I’ll let you imagine how an ultra-advanced civilization might use such complex orbits. In my science interpretations of the movie, for simplicity I mostly eschew them and focus primarily on circular, equatorial orbits (those of the parked Endurance, Miller’s planet, and the critical orbit), and on simple trajectories for the Endurance as it travels from one circular equatorial orbit to another. An exception is the orbit of Mann’s planet, discussed in Chapter 19.
NASA’s Gravitational Slingshots in the Solar System
Let’s return from the world of the possible (what the laws of physics allow) to hard-nosed, real-life gravitational slingshots in the comfy confines of our solar system (what humans have actually achieved as of 2014).
You may be familiar with NASA’s Cassini spacecraft (Figure 7.7). It was launched from Earth on October 15, 1997, with too little fuel to reach its destination, Saturn. The deficit was dealt with by slingshots: around Venus on April 26, 1998; a second slingshot around Venus on July 24, 1999; around Earth on August 18, 1999; and around Jupiter on December 30, 2000. Arriving at Saturn on July 1, 2004, Cassini slowed down with the aid of a slingshot around Saturn’s closest moon, Io.
None of these slingshots looked like the ones I described above. Instead of strongly deflecting the spacecraft’s direction of motion, Venus, Earth, Jupiter, and Io deflected it only mildly. Why?
The deflectors’ gravity was too weak to produce a strong deflection. For Venus, Earth, and Io, the deflection was inevitably small because their gravity is intrinsically weak. Jupiter has much stronger gravity, but a large deflection would have sent Cassini in the wrong direction; reaching Saturn required a small deflection.
Despite the small deflections, Cassini got substantial kicks from the flybys, big enough to compensate for inadequate fuel. In each flyby (except Io), Cassini traveled behind the deflecting planet but at an angle, so the planet’s gravity optimally pulled Cassini forward, speeding it up. In Interstellar, the Endurance does a similar slingshot around Mars.
Cassini has been exploring Saturn and Saturn’s moons for the past ten years, sending back amazing images and information—a treasure trove of beauty and science. For a glimpse, see http://www.nasa.gov/mission_pages/cassini/main/.
By contrast with these weak slingshots in the solar system, Gargantua’s intense gravity can grab even objects moving at ultrahigh speeds and throw them around on strongly bent slingshots. Even a light ray. This produces gravitational lensing, the key to seeing Gargantua.
8
Imaging Gargantua
Black holes emit no light, so the only way to see Gargantua is by its influence on light from other objects. In Interstellar the other objects are an accretion disk (Chapter 9) and the galaxy in which it lives including nebulae and a rich field of stars. For the sake of simplicity, let’s include only the stars for now.
Gargantua casts a black shadow on the field of stars and it also deflects the light rays from each star, distorting the stellar pattern that the camera sees. This distortion is the gravitational lensing discussed in Chapter 3.
Figure 8.1 shows a rapidly spinning black hole (let’s call it Gargantua) in front of a field of stars, as it would appear to you if you were in Gargantua’s equatorial plane. Gargantua’s shadow is the totally black region. Immediately outside the shadow’s edge is a very thin ring of starlight called the “ring of fire” that I intensified by hand to make the edge of the shadow more distinct. Outside that ring we see a dense sprinkling of stars with a pattern of concentric shells, a pattern produced by the gravitational lensing.
As the camera orbits around Gargantua, the field of stars appears to move. This motion combined with the lensing produces dramatically changing patterns of light. The stars stream at high speed in some regions, they float gently in others, and they’re frozen in still other regions; see the film clip on this book’s page at Interstellar.withgoogle.com.
In this chapter I explain all these features, beginning with the shadow and its ring of fire. Then I describe how the black-hole images in Interstellar were actually produced.
When imaging Gargantua in this chapter, I treat it as a fast-spinning black hole, as it must be to produce the extreme loss of time that the Endurance’s crew experience relative to Earth (Chapter 6). However, for fast spin, a mass audience could be confused by the flattening of the left edge of Gargantua’s shadow (Figure 8.1) and by some peculiar features of the star streaming and the accretion disk, so Christopher Nolan and Paul Franklin chose a smaller spin, 60 percent of the maximum, for their Gargantua images in the movie. See the last section in Chapter 9.
Warning: The explanations in the following three sections may require a lot of thought; you can skip them without losing pace with the rest of the book. Not to worry!
The Shadow and Its Ring of Fire
The shell of fire (Chapter 6) plays a key role in producing Gargantua’s shadow and the thin ring of fire alongside it. The shell of fire is the purple region surrounding Gargantua in Figure 8.2, and it contains nearly trapped photon orbits (light rays) such as the one in the upper right inset.[20]
Suppose you are at the location of the yellow dot. The white light rays A and B and others like them bring you the image of the ring of fire, and the black light rays A and B bring you the image of the shadow’s edge. For example, the white ray A originates at some star far from Gargantua, it travels inward and gets trapped on the inner edge of the shell of fire in Gargantua’s equatorial plane, where it flies round and round, driven by the whirl of space, and then escapes and comes to your eyes. The black ray also labeled A originates on Gargantua’s event horizon, it travels outward and gets trapped on that same inner edge of the shell of fire, where it goes round and round, then escapes and reaches your eyes alongside the white ray A. The white ray brings you an image of a bit of the thin ring; the black ray, an image of a bit of the shadow’s edge. The shell of fire is responsible for merging the rays side by side and directing them toward your eyes.