  We show that photon spheres of supermassive black holes generate
high-frequency stochastic gravitational waves through the photon-graviton
conversion. Remarkably, the frequency is universally determined as
$m_e\sqrt{m_e /m_p} \simeq 10^{20} \text{Hz}$ in terms of the proton mass $m_p$
and the electron mass $m_e$.
  It turns out that the density parameter of the stochastic gravitational waves
$ \Omega_{ \text{gw}}$ could be $ 10^{-12}$. Since the existence of the
gravitational waves from photon spheres is robust, it is worth seeking methods
of detecting high-frequency gravitational waves around $10^{20}$Hz.
