SIGNALS

05 / 13

Tune the galaxy

How can something you cannot see draw a map of the galaxy?

The sky is mostly noise. This is a radio dial, not a speaker. Sweep toward fourteen twenty point four zero five megahertz. If the teaching sky blooms, that is the hydrogen line. You are not hearing space with your ears.

Point the dish at the sky. Then tune.

Point the dish

More gas along the line of sight, and more than one speed.

You cannot see this hydrogen. Point the dish, then sweep toward the amber mark. The dial is still radio, not sound.

Spectrum (teaching model). Amber line: rest frequency 1420.405751768 MHz.

  1. Source

    Neutral hydrogen

  2. Medium

    Radio waves

  3. Signal

    21 cm line

  4. Detector

    Radio telescope

  5. Measurement

    Intensity and frequency

  6. Meaning

    Where the gas is, and how it moves toward or away from us