The Science of Interstellar

Black holes (Chapter 5), by contrast, are made fully and solely from warped space and warped time (I’ll explain this weird claim in Chapter 4). They contain no matter whatsoever, but they have surfaces, called “event horizons,” or just “horizons,” through which nothing can escape, not even light. That’s why they are black. A black hole’s circumference is proportional to its mass: the heavier it is, the bigger it is.

A black hole with about the same mass as a typical neutron star or white dwarf (say 1.2 times as heavy as the Sun) has a circumference of about 22 kilometers: a fourth that of the neutron star and a thousandth that of the white dwarf. See Figure 2.5.

Fig. 2.5. A white dwarf (left), neutron star (middle), and black hole (right) that all weigh as much as 1.2 Suns. For the white dwarf I show only a tiny segment of its surface.

Since stars are generally no heavier than about 100 Suns, the black holes to which they give birth are also no heavier than 100 Suns. The giant black holes in the cores of galaxies, a million to 20 billion times heavier than the Sun, therefore, cannot have been born in the death of a star. They must have formed in some other way, perhaps by the agglomeration of many smaller black holes; perhaps by the collapse of massive clouds of gas.

Magnetic, Electric, and Gravitational Fields

Because magnetic force lines play a big role in our universe and are important for Interstellar, let’s discuss them, too, before diving into Interstellar’s science.

As a student in science class, you may have met magnetic force lines in a beautiful little experiment. Do you remember taking a sheet of paper, placing a bar magnet under it, and sprinkling iron filings (elongated flakes of iron) on top of the paper? The iron filings make the pattern shown in Figure 2.6. They orient themselves along magnetic force lines that otherwise are invisible. The force lines depart from one of the magnet’s poles, swing around the magnet, and descend into the other pole. The magnetic field is the collection of all the magnetic force lines.

Fig. 2.6. Magnetic force lines from a bar magnet, made visible by iron filings sprinkled on a sheet of paper. [Drawing by Matt Zimet based on a sketch by me; from my book Black Holes & Time Warps: Einstein’s Outrageous Legacy.]

When you try to push two magnets together with their north poles facing each other, their force lines repel each other. You see nothing between the magnets, but you feel the magnetic field’s repulsive force. This can be used for magnetic levitation, suspending a magnetized object—even a railroad train (Figure 2.7)—in midair.

The Earth also has two magnetic poles, north and south. Magnetic force lines depart from the south magnetic pole, swing around the Earth, and descend into the north magnetic pole (Figure 2.8). These force lines grab a compass needle, just as they grab iron filings, and drive the needle to point as nearly along the force lines as possible. That’s how a compass works.

Fig. 2.7. The world’s first commerical magnetically levitated train, in Shanghai, China.

Fig. 2.8. The Earth’s magnetic force lines.

The Earth’s magnetic force lines are made visible by the Aurora Borealis (the Northern Lights; Figure 2.9). Protons flying outward from the Sun are caught by the force lines and travel along them into the Earth’s atmosphere. There the protons collide with oxygen and nitrogen molecules, making the oxygen and nitrogen fluoresce. That fluorescent light is the Aurora.

Fig. 2.9. The Aurora Borealis in the sky over Hammerfest, Norway.

Fig. 2.10. Artist’s conception of a neutron star with its donut-shaped magnetic field and its jets.