The Science of Interstellar

A typical accretion disk and its jet emit radiation—X-rays, gamma rays, radio waves, and light—radiation so intense that it would fry any human nearby. To avoid frying, Christopher Nolan and Paul Franklin gave Gargantua an exceedingly anemic disk.

Now, “anemic” doesn’t mean anemic by human standards; just by the standards of typical quasars. Instead of being a hundred million degrees like a typical quasar’s disk, Gargantua’s disk is only a few thousand degrees, like the Sun’s surface, so it emits lots of light but little to no X-rays or gamma rays. With gas so cool, the atoms’ thermal motions are too slow to puff the disk up much. The disk is thin and nearly confined to Gargantua’s equatorial plane, with only a little puffing.

Disks like this might be common around black holes that have not torn a star apart in the past millions of years or more—that have not been “fed” in a long time. The magnetic field, originally confined by the disk’s plasma, may have largely leaked away. And the jet, previously powered by the magnetic field, may have died. Such is Gargantua’s disk: jetless and thin and relatively safe for humans. Relatively.

Gargantua’s disk looks quite different from the pictures of thin disks that you see on the web or in astrophysicists’ technical publications, because those pictures omit a key feature: the gravitational lensing of the disk by its black hole. Not so in Interstellar, where Chris insisted on visual accuracy.

Eugénie von Tunzelmann was charged with putting an accretion disk into Oliver James’ gravitational lensing computer code, the code I described in Chapter 8. As a first step, just to see what the lensing does, Eugénie inserted a disk that was truly infinitesimally thin and lay precisely in Gargantua’s equatorial plane. For this book she has provided a more pedagogical version of that disk, made of equally spaced color swatches (Inset in Figure 9.7).

If there had been no gravitational lensing, the disk would have looked like the inset. The lensing produced huge changes from this (body of Figure 9.7). You might have expected the back portion of the disk to be hidden behind the black hole. Not so. Instead, it is gravitationally lensed to produce two images, one above Gargantua and the other below; see Figure 9.8. Light rays emitted from the disk’s top face, behind Gargantua, travel up and over the hole to the camera, producing the disk image that wraps over the top of Gargantua’s shadow in Figure 9.7; and similarly for the disk image that wraps under the bottom of Gargantua’s shadow.

^{Fig. 9.7. An infinitesimally thin disk in Gargantua’s equatorial plane, gravitationally lensed by Gargantua’s warped space and time. Here Gargantua spins very fast. Inset: The disk in the absence of the black hole. [From Eugénie von Tunzelmann’s artistic team at Double Negative.]}

^{Fig. 9.8. Light rays (red) bring to the camera images of the back part of the accretion disk, behind Gargantua: one image above the hole’s shadow, the other below the hole’s shadow.}

Inside these primary images, we see thin secondary images of the disk, wrapping over and under the shadow, near the shadow’s edge. And if the picture were made much larger, you would see tertiary and higher-order images, closer and closer to the shadow.

Can you figure out why the lensed disk has the form you see? Why is the primary image wrapping under the shadow attached to the thin secondary image wrapping over it? Why are the paint swatches on the over-wrapping and under-wrapping images widened so greatly, and those on the sides squeezed?…

Gargantua’s space whirl (space moving toward us on the left and away on the right) distorts the disk images. It pushes the disk away from the shadow on the left and toward the shadow on the right, so the disk looks a bit lopsided. (Can you explain why?)

To get further insight, Eugénie von Tunzelmann and her team replaced their variant of the color-swatch disk (Figure 9.7) with a more realistic thin accretion disk: Figure 9.9. This was much more beautiful, but it raised problems. Chris did not want his mass audience to be confused by the lopsidedness of the disk and black-hole shadow, and the shadow’s flat left edge, and the complicated star-field patterns near that edge (discussed in Chapter 8). So he and Paul slowed Gargantua’s spin to 0.6 of the maximum, making these weirdnesses more modest. (Eugénie had already omitted the Doppler shift caused by the disk’s motion toward us on the left and away on the right. It would have made the disk far more lopsided: bright blue on the left and dim red on the right—totally confusing to a mass audience!)

^{Fig. 9.9. Gargantua with the infinitesimally thin paint-swatch disk (Fig. 9.7) replaced by a more realistic, infinitesimally thin accretion disk. [From Eugénie von Tunzelmann’s artistic team at Double Negative.]}

The artistic team at Double Negative then gave the disk the texture and surface relief that we expect a real, anemic accretion disk to have, puffing it up a bit in a manner that varied from place to place. They made the disk hotter (brighter) near Gargantua and cooler (dimmer) at larger distances. They made it thicker at larger distances because it is Gargantua’s tidal gravity that squeezes the disk into the equatorial plane, and tidal gravity is much weaker farther from the black hole. They added the background galaxy: many layers of artwork (dust, nebulae, stars). And they added lens flare—the haze and glare and streaks of light that would arise from scattering of the disk’s bright light in a camera lens. The results were the wonderful and compelling images in the movie (Figures 9.10 and 9.11).

^{Fig. 9.10. Gargantua and its accretion disk, with Miller’s planet above the disk’s left edge. The disk is so bright that the stars and nebulae are barely visible. [From Interstellar, used courtesy of Warner Bros. Entertainment Inc.]}

^{Fig. 9.11. A segment of Gargantua’s disk seen up close, with the Endurance passing over it. The black region is Gargantua, framed by the disk and with some white scattered light in the foreground. [From Interstellar, used courtesy of Warner Bros. Entertainment Inc.]}

Eugénie and her team also, of course, made the disk’s gas orbit Gargantua, as it must to avoid falling in. When combined with gravitational lensing, the gas’s orbital motion produced the impressive streaming effects in the movie—streaming effects that are hinted at by the gas’s streamlines in Figure 9.11.

What a joy it was when I first saw these images! For the first time ever, in a Hollywood movie, a black hole and its disk depicted as we humans will really see them when we’ve mastered interstellar travel. And for the first time for me as a physicist, a realistic disk, gravitationally lensed, so it wraps over the top and bottom of the hole instead of being hidden behind the hole’s shadow.

With Gargantua’s disk anemic, though gorgeously beautiful, and with no jet, is Gargantua’s environment truly benign? Amelia Brand thinks so…

10 Accident Is the First Building Block of Evolution

In Interstellar, upon finding Miller’s planet sterile, Amelia Brand argues for going next to a planet very far from Gargantua, Edmunds’ planet, instead of the closer Mann’s planet: “Accident is the first building block of evolution,” she tells Cooper. “But when you’re orbiting a black hole, not enough can happen—it sucks in asteroids and comets, other events that would otherwise reach you. We need to go further afield.”