Radiation {coherent radiation}| travels outward from source, but coherent radiation does not travel inward to source, macroscopically. At classical levels, emitters add but absorbers cancel. Why is there radiation gradient, radiation flow, with respect to time? At quantum level, this condition does not necessarily hold. Perhaps, quantum-level emitters and absorbers can emit and receive coherent radiation, or coherence concept does not apply. Why are there coherent-radiation sources?
Physical Sciences>Physics>Time>Asymmetry
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Date Modified: 2022.0224