The Science of Interstellar

Below the Ranger is an outflying singularity, created by stuff that fell into the black hole long ago and was backscattered upward toward the Ranger (Chapter 27).

The Ranger is sandwiched between the two singularities (Figure 28.2). Inevitably, it will be hit by one or the other.

^{Fig. 28.2. An icon representing the Ranger sandwiched between Gargantua’s infalling and outflying singularities. The Ranger is drawn far larger than it should be, so you can see it.}

When I explained the two singularities to Chris, he immediately knew which one should hit the Ranger. The outflying singularity. Why? Because Chris had already adopted, for Interstellar, a variant of the laws of physics that prevents physical objects from ever traveling backward in time (Chapter 30). The infalling singularity is produced by stuff that falls into Gargantua long after Cooper falls in (long after, as measured by the external universe’s time; Earth’s time). If Cooper is hit by that singularity and survives, the universe’s far future will be in his past. He will be so far in our future that, even with the help of bulk beings, he won’t be able to return to the solar system until billions of years after he left, if ever. That would prevent him from ever reuniting with his daughter, Murph.

So Chris firmly chose Cooper to be hit by the outflying singularity, not the infalling one—hit by the singularity arising from stuff that fell into Gargantua before the Ranger, not after it.

Chris’s choice, though, presents a bit of a problem for my scientist’s interpretation of the movie. But not a problem so severe as backward time travel. If the Ranger falls directly into Gargantua from the critical orbit, then its infall is slow enough that the infalling singularity will catch up to it and hit it. For the Ranger to hit the outflying singularity instead, as Chris wants, the Ranger must nearly outrun the infalling singularity, which is descending at the speed of light. The Ranger can do so, if it is given a large, inward kick. How? The usual: by a slingshot around a suitable intermediate-mass black hole soon after leaving the Endurance.

What Does Cooper See Inside Gargantua?

Looking up as he falls inward, Cooper sees the external universe. Because his infall has been sped up, he sees time in the external universe flow at roughly the same rate as his own time,[51] and he sees the image of the external universe reduced in size,[52] from about half of the sky to roughly a quarter.

When I was first shown the movie’s depiction of this, I was pleased to discover that Paul Franklin’s team got it right, and also got right something I had missed: In the movie, the image of the universe above is surrounded by Gargantua’s accretion disk (Figure 28.3). Can you explain why this must be so?

Cooper sees all this above him, but he doesn’t see the infalling singularity. It is moving downward toward him at the speed of light, chasing but not catching the light rays that bring him images of the disk and universe above.

Because we are rather ignorant of what goes on inside black holes, I told Chris and Paul that I’d be comfortable if they used their imaginations in depicting what Cooper sees coming up at him from below, as he falls. I made only one request: “Please don’t depict Satan and the fires of Hades inside the black hole like the Disney Studios did in their Black Hole movie.” Chris and Paul chuckled. They weren’t tempted in the least.

When I saw what they did depict, it made great sense. Looking downward, Cooper should see light from objects that fell into Gargantua before him and are still falling inward. Those objects need not emit light themselves. He can see them in reflected light from the accretion disk above, just as we see the Moon in reflected sunlight. I expect those objects to be mostly interstellar dust, and this could explain the fog he encounters in the movie as he falls.

^{Fig. 28.3. The universe above, surrounded by the accretion disk, as seen by Cooper inside Gargantua, looking upward across his Ranger’s fuselage. Gargantua’s shadow is the black region on the left. [From Interstellar, used courtesy of Warner Bros. Entertainment Inc.]}

Cooper can also overtake stuff that’s infalling more slowly than he. This may explain the white flakes that hit and bounce off his Ranger in the movie.

Rescued by the Tesseract