Changing Speed Of Light: Scientists drag light by slowing it to speed of sound

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Does light travel at a constant speed in any medium? No. There is no doubt that thunder follows lightning, nevertheless, the speed of light can decrease depending on the medium in which it travels. A new paper in Science report that scientists have been able to drag light by slowing it down to the speed of sound and sending it through a rotating crystal.

The speed of light is constant only in the vacuum where it travels at 671million mph.  When it travels through different substances, such as water or solids, its speed is reduced, with different wavelengths (colors) travelling at different speeds.


The green laser is shown as it leaves the ruby crystal

In addition, it has also been observed, but is not widely appreciated, that light can be dragged when it travels through a moving substance, such as glass, air or water – a phenomenon first predicted by Augustin-Jean Fresnel in 1818 and observed a hundred years later.

The paper was published in the recent issue of the journal Science.

Prof. Miles Padgett in the Optics Group in the School of Physics & Astronomy, said: “The speed of light is a constant only in vacuum. When light travels through glass, movement of the glass drags the light with it too.

“Spinning a window as fast as you could is predicted to rotate the image of the world behind it ever so slightly. This rotation would be about a millionth of a degree and imperceptible to the human eye.”

Researchers Dr Sonja Franke-Arnold, Dr Graham Gibson and Prof Padgett, in collaboration with their colleague Professor Robert Boyd at the Universities of Ottawa and Rochester, took a different approach and set up an experiment: shining a primitive image made up of the elliptical profile of a green laser through a ruby rod spinning on its axis at up to 3,000 rpm.

Once the light enters the ruby, its speed is slowed down to around the speed of sound (approximately  741mph) and the spinning motion of the rod drags the light with it, resulting in the image being rotated by almost five degrees: large enough to see with the naked eye.

Dr Franke-Arnold, who came up with the idea of using slow light in ruby to observe the photon drag, said: “We mainly wanted to demonstrate a fundamental optical principle, but this work has possible applications too.

“Images are information and the ability to store their intensity and phase is an important step to the optical storage and processing of quantum information, potentially achieving what no classical computer can ever match.

“The option to rotate an image by a set arbitrary angle presents a new way to code information, a possibility not accessed by any image coding protocol so far.”

On, a user interpreted the observation: "It can be mathematically described as virtual absorption/remission, but this is not actual absorption, simply some particular matrix elements of interaction operator are non-zero, the same elements that would be responsible for actual absorption."

Source article: Rotary Photon Drag Enhanced by a Slow-Light Medium. Sonja Franke-Arnold,Graham Gibson, Robert W. Boyd and Miles J. Padgett. Science 1 July 2011:Vol. 333 no. 6038 pp. 65-67 DOI: 10.1126/science.1203984


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