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Precursor (physics)
Precursors are characteristic wave patterns caused by dispersion of an impulse's frequency components as it propagates through a medium. Classically, precursors precede the main signal, although in certain situations they may also follow it. Precursor phenomena exist for all types of waves, as their appearance is only predicated on the prominence of dispersion effects in a given mode of wave propagation. This non-specificity has been confirmed by the observation of precursor patterns in different types of electromagnetic radiation (microwaves,〔(), P. Pleshko and I. Palocz, "Experimental Observation of Sommerfeld and Brillouin Precursors in the Microwave Domain," Phys. Rev. Lett. 22, 1201-1206, (1969).〕 visible light,〔(), J. Aaviksoo, J. Kuhl, and K. Ploog, "Observation of Optical Precursors at Pulse Propagation in GaAs," Phys. Rev. A 44, 5353-5356 (1991).
〕 and terahertz radiation〔(), X. Ni and R. R. Alfano, "Brillouin Precursor Propagation in the THz Region in Lorentz Media," Optics Express 14, 4188-4194 (2006).〕) as well as in fluid surface waves〔(), E. Falcon, C. Laroche, and S. Fauve, "Observation of Sommerfeld Precursors on a Fluid Surface," Phys. Rev. Lett. 91, 064502 (2003).
〕 and seismic waves.〔(), S. Rost, E. J. Garnero, Q. Williams, and M. Manga, "Seismological Constraints on a Possible Plume Root at the Core-Mantle Boundary," Nature 435, 666-669 (2005).〕
==History==
Precursors were first theoretically predicted in 1914 by Arnold Sommerfeld for the case of electromagnetic radiation propagating through a neutral dielectric in a region of normal dispersion.〔See L. Brillouin, ''Wave Propagation and Group Velocity'' (Academic Press, New York, NY, 1960), Ch. 1.〕 Sommerfeld's work was expanded in the following years by Léon Brillouin, who applied the saddle point approximation to compute the integrals involved.〔 However, it was not until 1969 that precursors were first experimentally confirmed for the case of microwaves propagating in a waveguide,〔 and much of the experimental work observing precursors in other types of waves has only been done since the year 2000. This experimental lag is mainly due to the fact that in many situations, precursors have a much smaller amplitude than the signals that give rise to them (a baseline figure given by Brillouin is six orders of magnitude smaller).〔 As a result, experimental confirmations could only be done after technology became available to detect precursors.
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