A very difficult book. In hindsight, this wouldn’t have been all that surprising had I known that it was Kip Thorne, the Nobel-winning theoretical physicist that worked on the science front of this movie: Black holes, gravitational slingshots, disks and jets, wormholes, the fourth and fifth dimensions, brane and bulk, the tesseract, etc.
I’ll only quote (or rephrase) three ideas I find most fascinating.
1 Gravitational Slingshots
The gravity-assisted maneuver is called a “gravitational slingshot,” and has often been used by NASA in the solar system—though with the gravity coming from planets rather than a black hole. Take a look at the trajectory of NASA’s Cassini spacecraft from Earth to Saturn 🪐 (In the NASA app, go to missions, then Cassini). In each flyby (except Io), Cassini traveled behind the deflecting planet but at an angle, so the planet’s gravity optimally pulled Cassini forward, speeding it up. In Interstellar, the Endurance does a similar slingshot around Mars.
To fly across much of the universe (not just interstellar travel, but intergalactic travel) at near light speed in a few years of our own life, we will need to do so with the aid of two black holes that are orbiting each other, a black-hole binary.
This is a bit hopeless. Unlike black holes, no objects in our universe could become wormholes as they age.
There is a very tiny hope that wormholes exist naturally on submicroscopic scales in the form of “quantum foam”, a hypothesized network of wormholes that is continually fluctuating in and out of existence in a manner governed by the laws of quantum gravity. The foam is probabilistic in the sense that, at any moment, there is a certain probability the foam has one form and also a probability that it has another form, and these probabilities are continually changing. And the foam is truly tiny: the typical length of a wormhole would be the so-called Planck length, 0.000000000000000000000000000000001 centimeters.
Besides, any traversable, spherical wormhole must be threaded by some sort of material that has negative energy. Kip Throne called such material “exotic matter”, and challenged his physicist colleagues to figure out whether the combined quantum laws and relativistic laws permit a very advanced civilization to collect enough exotic matter inside a wormhole to hold it open. This triggered a lot of research by a lot of physicists; but today, nearly thirty years later, the answer is still unknown. The only real hope for forming traversable wormholes artificially is in the hands of an ultra-advanced civilization. (In Interstellar, the wormhole is thought to have been made, held open, and placed near Saturn by a civilization that lives in the bulk, a civilization whose beings have four space dimensions, like the bulk.)
3 The Tesseract (四维超正方体)
I had to watch a few YouTube videos to understand each of the 8 sides of a Tesseract is a cube.
In Interstellar, the extrusions and the latticework of bedrooms and chambers are nothing but time represented as a physical dimension.
In the movie we see Cooper pushing with his finger on the world tube* of a watch’s second hand. His pushes produce a backward-in-time gravitational force, which makes the second-hand twitch in a Morse-encoded pattern that carries the quantum data. The tesseract stores the twitching pattern in the bulk so it repeats over and over again.
(* Physicists call the extrusion of each particle of matter the particle’s “world line”, and a variant of this extrusion the “world tube.”)
To beings in the bulk, time may be just another physical dimension, “the past might be a canyon They (bulk beings) can climb into and the future a mountain They can climb up.”
Einstein’s laws, extended into the bulk, tell us that local bulk time can’t behave this way. Nothing in the bulk can go backward in local bulk time. However, when looking into our brane from the bulk, our brane’s time can look like just another physical dimension.
And Chris Nolan did just that. He presented our brane’s time as seen from the bulk.
Last, maybe I should mention Kip Thorn’s mentor, John Wheeler, as in the Wheeler–DeWitt equation, an equation accounting for quantum gravity without any time variable. At the fundamental level, no such special variable exists— time doesn’t exist.