If the photon had a mass M, the dispersion relation for electromagnetic waves in vacuum would be A limit on  can be determined by measuring the difference in arrival times of the highest- and lowest-frequency components of a wave packet received from an astrophysical source that emits EM radiation in bursts. Estimate δt from the proposed dispersion relation. Should one collect data from a radio source or from a γ-ray source to obtain the lowest limit on M? Why?

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w = ck + (Mc/h).