Experiment resolves century-old optics mystery

13 Jan 2009

After 100 years of conflicting experimental results, a team from China believe that it has finally unravelled the puzzle surrounding the momentum of light.

Since the early 20th century physicists have known that light carries momentum, but the way that this momentum changes as light passes through different media is much less clear. Two rival theories of the time predicted precisely the opposite effect for light incident on a dielectric: one suggesting that it pushes the surface in the direction light is travelling; the other suggesting that it drags the surface backwards towards the source of light (Phys Rev Lett 101243601).

Weilong She and his colleagues from Sun Yat-Sen University have studied the effect of light at the interface of air and a silica filament and they find that light exerts a push force on the surface “This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light” said Ulf Leonhardt a researcher in transformation optics at the University of St Andrews, UK.

The authors suggest that this finding could now pave the way for new applications like highly efficient fusion using laser "compression".

100 year riddle
Hermann Minkowski had proposed in 1908 that light momenta is proportional to a material's refractive index then the following year, another German theorist, Max Abraham proposed the opposite – momentum is inversely proportional to a material's refractive index.

This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light Ulf Leonhardt, University of St Andrews

It was suggested that this debate should be resolved experimentally but it proved to be notoriously difficult to record the momentum of light in a dielectric. In the seventies it seemed like the mystery was finally solved using a simple experiment involving an air–water interface. Conservation of momentum inferred that if Abraham was right, the water surface would compress slightly as light rays pass through, but if Minkowski was correct it would bulge. A bulge was witnessed and Minkowski was declared the victor.

Unfortunately, later in the same year further analysis showed the bulge to be the result of an unrelated optical effect; the debate was once again thrown open.

21st century makeover
She and colleagues have now finally overcome these difficulties by replacing the water surface with a nanometre silica filament. "We report direct observation of a push force on the end face of the silica filament exerted by the outgoing light" said She. Given this result, Abraham has been declared the new winner and light momenta is directly proportional to the material that it is travelling through. "The experiment represents a modern form of a beautifully simple idea" said Leonhardt.

One application that may spring from this knowledge is a more precise technique for laser-induced inertially confined fusion: a method of producing fusion energy by compressing a fuel capsule made to high density. A series of incoherent laser beams incident on a transparent dielectric ball in a vacuum would cause it to shrink under pressure to achieve nuclear fusion.

Mansud Mansuripur from the University of Arizona recognizes the potential of radiation pressure for inertially confined fusion but he warns that She and colleagues have only considered electromagnetic pressure without taking account of mechanical forces. “A correct accounting for the deformation of the silica filament in the reported experiments would have required a complete balancing of the momenta” he said.