Here’s a conundrum for you. What is the momentum of light in a transparent dielectric medium?

If the answer doesn’t trip off your tongue, that might be because nobody else knows either. Amazingly, there are two lines of thought:

In 1908, the German mathematician Hermann Minkowski guessed that the momentum was equal to nE/c (where n is the refractive index, and E and c are the energy and speed of light in a vacuum).

A year later, his contemporary Max Abraham suggested that the momentum is equal to E/nc.

A century since then and we’re none the wiser. An embarrassing state of affairs for theoretical physics, wouldn’t you agree?

Today Weilong She and pals from Sun Yat-Sen University in Guangzhou, China, announce that they have the answer. And they got it by measuring the recoil on the end face of a nanometre-sized fibre exerted by outgoing light. (This isn’t the well known pressure caused by specular reflection but something a little more subtle.)

The experiment is impressive because it is designed in such a way that if Minkowski were correct the fibre should be pushed in one direction and in the other if Abraham were correct.

So the result is the first to unambiguously favour one theorist over the other.

And the winner is…drum roll…Abraham.

It’s about time.

Ref: arxiv.org/abs/0806.2442: Observation of a Push Force on the End Face of a nm Fiber Taper Exerted by Outgoing Light