Christian Doppler (1803–1853) was an Austrian physicist and mathematician who explained something you've heard a thousand times without thinking about it: the way a siren's pitch drops as an ambulance zooms past you. That everyday whoosh turned out to be a clue that reaches all the way out to the edge of the universe.
The Doppler effect says that when a source of waves is moving toward you the waves get bunched up
(higher pitch, bluer light) and when it moves away they get stretched out (lower pitch, redder
light). For light this stretching is the
Doppler worked out his effect on paper in 1842, but it sounded almost too strange to believe. A few years later a Dutch scientist put it to a wonderfully literal test: he hired a group of trumpet players, loaded them onto an open railway carriage, and had them hold a steady note while the train sped past a second set of musicians with perfect pitch standing beside the track. The listeners heard the note shift up as the train approached and down as it left — exactly as Doppler had predicted. Science, performed live, on rails.