Recently blazars have shown variability in total flux density, polarization and polarization position angle in both radio and optical bands. This demands brightness temperatures ~10^6^ times the limit set by incoherent synchrotron emission by relativistic electrons. We report laboratory experiments displaying just such enhancement when relativistic beam density exceeds 1% of background plasma density. These suggest a model for collective emission from jets explaining variability and spectral features. It implies that jets could have lower energy electrons in much lower numbers than conventional theory demands, lessening the requirements on the underlying engines. Rapid decay of the high-frequency end of the varying spectra could check these ideas.
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Gregory Benford (1992) studied this question.