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The Enigma of Optical Momentum

Barnett, S (University of Strathclyde)
Tuesday 07 May 2013, 11:00-12:00

Seminar Room 1, Newton Institute


It is more than 100 years since the battle began to determine the correct form of the momentum of light inside a material medium. The two principal contenders for the momentum density are those attributed to Abraham

g$_Abr = $$ExH\over c$^2$

and to Minkowski

g$_Min = DxB

The work of Minkowski suggests that the momentum of each photon exceeds its value in free space by a factor of the refractive index. Abraham's approach, however, requires that the momentum is less than that in free space by the same factor.

Compelling arguments have been proposed for each of the two candidate momenta. A simple application of Newton's first law of motion to the combined system of a medium and a single photon, for example, leads unambiguously to the Abraham form. An equally simple analysis of single or double slit diffraction, however, leads equally powerfully to the Minkowski form. The problem has not been resolved by experiment, moreover, with experiments supporting both candidate momenta being reported.

The resolution, of course is that both are correct


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