Why 21 cm is our Universe’s “magic length” | by Ethan Siegel | Starts With A Bang! | May, 2025

Photons come in every wavelength you can imagine. But one particular quantum transition makes light at precisely 21 cm, and it’s magical.

In our Universe, quantum transitions are the governing rule behind every nuclear, atomic, and molecular phenomenon. Unlike the planets in our Solar System, which could stably orbit the Sun at any distance if they possessed the right speed, the protons, neutrons, and electrons that make up all the conventional matter we know of can only bind together in a specific set of configurations. These possibilities, although numerous, are finite in number, as the quantum rules that govern electromagnetism and the nuclear forces restrict how atomic nuclei and the electrons that orbit them can arrange themselves.

In all the Universe, the most common atom of all is hydrogen, with just one proton and one electron. Wherever new stars form, hydrogen atoms become ionized, becoming neutral again if those free electrons can find their way back to a free proton. Although the electrons will typically cascade down the allowed energy levels into the ground state, that normally produces only a specific set of infrared, visible, and ultraviolet light. But more importantly, a special transition occurs in hydrogen that produces light of about the…