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 ...

In any experiment, specifying a physical quantity of interest always relies on a reference frame. For example, identifying the time at which an event happens only makes sense relative to a clock.

Quantum mechanics forms the bedrock of modern physics, describing the behaviour of matter and energy at the smallest scales. Its counterintuitive concepts, such as wave‐particle duality, superposition ...