Physics, at its most basic, is just a description

of the motion of the stuff in our universe. “This planet goes this way, that rocket

goes that way” – except that some – in fact, many – objects move without moving.

Or, more precisely, they move without going anywhere. I’m talking objects that spin, revolve,

rotate, pirouette, orbit, circle, gyrate, whirl, twirl, cartwheel, and so on. Like a

planet around a star, an electron in an atom, or even our solar system going around the

gravitational center of the milky way: from up close they’re certainly moving, but in

the grand scheme of things, that motion doesn’t take them anywhere. We can still talk about it, though: just like

“momentum” is a concept that describes how much oomph an object has when it moves

in a straight line, “angular momentum” is a way to account for how much oomph objects

have when they’re going in circles – figuratively, or literally. And angular momentum is simple, in theory:

pick a point, any point. Pretend your object is moving in a circle around that point. Figure

out how fast the object is moving along the circle (never mind that it probably isn’t

moving exactly along the circle, and that the circle might have to change size over

time to follow the object), then multiply that speed times the size of the circle and

the object’s mass, and there you have it: angular momentum. For example, a 2 kilogram 60 cm-diameter bicycle

wheel going 20 km per hour would have an angular momentum of about 7 kilogram meters squared

per second. The reason we care about angular momentum

is that if you take a bunch of objects that are interacting electromagnetically or gravitationally

or whatever, and add up all of their angular momenta into one number, then that total value

won’t change over time (unless some other objects from outside come in and mess things

up). So earth, which is 150 million kilometers

from the sun, orbits at 30 km/s and has a mass of 6*10^24 kilograms, has an angular

momentum of 2.7 * 10^40 kilogram meters squared per second. That’s four thousand quintillion

quintillion bicycle wheels! And this angular momentum stays roughly constant over the course

of the earth’s orbit year in and year out. But what’s amazing is that even if the sun

and the rest of the solar system were to suddenly disappear, the earth would STILL have that

same angular momentum about the point where the sun WAS. Without the sun’s gravity,

the earth would of course now move in a straight line, requiring an ever-larger imaginary circle

as it got farther from the point where the sun used to be. But as the earth continued

through space, its 30km/s velocity would also point less and less along the circle, so when

you calculated the angular momentum, the decrease in velocity would exactly cancel out the increase

in the size of the circle, and you’d always get the same answer. 2.7 * 10^40 kilogram

meters squared per second. So even when nothing is rotating at all, angular

momentum is still conserved. And that’s the beauty of a law of physics – it works

even when you try to break it!