The original version of this story appeared in Quanta Magazine.
Imagine standing on a railway platform watching a trolley go past. A girl on the trolley drops a bright red ball. To her, the ball falls straight down. But from the platform, you see the ball traverse an arc before hitting the trolley floor. The two of you observe the same event, but from different reference frames: one anchored to the trolley and the other to the platform.
The idea of reference frames has a storied history in classical physics: Isaac Newton, Galileo, and Albert Einstein all relied on them for their studies of motion. A reference frame is essentially a