We explore the fundamental but untested possibility that the photon is a continuous spin particle (CSP) with a small but nonzero spin Casimir $\rho$. When $\rho \ne 0$, the familiar polarization modes of the photon transform nontrivially under Lorentz boosts, leading to deviations from familiar QED. Surprisingly, these deviations are strongest at low energy, but smoothly vanish in the $\rho \to 0$ limit. In this letter, we compute corrections to the hydrogen 21-cm transition rate, which is expected to be particularly sensitive given the small hyperfine energy splitting $\omega$. We find deviations from $\mathrm{QED}\propto {\rho }^2{\alpha }^2/{\omega }^2$ at leading order, suggesting experimental constraints $\rho \lesssim 1\mathrm{meV}$. Building on this work, we expect that a range of other atomic, molecular, or condensed matter systems could be used to provide even more stringent tests of $\rho$ in electromagnetic interactions.

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• Angular momentum of light
• Atomic & molecular processes in external fields
• Extensions of gauge sector
• Fine & hyperfine structure
• Masers
• Particle interactions
• Phenomenology
• Photoemission
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions