  The recent remark by G. Mourou and T. Tajima (Science 331, 41 (2011)) on the
intensity of the driver laser pulse and the duration of the created pulse that
higher driver beam intensities are needed to reach shorter pulses of radiation
remains a conjecture without clear theoretical reasoning so far. Here we offer
its extension to the case of relativistic electron bunches as the laser's
radiating medium (free-electron laser). This also bolsters the understanding of
the underlying physical principle of the Conjecture.
