Classical Mechanics
The Dark Side of the Moon
Every time you've ever looked up at a full moon, it's looked just like this:
This is the only moon that mankind has ever known, the only side of our satellite that ever faces Earth. The moon is stuck in a perpetual staring contest with the Earth due to a phenomenon called tidal locking, where the same forces responsible for tides on Earth have conspired to slow down the moon's rotation until it revolves once around its own axis in exactly the same amount of time it takes the moon to orbit once around the earth.
This is no cosmic coincidence, but rather the rate of rotation that naturally minimizes the torque on the moon. Minimums, in general, are good—they're why balls roll down hills (minimum potential energy), why planets are spherical (minimum surface area)2, why a quarter pounder is so cheap (minimum wage). Nature likes minimums, and so it's not surprising that she strives for one in satellite orbits, as well.
How, exactly, nature manages to change the moon's rotational speed is trickier. It's very similar to tides on Earth—the water on Earth directly beneath the moon is pulled towards our satellite by gravity, contributing to"high tide":
The force of gravity between the Earth and moon is so strong that the moon itself is stretched in much the same way, so that it's not truly a sphere but rather what mathematicians and rugby players call a prolate spheroid (though admittedly just barely).
Let's rewind the clock about four billion years. (Please don't try this at home. Or if you do, at least wear safety goggles.) The moon, newly formed—or perhaps collected, ejected, trapped, summoned, etc3—spins rapidly around its axis, revolving many times during a single orbit around the earth. It spins so rapidly, in fact, that the deformation caused by Earth's gravity can't keep up with its rotation. The moon starts to bulge, pulled towards the Earth, but rotates quickly away—and the next section of the moon is pulled, but then rotates away, so the next section is pulled, and so on and so on.
This means the mass of our early, rapidly spinning moon is never evenly distributed. It sticks out to the side like a rocky cowlick, and so the earth's gravity tugs at the moon unevenly, too.
The uneven force of gravity pulling on this stretched lump of moon rock creates a torque around the moon's axis:
This torque is directed in the opposite direction of the moon's rotation—gradually slowing the rotation of the moon. In fact, the torque only disappears when the rotational period of the moon about its own axis is exactly the same as its orbital period. When that happens, the prolate spheroid shape of the moon is directed straight along the line between the earth and the moon, and its mass is evenly distributed.
Once the periods are the same, only one side of the moon ever faces the earth. It wasn't until 1959 that a Soviet lunar probe, Luna 3, took the first recorded images of the dark side of the moon4.
Spoiler alert—it looks like this:
