Coming or Going

Crests arriving sooner

Christian Doppler published the idea in 1842. A source emitting waves while moving toward you crowds the crests together: they arrive more often, so the frequency you measure is higher. Moving away, they are stretched and the frequency drops. Nothing about the source changes — only the geometry between you and it.

Buys Ballot tested it in 1845 by putting trumpeters on an open railway carriage and musicians with perfect pitch on the platform, which remains the most charming experiment in physics.

The same effect, everywhere

Light does it too. A star approaching has its spectral lines shifted toward blue, receding toward red. In 1929 Hubble showed that almost every distant galaxy is redshifted, and the further away it is the more so — the observation that the universe is expanding.

It is also how weather radar sees the wind inside a storm, how police radar and speed cameras work, how an ultrasound machine images blood flow and colours it by direction, and how astronomers detect planets around other stars: the star wobbles a few metres per second as its planet pulls it about, and the shift is measurable.

Early and late is the same measurement

A shift in frequency and a shift in timing are the same fact viewed two ways. If a source sends one crest every second and you receive them 999,999 microseconds apart, you may call it a frequency shift or you may call it each tick arriving early. The instrument in Enspinors does the latter and colours them the way an astronomer would — early toward blue, late toward red — which is a real convention applied to an invented signal.