Moti Segev and his team from Princeton University were the first into the field of self-trapped (soliton), coherent light beams in photorefractive materials. Not content with that, on page 880 of this issue Mitchell and Segev now announce the trapping of incoherent light beams (Nature 387 880–883; 1997). This result lends great flexibility to their pioneering work, and points the way towards the development of devices based on the principle. An appreciation of this advance demands knowing what a soliton is, and why photorefractive materials are so useful, which in turn requires stepping back in time.
One of the most significant observations in science was made 153 years ago by a naval architect, called John Scott Russell. This was his sighting of a very peculiar wave on the Edinburgh to Glasgow canal. For some reason, a boat he was observing stopped suddenly and caused a hump of water to accumulate around its front. This hump then took off down the canal. Russell, a man blessed with acute observational powers, soon noticed that this heap of water was nicely rounded and, rather strangely, propagated without change of shape. The hump had no disturbance in front of it and none behind it. It was, in fact, alone (that is, solitary) and did not conform to common sense by dying away, as wave disturbances caused by striking water with a stone, or in this case a boat, are expected to do.
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