The Science of Interstellar

In a second variant of the explanation (Figure 9.4), the whirling magnetic field is anchored in the accretion disk instead of the hole, and is dragged around by the disk’s orbital motion. Otherwise, the story is the same: dynamo action; plasma flung out. This variant works well even if the black hole isn’t spinning. But we’re pretty sure that most black holes spin fast, so I suspect the Blandford-Znajek mechanism (Figure 9.3) is the most common one in quasars. However, I may be prejudiced. I spent much time in the 1980s exploring aspects of the Blandford-Znajek ideas and even coauthored a technical book about them.

^{Fig. 9.4. Like Figure 9.3 but with magnetic field anchored in the accretion disk. [Drawing by Matt Zimet based on a sketch by me; from my book Black Holes & Time Warps: Einstein’s Outrageous Legacy.]}

Whence Comes the Disk? Tidal Forces Tear Stars Apart

Lynden-Bell, in 1969, speculated that quasars live at the centers of galaxies. We don’t see a quasar’s host galaxy, he said, because its light is so much fainter than the quasar’s light. The quasar drowns the galaxy out. In the decades since then, with improving technology, astronomers have found the galaxy’s light around many quasars, confirming Lynden-Bell’s speculation.

In those recent decades we also learned where most of the disk’s gas comes from. Occasionally a star strays so close to the quasar’s black hole that the hole’s tidal gravity (Chapter 4) tears the star apart. Much of the shredded star’s gas is captured by the black hole and forms an accretion disk, but some of the gas escapes.

In recent years, thanks to improving computer technology, astrophysicists simulated this. Figure 9.5 is from a recent simulation by James Guillochon, Enrico Ramirez-Ruiz, and Daniel Kasen (University of California at Santa Cruz) and Stephan Rosswog (University of Bremen).[22] At time zero (not shown) the star was headed almost precisely toward the black hole and the hole’s tidal gravity was beginning to stretch the star toward the hole and squeeze it from the sides, as in Figure 6.1. Twelve hours later the star is strongly deformed and at the location shown in Figure 9.5. Over the next few hours, it swings around the hole along the blue gravitational-slingshot orbit and deforms further as shown. By twenty-four hours the star is flying apart; its own gravity can no longer hold it together.

^{Fig. 9.5. Tidal disruption of a red giant star by a black hole similar to Gargantua.}

The star’s subsequent fate is shown in Figure 9.6, from a different simulation by James Guillochon together with Suvi Gezari (Johns Hopkins University). For a movie of this simulation, see http://hubblesite.org/newscenter/archive/releases/2012/18/video/a/.

^{Fig. 9.6. Subsequent fate of the star in Figure 9.5.}

The top two images are shortly before the beginning and shortly after the end of Figure 9.5; I enlarged these two images tenfold compared to the others, to make the hole and the disrupting star visible.

As the whole set of images shows, over the subsequent several years much of the star’s matter is captured into orbit around the black hole, where it begins to form an accretion disk. And the remaining matter escapes from the hole’s gravitational pull along a streaming, jetlike trajectory.

Gargantua’s Accretion Disk and Missing Jet

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.