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Black hole spectroscopy from null geodesics

The quasinormal mode frequencies can be understood from the massless particles trapped at the unstable circular null geodesics and slowly leaking out to infinity. Base on this viewpoint, in this paper, we construct the quantum entropy spectrum of the stationary black holes from the null geodesics using the new physical interpretation of the quasinormal mode frequencies proposed by Maggiore. Following this idea, we calculate the spacing of the entropy spectrum for various types of black holes with or without the charge and spin in any dimension $d$ of the spacetime. The result shows that the spacing closely depends on the charge, spin, and the dimension of the spacetime. Moreover, for a black hole far from the extremal case, the spacing is found to be larger than $2π\hbar$ for $d=4$, while smaller than $2π\hbar$ for $d\geq 5$, which is very different from the result of the previous work by using the usual quasinormal mode frequencies, where the spacing of the entropy spectrum is $2π\hbar$ and is independent of the black hole parameters and dimension $d$.