Flipwarden

Flipwarden, a cowled monk-warden, waits beside an almost-full hourglass while one hydrogen atom above his palm flips its spin.

The Physics

In ground-state hydrogen the electron and proton spins couple through their magnetic moments. The level with total spin F = 1 (spins parallel) lies just above F = 0 (antiparallel), and the magnetic-dipole spin flip between them emits a photon at 1420.405751768 MHz, a wavelength of 21.106 cm. The spontaneous rate is tiny, so each atom waits about 11 million years on average.

ΔE = hν = 5.874 µeV, λ = c/ν = 21.106 cm

A₁₀ ≈ 2.9 × 10⁻¹⁵ s⁻¹ ⇒ τ = 1/A₁₀ ≈ 3.5 × 10¹⁴ s ≈ 11 Myr

• ΔE: hyperfine splitting of the hydrogen 1s level (eV)
• h: Planck constant, 6.626 070 15 × 10⁻³⁴ J s
• ν: transition frequency, 1420.405751768 MHz
• λ: wavelength (m)
• c: speed of light, 299 792 458 m s⁻¹
• A₁₀: Einstein coefficient for spontaneous F = 1 → F = 0 decay (s⁻¹)
• τ: mean radiative lifetime of the upper level, per atom (s)

Rare per atom, the line is still radio astronomy’s principal tracer of neutral hydrogen, because the Galaxy holds so much of it. The frequency is known to 13 significant figures thanks to hydrogen masers, which run on this very transition by stimulated emission. Sources: Kulkarni, ASTR501 lecture notes (A = 2.8843 × 10⁻¹⁵ s⁻¹); Zaroubi, The Epoch of Reionization (A₁₀ = 2.85 × 10⁻¹⁵ s⁻¹).

Honest caveat: The 11 Myr is a per-atom average lifetime, not a waiting time anyone has watched. The violet glow is symbolic: 21 cm is a radio wave and has no visible colour.

In character

One grain falls. One spin turns. I have kept this watch eleven million years, and the sky still hears every flip I make.

On the hoodie

Front: two violet spin-arrow pairs, ↑↑ to ↑↓. Back: Flipwarden in full pixel art with “FORBIDDEN. OBSERVED ANYWAY.” and “ONE FLIP PER 11 MILLION YEARS.” Gildan 18500 Heavy Blend hoodie, Black, S to 5XL.

Part of the Forbidden Transitions set with Nebulia, Coronix, Viridael, Sanguora, Sulphra. Browse all physics hoodies, more astrophysics & cosmology designs, or the Deep Cuts.