Abstract
It is 100 years since Minkowski and Abraham first gave rival expressions for the momentum of light in a material medium. At the single-photon level, these correspond, respectively, either to multiplying or dividing the free-space value () by the refractive index (n). The debate that this work started has continued till the present day, punctuated by the occasional publication of ‘decisive’ experimental demonstrations supporting one or other of these values. We review the compelling arguments made in support of the Minkowski and Abraham forms and are led to the conclusion that both momenta are correct. We explain why two distinct momenta are needed to describe light in a medium and why each appears as the natural, and experimentally observed, momentum in appropriate situations.
Highlights
It has long been appreciated that light has mechanical properties
It is not necessary to quantize the electromagnetic field in order to appreciate the problem, but it is helpful to understand it in terms of the properties of a single photon of angular frequency ω
Simple conservation laws have led us to conclude that the photon momentum is that given by Minkowski. These arguments in support of the Minkowski momentum are of a different character from that made in support of the Abraham form, but they are no less convincing for that
Summary
It has long been appreciated that light has mechanical properties. Maxwell (1891) presented a simple calculation of the pressure exerted by sunlight at the surface of the Earth. It is not necessary to quantize the electromagnetic field in order to appreciate the problem, but it is helpful to understand it in terms of the properties of a single photon of angular frequency ω. We can do this by means of a simple scaling argument. These arguments in support of the Minkowski momentum are of a different character from that made in support of the Abraham form, but they are no less convincing for that. Both forms are well supported, and we have a dilemma
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