How do we know when we're going fast? Generally speaking, we don't. Observers on Earth's surface rotate around its axis at 40 meters per second; Earth itself revolves around the Sun (30 kilometers/second); and the Sun rotates around the galactic center at about (220 km/s). Meanwhile, the whole kit is being dragged in a stately gyre toward the constellation Virgo at about 370 km/s. What, you don't feel that?

We don't because we judge motion by acceleration, that is, a change in velocity. This information comes to us in several...