1420 MHz

Plate 42 speaks for itself: “The note the whole galaxy hums.”

The Physics

Neutral hydrogen's electron and proton spins can flip from parallel to antiparallel, emitting a 21 cm radio photon. The flip is very rare per atom, but the galaxy holds so much hydrogen that the line maps its spiral arms.

λ = c / ν

• λ: wavelength ≈ 0.211 m
• c: speed of light, m s⁻¹
• ν: frequency 1420.405751768 MHz, Hz

Ewen & Purcell, 25 March 1951. Colours symbolic; radio is invisible.

Honest caveat: 'Hum' is a metaphor: the line is a radio signal at 1.42 GHz, not sound, and nothing in the galaxy is audible.

Sources

• H. I. Ewen, E. M. Purcell, Nature 168, 356 (1951).

On the poster

Plate 042, “1420 MHz”: the pixel-art plate with its slogan “The note the whole galaxy hums.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.