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Physics World

Beyond the speed of light

Published Jul 1, 2000• 0 citations
Publisher

Abstract

One of the most fundamental laws in physics is that nothing can travel faster than the speed of light. However, two teams of researchers have now shown experimentally that electromagnetic pulses can be made to travel faster than c, the velocity of light in vacuum, and over relatively long distances.
DOI
10.1088/2058-7058/13/7/1

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Nothing can travel faster than light. Despite a recent raft of reports in the media, this statement is as true now as it ever was. Nonetheless, experiments over the past 20 years have been forcing us to re-examine what we mean by the word "nothing". In the latest experiment, a group of researchers at the NEC Research Institute in Princeton, US, observed the peak of a laser pulse leave a small cell filled with caesium gas before it had even entered the cell (L J Wang, A Kuzmich and A Dogariu 2000 Nature 406 277). Apparently, the peak of this pulse is simply not the kind of "thing" to which Einstein's famous law applies.
WHETHER we use it to estimate how far away a thunderstorm is, or simply take it for granted that we can have a conversation with someone on the other side of the world, we all know that light travels extremely fast. Indeed, special relativity teaches us that nothing in the universe can ever move faster than the speed of light in a vacuum: 2,99792,458ms -1 . However, there is no such limitation on how slowly light can travel. For the last few years, researchers have been routinely slowing light to just a few metres per second, and have recently even stopped it dead in its tracks so that it can be stored for future use.
Einstein's theory of relativity establishes the speed of light in vacuum, c, as a fundamental constant. However, in optically dense media, light can be made to travel at speeds lower than c, e.g. light in water propagates at 3/4c. More dramatically, the speed of light pulses can be altered by orders of magnitude in dispersive media such as cold atomic clouds [1], atomic vapours [2] and structurally engineered materials [3]. Under exotic conditions, the velocity of these pulses can be rendered greater or smaller than c, or even negative [4]. While significant control can be exerted over the speed of light in dispersive media, no experimental demonstration of altered light speed has hitherto been achieved in vacuum for propagating physical beams of finite energy. Here we show that pulsed beams of twisted light can exhibit sub-luminal velocities upon vacuum propagation. Our experiment reveals changes in the range of 0.1% in the velocity of propagating twisted pulsed beams relative to c. Though our work poses no challenge to relativity theory, it experimentally confirms a counterintuitive body of theoretical work on the group velocity of structured pulses in vacuum, and introduces a novel theory of pulse velocity for twisted light. Our results are particularly important given the recent surge of interest in the application of structured beams to telecommunication [5], quantum key distribution [6] and quantum information [7].
(1) Mathematically, all true science findings must be internally self-consistent with each other via a distance-time-only reference frame (RF); (2) No human qualities of consciousness, intention, emotion, mind or spirit can significantly influence a well-designed target experiment in physical reality (thought to be first proposed by Descartes in ~1600 AD); (3) Nothing with physical mass can travel at or faster than the velocity of electromagnetic (EM) light through physical vacuum (4) Today’s quantum mechanics (QM) can be meaningfully applied to any phenomenon of nature and (5) The “big bang” creation of our physical cosmos grew out of a completely empty physical vacuum.