Tag Archives: Minkowski-Abraham Controversy

I Get No Kick from Champagne

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So tell me why should it be true?
That I get a kick out of you

gg writes up the recent paper on the Abraham-Minkowski controversy

Measuring the ‘kick’ of a photon leaving a fiber!

The difficulty lies in the fact that any discussion of the momentum of light in a medium must properly account for the total momentum of the system, which includes the momentum of the medium itself. When traveling into a medium of refractive index much greater than unity, the light is strongly interacting with the material and it becomes almost arbitrary to distinguish between the momentum of the photon and that of the matter: the two are completely intertwined. With this perspective, one would say that the designation of ‘light momentum’ and ‘medium momentum’ are completely arbitrary, merely different ways to slice ‘total momentum pie’. Differences in experimental results can be explained away as a failure to completely account for the interaction between the light and the medium.

The Photon Push-Me Pull-You Update

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Back in June I wrote up a post an the Abraham-Minkowski controversy, which concerns the momentum of a photon when it’s in a medium.

Depending on the assumptions one makes, one can show that the momentum increases or decreases inside the medium, and obviously both solutions can’t be correct. But for a long time it was unclear which assumptions were faulty, because it was such a delicate experiment to do.

I just ran across a post at Everyday Scientist, and the paper (based on the ArXiv preprint I cite in the link) was published last month … and there’s a video.

Light Bends Glass

The researchers performed a second experiment with a longer fiber and continuous–rather than pulsed–laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber’s movement.

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The Photon Push-Me Pull-You

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A few weeks ago, over at Built on facts, I threw Matt a bit of a knuckleball in the comments.

[C]onsider a solid bar of the same index [as water]. You send in the pulse of light (assume a really good AR coating so there’s no reflection). What happens to the speed of the bar?

This was sneaky because it is one of the unsolved issues in physics (I feel no remorse for doing this, and Matt realized that something was up) — the theory is complicated enough that it’s really easy to miss out on some of the subtleties and end up with an invalid answer. There are two schools of thought: Minkowski, who had taken the approach that the photon’s momentum in the medium should be nE/c, and Abraham, whose approach gave the momentum as E/nc. Clearly, the results are at odds, and this came to be known as the Minkowski-Abraham momentum controversy.

I found a number of articles on the topic, but perhaps the best one is a review article from Reviews of Modern Physics. Momentum of an electromagnetic wave in dielectric media by Pfeifer et. al, No. 4, October–December 2007 pp. 1197-1216. (link is to a pdf file) The article points out that this isn’t a simple problem, because a photon in a medium can’t be naively treated as just a photon — both solutions have merit, but must include the interactions with the medium, which are obviously different depending on the approach you take — in the end there can be only one you can only have one answer for the momentum of the system.
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