The Science of Interstellar

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.

^{Fig. 9.1. Top: Photograph of 3C273 taken by NASA’s Hubble Space Telescope. The star (upper left) looks big only because the photo is overexposed in order to see the faint jet (lower right). It is actually so small that its size cannot be measured. Bottom: Maarten Schmidt’s spectral lines from 3C273 (upper panel) compared with spectral lines of hydrogen measured in an Earth laboratory. The quasar’s three lines are the same as hydrogen’s lines called Hβ, Ηγ, and Ηδ, but with wavelengths increased by 16 percent. (The images of the spectral lines are photographic negatives: black lines are really bright.)}

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.