Thermodynamics of continuous spin photons
Year: 2025
Type: article
Abstract: Special relativity allows massless particles to have states of different integer (or half-integer) helicities that mix under boosts, much like the spin states of a massive particle. Such massless particles are known as “continuous spin” particles (CSPs), a term coined by Wigner, and they are notable for their infinite tower of spin polarizations. The mixing under boosts is controlled by a spin scale ρ with units of momentum. Normally, we assume ρ = 0 . The interactions of CSPs are known to satisfy certain simple properties, one of which is that the ρ → 0 limit recovers familiar interactions of massless scalars, photons, or gravitons, with all other polarizations decoupling in this limit. Thus, one can ask if the photon of the Standard Model is a CSP at small but nonzero ρ . One concern about this possibility—originally raised by Wigner—is that the infinite tower of polarizations could pose problems for thermodynamics. To address this question, we study the thermal evolution of a CSP photon gas coupled to isothermal matter, across CSP helicity modes and phase space. We find that the structure of the interactions dictated by Lorentz symmetry implies well-behaved thermodynamics. When the CSP photon’s interactions with charged matter are turned on, the primary h = ± 1 helicity modes thermalize quickly, while the other modes require increasingly long time scales to thermalize, set by powers of T / ρ . In familiar thermal systems, the CSP photon behaves like the QED photon with small ρ - and time-dependent corrections to its effective relativistic degrees of freedom. Sizable departures from familiar thermal behavior arise at energy scales comparable to ρ and could have testable experimental consequences.
Institutions: SLAC National Accelerator Laboratory, Stanford University
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W4408505289