We show that in order to avoid a breakdown of general covariance at the
quantum level the total flux in each outgoing partial wave of a quantum field
in a black hole background must be equal to that of a (1+1)-dimensional
blackbody at the Hawking temperature.Comment: 5 pages, 1 figure; v2: typo corrected, reference added; v3: comment
added, minor editorial changes to agree with published versio