Linewell

Meet Linewell, a hydrogen atom who hums a single radio note when his spin flips.

Both the proton and the electron in a hydrogen atom behave like tiny spinning magnets. The atom has slightly more energy when the two spins point the same way than when they point opposite ways. Occasionally the electron flips and the atom drops to the lower energy, giving out a radio photon.

That photon has a frequency of about 1420.4 MHz, so its wavelength is λ = c/f ≈ 3.0 × 10⁸ / 1.42 × 10⁹ ≈ 0.21 m, which gives the 21 cm line its name. A single atom waits on average about 10 million years before flipping.

There is so much hydrogen in space that the hum is still strong. Hendrik van de Hulst predicted it in 1944, and it was first detected in 1951.

Radio telescopes use the 21 cm line to map cold hydrogen gas through dust that blocks visible light, which is how astronomers traced the spiral arms of our Milky Way.