Photon gas normally obeys the ideal gas law. IRL almost no gases do, there's some non-ideality and this is why we can have refigerators. They let the photon ga

Photon gas normally obeys the ideal gas law. IRL almost no gases do, there's some non-ideality and this is why we can have refigerators. They let the photon gas move around inside a crystal where they behave nonlinearly so that photons are bound or repelled to some degree, so that they behave more like CO2 or a refrigerant than like helium.