Scientists drag light by slowing it to the speed of sound

Scientists have — for the first time — been able to rotate an image by dragging light to close to the speed of sound. University of Glasgow researchers dragged light by first slowing it down to the speed of sound in a ruby crystal and then spinning it at almost 3,000 rpm. They confirmed the drag by noting the shift of an image in the light, visible to the naked eye. Details of their study appear in the July 1, 2011 issue of Science.

Most people may think the speed of light is constant, but this is only the case in a vacuum, such as space, where it travels at 671 million mph. However, when light travels through different substances, such as water or solids, its speed slows down, with different wavelengths (colors) traveling at different speeds. It has also been observed — but is not widely appreciated — that a moving substance such as glass, air or water can drag light that passes through it –- a phenomenon first predicted by Augustin-Jean Fresnel in 1818 and observed a hundred years later.

Miles Padgett of the School of Physics and Astronomy Optics Group, 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.

The Glasgow researchers used light from a green laser and shone an elliptical image through a ruby crystal rod spinning on its axis at up to 3,000 rpm. When the light first entered the ruby, its speed slowed down to around the speed of sound (approximately 741 mph). The spinning motion of the rod dragged the light with it, rotating the image by almost five degrees — large enough to see with the naked eye.

Sonja 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.

Bottom line: University of Glasgow research scientists were able to drag light by first slowing it down to the speed of sound in a ruby crystal and then spinning it at 3,000 rpm. Results of their study appear in the July 1, 2011 issue of Science.

Via University of Glasgow News

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