To tackle the problem, the researchers looked back at work by Lord Rayleigh over 100 years ago, who performed a similar thought experiment with the speed of sound. Following this line of thought, we devised a way to experimentally investigate the of superluminal motion.” “The existence of an absolute limit, the speed of light, is the natural source of the question: what would happen if we cross this limit?” lead author Mattero Clerici told IFLScience. “Light sources, however, may move faster than the speed of light when their speed is not associated with the physical motion of matter. In their paper, published in Science Advances, they demonstrated that if a light source approaches the observer faster than the speed of light, then the images appear to move backwards in time. And that’s when the fun starts.Īn international team of scientists set up an experiment to simulate what a stationary observer would see when looking at a superluminal (faster than light) event. Nothing carrying information can travel faster than the speed of light, but by playing with geometries physicists can create faster-than-light motion. 83, 1767–1770 (1999).The fact that the speed of light is constant is one of the cornerstones of physics. Nonlinear magneto-optics and reduced group velocity of light in atomic vapor with slow ground state relaxation. Ultraslow group velocity and enhanced nonlinear optical effects in a coherently driven hot atomic gas. Light speed reduction to 17 meters per second in an untracold atomic gas. Measurement of dispersion properties of electromagnetically induced transparency in rubidium atoms. Electromagnetically induced transparency. Measurement of the single-photon tunneling time. Optical pulse propagation at negative group velocities due to a nearby gain line. Causality and negative group delays in a simple bandpass amplifier. Dispersionless, highly superluminal propagation in a medium with a gain doublet. Two theorems for the group velocity in dispersive media. Superluminal (but causal) propagation of wave packets in transparent media with inverted atomic populations. Superluminous laser pulse in an active medium. Observation of dissipative superluminous solitons in a Brillouin fiber ring laser. Picholle, E., Montes, C., Leycuras, C., Legrand, O. Propagation of light pulses in a laser amplifier. Steep anomalous dispersion in coherently prepared Rb vapor. Linear pulse propagation in an absorbing medium. Propagation of a gaussian light pulse through an anomalous dispersion medium. in Amazing Light, a Volume Dedicated to C. Wave Propagation and Group Velocity (Academic, New York, 1960).Ĭhiao, R. Electrodynamics of Continuous Media (Pergamon, Oxford, 1960).īrillouin, L. Principle of Optics 7th edn (Cambridge Univ. The Principle of Relativity, Collected Papers (Dover, New York, 1952).īorn, M. The observed superluminal light pulse propagation is not at odds with causality, being a direct consequence of classical interference between its different frequency components in an anomalous dispersion region.Įinstein, A., Lorentz, H.
We measure a group-velocity index of n g = -310(±5) in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. The group velocity of a laser pulse in this region exceeds c and can even become negative 16, 17, while the shape of the pulse is preserved. Here we use gain-assisted linear anomalous dispersion to demonstrate superluminal light propagation in atomic caesium gas. However, in all previous experimental demonstrations, the light pulses experienced either very large absorption 7 or severe reshaping 9, 19, resulting in controversies over the interpretation. Nevertheless, there exist various proposals 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 for observing faster-than- c propagation of light pulses, using anomalous dispersion near an absorption line 4, 6, 7, 8, nonlinear 9 and linear gain lines 10, 11, 12, 13, 14, 15, 16, 17, 18, or tunnelling barriers 19. Einstein's theory of special relativity and the principle of causality 1, 2, 3, 4 imply that the speed of any moving object cannot exceed that of light in a vacuum ( c).