Imaging a Gravitational Slingshot
Although Chris chose not to show any gravitational slingshots in Interstellar, I wondered what they would look like to Cooper as he piloted the Ranger toward Miller’s planet. So I used my equations and Mathematica to simulate them and produce images. (My images have far lower resolution than Oliver’s and Eugénie’s due to my code’s slowness.)
Figure 8.7 shows a sequence of images, as Cooper’s Ranger swings around an intermediate-mass black hole (IMBH) to initiate its descent toward Miller’s planet—in my scientist’s interpretation of Interstellar. This is the slingshot described in Figure 7.2.
In the top image, Gargantua is in the background with the IMBH passing in front of it. The IMBH grabs light rays from distant stars that are headed toward Gargantua, swings the rays around itself, and ejects them toward the camera. This explains the donut of starlight that surrounds the IMBH’s shadow. Although the IMBH is a thousand times smaller than Gargantua, it is far closer to the Ranger than is Gargantua, so it looks only modestly smaller.
As the IMBH appears to move rightward, as seen by the slingshot-moving camera, it leaves Gargantua’s primary shadow behind itself (middle picture in Figure 8.7), and it pushes a secondary image of Gargantua’s shadow ahead of itself. These two images are completely analogous to the primary and secondary images of a star gravitationally lensed by a black hole; but now it is Gargantua’s shadow that is being lensed, by the IMBH. In the bottom picture, the secondary shadow is shrinking in size, as the IMBH moves onward. By this time the slingshot is nearly complete, and the camera, on board the Ranger, is headed downward, toward Miller’s planet.
As impressive as these images may be, they can be seen only up close to the IMBH and Gargantua, not from the great distance of Earth. To astronomers on Earth, the most visually impressive things about gigantic black holes are jets that stick out of them and the light from brilliant disks of hot gas that orbit them. To these we’ll now turn.
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Disks and Jets
Quasars
Most of the objects seen by radio telescopes are huge clouds of gas, clouds far larger than any star. But in the early 1960s a few tiny objects were found. Astronomers named these objects quasars for “quasi-stellar radio sources.”
In 1962 the Caltech astronomer Maarten Schmidt, looking through the world’s largest optical telescope on Palomar Mountain, discovered light coming from a quasar called 3C273. It looked like a bright star with a faint jet shooting out of it (Figure 9.1). This was weird!
When Schmidt split 3C273’s light into its various colors (as is sometimes done by sending light through a prism), he saw the set of spectral lines in the bottom of Figure 9.1. At first sight, these were unlike any spectral lines he had ever seen. But in February 1963, after a few months’ struggle, he realized the lines were unfamiliar simply because their wavelengths were 16 percent larger than normal. This is called the Doppler shift; it was caused by the quasar’s moving away from Earth at 16 percent the speed of light, about c/6. What could cause that ultrafast motion? The least crazy explanation Schmidt could find was the expansion of the universe.
As the universe expands, objects far from Earth move apart from us at very high speed, and objects nearer move away more slowly. 3C273’s enormous speed, one-sixth that of light, meant that 3C273 was 2 billion light-years from Earth, nearly the farthest object that had ever been seen at that time. From its brightness and its distance, Schmidt concluded that 3C273 puts out 4 trillion times more power in light than the Sun, and a hundred times more power than the brightest galaxies!
This prodigious power fluctuated on times as short as a month, so most of the light must be coming from an object so small that the light can travel across it in one month’s time—far smaller than the distance from Earth to the nearest star, Proxima Centauri. And other quasars with almost as much power fluctuated on times of a few hours, so they had to be not much larger than our solar system. One hundred times the power of a bright galaxy, coming from a region the size of our solar system; that was phenomenal!
Black Holes and Accretion Disks
How could so much power come out of a region so small? When we think about the fundamental forces in Nature, there are three possibilities: chemical energy, nuclear energy, or gravitational energy.
Chemical energy is the energy released when molecules combine together to make new kinds of molecules. An example is burning gasoline, which combines oxygen from the air with gasoline molecules to make water and carbon dioxide, and a lot of heat. The power from that would be far, far, far too little though.
Nuclear energy results when atomic nuclei combine together to make new atomic nuclei. Examples are an atomic bomb, a hydrogen bomb, and the burning of nuclear fuel inside a star. Though this can be far more powerful than chemical energy (think of the difference between a gasoline fire and a nuclear bomb), astrophysicists couldn’t see any plausible way for nuclear energy to power quasars. It was still too puny.
So the only possibility left was gravitational energy, the same kind of energy we were driven to, when navigating the Endurance around Gargantua. For the Endurance, gravitational energy was harnessed by a slingshot around an intermediate-mass black hole (Chapter 7). The black hole’s intense gravity was key. For quasars, similarly, the power must come from a black hole.
For several years, astrophysicists struggled to figure out how a black hole could do the job. The answer was found in 1969, by Donald Lynden-Bell at the Royal Greenwich Observatory in England. A quasar, Lynden-Bell hypothesized, is a gigantic black hole surrounded by a disk of hot gas (an accretion disk) that is threaded by a magnetic field (Figure 9.2).
Hot gas in our universe is almost always threaded by magnetic fields (Chapter 2). These fields are locked into the gas; the gas and fields move together, in lockstep.
When threading an accretion disk, a magnetic field becomes a catalyst for converting gravitational energy into heat and then light. The field provides ultrastrong friction[21] that slows the gas’s circumferential motion, reducing the centrifugal force that holds it out against the pull of gravity, so the gas moves inward, toward the black hole. As the gas moves inward, the hole’s gravity speeds up its orbital motion by even more than the friction slowed it. In other words, gravitational energy is converted into kinetic energy (energy of motion). Magnetic friction then converts half that new kinetic energy into heat and light, and the process repeats.
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The friction arises through a complex process where moving gas winds the field up, strengthening it and thereby converting energy of motion into magnetic energy; and then the magnetic field, pointing in opposite directions in neighboring regions of space, reconnects and in the process converts magnetic energy into heat. That’s the nature of friction: a conversion of motion into heat.