside the black hole backward through time to that very daughter (a celebrated physicist in adulthood) in time for her to figure out how one might exploit “gravitational anomalies” to propel county-sized space stations into solar orbit. Thorne supplies the missing explanations in the final chapters of this well-written book, a must read for all who refuse to abandon the dream of travel through interstellar space. 

Jim Case writes from Baltimore, Maryland.

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