How can the group velocity of a pulse of light propagating through a dispersive material exceed the speed of light in vacuum without violating Einstein’s special theory of relativity?Daniel J. Gauthier, Duke University, and Robert W. Boyd, University of RochesterThe speed of light in vacuum (c ≈ 3 × 108 m/s) is an important physical constant that appears in Maxwell’s theory of electromagnetism. For this reason, scientists have endeavored to measure it with very high precision, making it one of the most accurately known of all physical constants. The situation becomes murkier for a slightly different situation: Send a pulse of light through a dispersive optical material rather than vacuum, and bizarre things start to appear. For example, under conditions such that the dispersion of the medium is anomalous over some spectral region,1...Read full articleRelated content from Photonics MediaARTICLESA History of the Laser: 1960 - 2019In 2020, the laser will celebrate its 60th anniversary. 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