  Bowman et al. (2019) reported low-frequency photometric variability in 164 O-
and B-type stars observed with K2 and TESS. They interpret these motions as
internal gravity waves, which could be excited stochastically by convection in
the cores of these stars. The detection of internal gravity waves in massive
stars would help distinguish between massive stars with convective or radiative
cores, determine core size, and would provide important constraints on massive
star structure and evolution. In this work, we study the observational
signature of internal gravity waves generated by core convection. We calculate
the \textit{wave transfer function}, which links the internal gravity wave
amplitude at the base of the radiative zone to the surface luminosity
variation. This transfer function varies by many orders of magnitude for
frequencies $\lesssim 1 \, {\rm d}^{-1}$, and has regularly-spaced peaks near
$1 \, {\rm d}^{-1}$ due to standing modes. This is inconsistent with the
observed spectra which have smooth ``red noise'' profiles, without the
predicted regularly-spaced peaks. The wave transfer function is only meaningful
if the waves stay predominately linear. We next show that this is the case: low
frequency traveling waves do not break unless their luminosity exceeds the
radiative luminosity of the star; and, the observed luminosity fluctuations at
high frequencies are so small that standing modes would be stable to nonlinear
instability. These simple calculations suggest that the observed low-frequency
photometric variability in massive stars is not due to internal gravity waves
generated in the core of these stars. We finish with a discussion of
(sub)surface convection, which produces low-frequency variability in low-mass
stars, very similar to that observed in Bowman et al. (2019) in higher mass
stars.
