Applications: Radar, Medicine and Cosmology

The Doppler effect is one of the most universal measuring tools in physics. Three fields illustrate it.

Radar and road safety. A radar emits a wave toward a vehicle; the reflected wave comes back shifted in frequency. The shift gives the speed. The same principle is used in weather radars, which track the motion of raindrops to measure the wind inside storms.

Medicine. Doppler ultrasound sends ultrasound toward moving blood. Red blood cells send back a shifted wave, whose shift reveals the speed and direction of the flow. This makes it possible to visualize blood flow in the arteries without any surgery.

Cosmology. This is the most spectacular application. In 1929, Hubble observed that the light of almost all galaxies is redshifted: they are moving away from us. Even more striking, the more distant a galaxy, the greater its redshift.

   nearby galaxy     small redshift   low recession speed
   distant galaxy    large redshift   high recession speed
   -> Hubble's law : v ≈ H₀ · d      (speed proportional to distance)

This observation leads to a dizzying conclusion: the Universe is expanding. It is not that galaxies are rushing through a fixed space; it is space itself that is stretching, carrying the galaxies along. Running this expansion backward in time leads to the Big Bang. Thus an effect observed on an ambulance siren, transposed to light, yielded one of the greatest discoveries in history: the Universe has a history, and it is growing.