  In a gedankenexperiment about the generalized second law (GSL) of black hole
thermodynamics, the buoyant force by black hole atmosphere (the acceleration
radiation) plays an important role, and then it is significant to understand
the nature of the buoyant force.
  Recently, Bekenstein criticizes that the fluid approximation of the
acceleration radiation which is often used in the estimation of the buoyant
force is invalid for the case that the size of the target is much less than a
typical wavelength of the acceleration radiation, due to the diffractive effect
of wave scattering.
  In this letter, we argue that even if it is correct that we should calculate
the buoyant force as a wave scattering process, its implication in the GSL
strongly depends on whether there exists any massless scalar field, that is,
S-wave scattering. By reconsidering the diffractive effect by S-wave
scattering, we show that if some massless scalar field exists, then the GSL can
hold without invoking a new physics, such as an entropy bound for matter.
