I’m reasonably well-versed with small stuff. I’ve studied quantum mechanics, spent two years researching electronic structure, that sort of thing. I imagine that I’m about as comfortable as I’ll ever be with the incomprehensible probabilistic weirdness that underlies reality.
But although I helped teach introductory calculus-based physics, I’ve never learned about big things. I took no geometry in college, and most big physics, I assume, is about transferring equations into spaces that aren’t flat. The basic principle seems straightforward – you substitute variables, like if you’re trying to estimate prices in another country and keep plugging in the exchange rate – but I’ve never sat down and worked through the equations myself.
There’s only so much physics you can understand without chugging through the math. Our numbers don’t quite describe the world – they can’t exactly express quantities like pi, or the solutions to most three-body problems – but they do a better job than our words.
Still, some excellent pop-science books on gravity have been published recently. My favorite of these was On Gravity by A. Zee – it’s quite short, and has everything I assume you’d want from a book like this: bad humor, lucid prose, excellent pacing. Zee has clearly had a lot of practice teaching this material to beginners, and his expertise shines through.
Near the end of the book, Zee introduces black holes – gravity at its weirdest. Gravity becomes stronger as the distance between objects decreases – it follows an “inverse square law.”
If our moon was closer to Earth, the tides would be more extreme. To give yourself a sense of the behavior of inverse square laws, you can play with some magnets. When two magnets are far apart, it seems as though neither cares about the existence of the other, but slide them together and suddenly the force gets so strong that they’ll leap through the air to clank together.
But because each magnet takes up space, there’s a limit to how close they can get. Once you hear them clank, the attractive magnetic force is being opposed by a repulsive electrostatic force – this same repulsion gives us the illusion that our world is composed of solid objects and keeps you from falling through your chair.
Gravity is much weaker than magnetism, though. A bar magnet can have a strong magnetic field but will have an imperceptible amount of gravity. It’s too small.
A big object like our sun is different. Gravity pulls everything together toward the center. At the same time, a constant flurry of nuclear explosions pushes everything apart. These forces are balanced, so our sun has a constant size, pouring life-enabling radiation into the great void of space (of which our planet intercepts a teensy tiny bit).
But if a big object had much more mass than our sun, it might tug itself together so ardently that not even nuclear explosions could counterbalance its collapse. It would become … well, nobody knows. The ultra-dense soup of mass at the center of a black hole might be stranger than we’ve guessed. All we know for certain is that there is a boundary line inside of which the force of gravity becomes so strong that not even light could possibly escape.
Satellites work because they fall toward Earth with the same curvature as the ground below – if they were going faster, they’d spiral outward and away, and if they were going slower, they’d spiral inward and crash. The “event horizon” of a black hole is where gravity becomes so strong that even light will be tugged so hard that it’ll spiral inward. So there’s almost certainly nothing there, right at the “edge” of the black hole as we perceive it. Just the point of no return.
If your friends encounter a black hole, they’re gone. Not even Morse-code messages could escape.
(Sure, sure, there’s “Hawking radiation,” quantum weirdness that causes a black hole to shrink, but this is caused by new blips in the fabric of reality and so can’t carry information away.)
The plot of Saga, by Brian K. Vaughan and Fiona Staples, revolves around a Romeo & Juliet-esque romance in the middle of intergalactic war, but most of the comic is about parenting. K read the entire series in two days, bawling several times, and then ran from the bedroom frantic to demand the next volume (unfortunately for her, Vaughan & Staples haven’t yet finished the series).
Saga is masterfully well-done, and there are many lovely metaphors for a child’s development.
For instance, the loss of a child’s beloved caretaker – babysitters, daycare workers, and teachers do great quantities of oft under-appreciated work. In Saga, the child and her first babysitter are linked through the spirit, and when the caretaker moves on, the child feels physical pain from the separation.
A hairless beast named “Lying Cat” can understand human language and denounces every untruth spoken in its present – allowing for a lovely corrective to a child’s perception that she is to blame for the traumas inflicted upon her.
Perhaps my favorite metaphor in Saga depicts the risk of falling into a black hole. Like all intergalactic travelers, they have to be careful – in Saga, a black hole is called a “timesuck” and it’s depicted as a developing baby.
My favorite scene in the film Interstellar depicts the nightmarish weirdness of relativistic time. A massive planet seems perfectly habitable, but its huge gravitational field meant that the years’ worth of “Everything’s okay!” signals had all been sent within minutes of a scout’s arrival. The planet was actually so dangerous that the scout couldn’t survive a full day, but decades would have passed on Earth before anyone understood the risk.
Gravity eats time.
So do babies. A child is born and the new parents might disappear from the world. They used to volunteer, socialize, have interests and hobbies … then, nothing.
They fell into the timesuck.