Recently, I have seen an increasing number of papers in which scientists claim to have proven extraordinary phenomena by applying the concept of group velocity to the anomalous dispersion of waves. Two of the greatest wave theorists of all time, Arnold Sommerfeld and Léon Brillouin, have dealt with the subject.In separate papers, Sommerfeld and Brillouin wrote that, in anomalous dispersion, the group velocity cannot be the signal velocity 1 1. A. Sommerfeld, Annalen der Physik 44, 177 (1914) https://doi.org/10.1002/andp.19143491002; L. Brillouin, Annalen der Physik 44, 203 (1914). For a lucid English-language digest of the two papers, see ref. 2, p. 334 https://doi.org/10.1002/andp.19143491003. Indeed, in anomalous dispersion, the group velocity goes through both negative and positive infinite values. It also goes through values greater than the speed of light 2 2. J. A. Stratton, Electromagnetic Theory, McGraw-Hill, New York (1941), p. 339, fig. 63. (as does the phase velocity).REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. A. Sommerfeld, Annalen der Physik 44, 177 (1914) https://doi.org/10.1002/andp.19143491002; Google ScholarCrossref L. Brillouin, Annalen der Physik 44, 203 (1914). For a lucid English-language digest of the two papers, see ref. 2, p. 334 https://doi.org/10.1002/andp.19143491003. , Google ScholarCrossref2. J. A. Stratton, Electromagnetic Theory, McGraw-Hill, New York (1941), p. 339, fig. 63. Google Scholar© 2003 American Institute of Physics.