We show that photon spheres of supermassive black holes generate high-frequency stochastic gravitational waves through photon-graviton conversion. Remarkably, the frequency is universally determined as m e ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi m e =m p p ≃ 10 20 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 Ω 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.
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