The Hyperfine Splitting in Hydrogen: Feynman's Physics Explained
Even hydrogen's ground state is not a single level. The magnetic moments of its electron and proton interact, and the energy differs slightly depending on whether their spins are parallel or anti-parallel. This tiny split is the hyperfine splitting, and transitions between the two levels emit radiation with a wavelength of 21 centimeters. That famous 21-cm line lets radio astronomers map vast clouds of hydrogen gas across our galaxy.
The big idea
A tiny spin flip in hydrogen lets us map the whole galaxy.
Think about it
Why is a faint signal from a simple spin flip so valuable for mapping gas we could never see with ordinary telescopes?
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