Finding A Solution To Photon Momentum Mystery

Over a century ago two physicists published separate theories on the behaviour of light when it enters a more optically dense material. The first was Hermann Minkowski in 1908 followed by Max Abraham in 1909. The reason these theories are so important and tied so inseparably together is because they both predict the exact opposite thing. Minkowski’s prediction is that the light will gain greater momentum as it passes into the material while Abraham claims that the momentum will be decreased. Although at first Abraham’s seems more realistic as in everyday life things decreasing in speed have less momentum and if light is seen as a particle this explanation is sound. But of course light can also be seen as a wave and due to waves less concrete characteristics an increase in momentum is reasonable.

p_{{\mathrm  {M}}}={\frac  {nh\nu }{c}};p_{{\mathrm  {A}}}={\frac  {h\nu }{nc}},

The difference between the two theoretical equation is minor but the changing location of the refractive index (n) has a massive effect.

Of course one has to be right and the other wrong and the most “basic” way of seeing this is to measure the distortion of a material’s surface as light passes into it. The material selected to experience the radiation pressure was a thin layer of graphene oxide that would be irradiated with laser of relatively low energy and a wavelength of 532 nm. If powers of greater than about 1.6 mW are used then the graphene oxide would probably burn into the surface. The extremely thin layer would be expected to bend one way or the other; inwards for Abraham’s theory and outwards for Minkowski’s theory. Atomic force microscopy, where a physical probe with a nanometre head is dragged over the surface, was used in order to detect the bend and its direction. Then a bit of Raman scattering was implemented so that the strain on the structure was also known. And so after going back and forth for a century it seems that the ultimate winner is: Max Abraham. The film bent inwards as a force was applied when the photons momentum dropped. This is the first ever observation of a nanomaterials surface bending due to radiation pressure and being able to solve an a hundred year mystery is brilliant also.

Paper links: Graphene Oxide Demonstrates Experimental Confirmation of Abraham Pressure on Solid Surface

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