Information about the collapsed matter in a black hole will be lost ifHawking radiations are truly thermal. Recent studies discover that informationcan be transmitted from a black hole by Hawking radiations, due to theirspectrum deviating from exact thermality when back reaction is considered. Inthis paper, we focus on the spectroscopic features of Hawking radiation from aSchwarzschild black hole, contrasting the differences between the nonthermaland thermal spectra. Of great interest, we find that the energy covariances ofHawking radiations for the thermal spectrum are exactly zero, while the energycovariances are non-trivial for the nonthermal spectrum. Consequently, thenonthermal spectrum can be distinguished from the thermal one by counting theenergy covariances of successive emissions, which provides an avenue towardsexperimentally testing the long-standing "information loss paradox".
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