Doppler Shifts
Changes in the frequency of waves (light, sound) when the source and observer are moving relative to each other—like how an ambulance siren sounds higher-pitched approaching you, then lower as it speeds away.
Not to be confused with: Redshift
The change in frequency or wavelength of waves—such as light or sound—caused by relative motion between the source and the observer. Named after Austrian physicist Christian Doppler, who described the effect in 1842. When a source moves toward an observer, waves compress and frequency increases (blueshift for light, higher pitch for sound); when moving away, waves stretch and frequency decreases (redshift for light, lower pitch for sound). Astronomers use Doppler shifts to measure how fast stars and galaxies move toward or away from Earth.
What this means in real life
When a fire truck races past you, its siren sounds higher-pitched as it approaches and lower-pitched as it drives away—even though the siren itself never changes tone. Astronomers use the same principle with starlight to detect planets orbiting distant stars.
What it isn’t
It's not a change in the actual speed or brightness of the wave source itself. The siren on a moving ambulance doesn't get louder or quieter because of the Doppler effect—only its pitch changes from your perspective.
Commonly misused online
Science memes often claim 'redshift means the universe is slowing down' or confuse Doppler shifts with time dilation. Redshift from galaxies moving away doesn't mean cosmic expansion is decelerating—it's evidence expansion happened, not that it's changing speed right now.