Comments on: Measuring the ‘kick’ of a photon leaving a fiber!

By: Blake Stacey

Blake Stacey — Wed, 28 Jan 2009 20:29:05 +0000

A lot of controversies in physics end this way: with the participants realizing that, in fact, the question they originally asked was ill-posed. With my relatively superficial understanding of the controversy, this seems like the most sensible resolution of the controversy.

So, we should denote the momentum of the photon by [tex\mu[/tex]?

By: IronMonkey

IronMonkey — Wed, 28 Jan 2009 15:31:09 +0000

Today I made a rapid reading of the colloquium by R. N. C. Pfeifer et al. (skipping the tensors’ stuff). Unsurprisingly, my initial understanding of the issue was oversimplifying the A-M problem which requires a rigorous account of all material, internal and external effects on the system. I must say the authors made a nice historical review. They also point out that the A-M controversy has already been resolved: each version of the electromagnetic momentum tensor is not adequate on its own, we must include the complete material momentum tensor such that the total momentum tensor is correctly evaluated. Hence our choice of A or M in the equation would be more a question of aesthetics.

By: skullsinthestars

skullsinthestars — Wed, 28 Jan 2009 03:29:07 +0000

stuwat: They do address the possibility of thermal expansion in the fiber in some detail. Basically, they adhered a colophony filament to their fiber and ran cw power through it for a period of time. The colophony decomposes at 300 C, and no decomposition was observed, putting an upper limit on the fiber temperature and thermal expansion. Of course, there could be subtler effects at play.

IronMonkey: My interpretation is very similar to yours! In fact, there seems to be little question about the microscopic definition of electromagnetic momentum. The problem comes in when one tries to apply the macroscopic equations, which are in a very real sense an oversimplification of the problem.

By: IronMonkey

IronMonkey — Tue, 27 Jan 2009 16:51:47 +0000

It also appears to me that the She et al. experiment does not resolve the issue. The observation made by stuwat is very interesting. A quick search on Wikipedia for the speed of light in a dense medium suggests:

“As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore when the speed of light in a medium is said to be less than c, this should be read as the speed of energy propagation at the macroscopic level. At the microscopic level electromagnetic waves always travel at c.”

It thus appears to me that the issue of light momentum could be similarly treated as above. Namely, that some experiments may measure the a “macroscopic light momentum” as M or A, however the momentum itself may never change and remain p=h/lambda as in vacuum… Hence, our interpretation might depend on the frame of reference used.

In any case, this is a very interesting physical question and I’m looking forward to see if it will be resolved (or has already been done).

By: stuwat

stuwat — Mon, 26 Jan 2009 21:32:06 +0000

I haven’t read the She, Yu, Feng paper and would really like to know how they ruled out thermal effects. It seems like there should be a more obvious explanation of why the fibre behaves the way it does, one which accounts for it based on the repeated absorption of light and dissipation of heat, in what is a very fine fibre. The movement may just happen to resemble the way it would move according to the Abraham prediction. It’s a lovely demonstration and fuel for curious minds.