Hydrogen 21 cm hyperfine transition
This is a change in the energy state of neutral hydrogen atoms that produces or absorbs radio waves, widely used in radio astronomy.
Also known as: 21 cm line, hydrogen line
How big is it?
5.873 × 10⁻⁶ eV · single-photon transition energy
Single-photon transition energy · measured constant
Typical recorded range: 5.867 × 10⁻⁶ eV to 5.88 × 10⁻⁶ eV.
A little perspective
Hydrogen 21 cm hyperfine transition vs Caesium-133 clock transition: energy comparison
The single-photon transition energy of the caesium-133 clock transition is about 6.47 times the single-photon transition energy of the hydrogen 21 cm hyperfine transition.
ENERGY STORED OR TRANSFERRED
This is a change in the energy state of neutral hydrogen atoms that produces or absorbs radio waves, widely used in radio astronomy.
The caesium-133 clock transition is the specific change in energy state of a caesium-133 atom that defines the duration of one second in the International System of Units.
Measurements, assumptions & sources +
Hydrogen 21 cm hyperfine transition
Single-photon transition energy: 5.873 × 10⁻⁶ eV; measured constant; range 5.867 × 10⁻⁶ eV–5.88 × 10⁻⁶ eV.
The neutral-hydrogen ground-state hyperfine line has a wavelength near 21.1 cm and photon energy near 9.41 × 10⁻²⁵ J. It is a transition wavelength, not an atom diameter.
NIST — Hydrogen hyperfine frequency reference
The marked single-photon transition energy is calibrated. Unmeasured illustration details are representative.
Caesium-133 clock transition
Single-photon transition energy: 3.802 × 10⁻⁵ eV; derived.
The SI second is defined from exactly 9,192,631,770 periods of the unperturbed caesium-133 ground-state hyperfine transition. The cycle duration and photon energy here are derived from that exact frequency.
BIPM — The International System of Units
The marked single-photon transition energy is calibrated. Unmeasured illustration details are representative.
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Why this measurement?
Why we use 5.873 × 10⁻⁶ eV
We compare the single-photon transition energy of the hydrogen 21 cm hyperfine transition. This is a measured value; check the notes for its source and scope. The recorded range is 5.867 × 10⁻⁶ eV to 5.88 × 10⁻⁶ eV.
The neutral-hydrogen ground-state hyperfine line has a wavelength near 21.1 cm and photon energy near 9.41 × 10⁻²⁵ J. It is a transition wavelength, not an atom diameter.
The picture identifies the subject; the diagram compares the recorded values. How calibrated comparisons work →
Recorded properties
Dimensions and measurements
| Property | Selected value | Recorded range | Basis |
|---|---|---|---|
| 5.873 × 10⁻⁶ eV | 5.867 × 10⁻⁶ eV to 5.88 × 10⁻⁶ eV | measured constant | |
| 21.1 cm | 21 cm to 21.2 cm | measured constant |
Around this scale
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Sources and method
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The source records and measurement method remain available for inspection.
- NIST — Hydrogen hyperfine frequency referenceGround-state hydrogen hyperfine transition; wavelength and photon energy are derived from the frequency and exact c and h.
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