Ben (narrator/author of the LessWrong post)

Going back to our runner example, the slight shift of the block from left to right can be thought of as the splash as the runner leaves the water.

There's an image here.

Putting the water center of mass slightly to the right.

Although this may be taking the analogy too far.

In contrast, Minkowski's momentum would have us believe the glass block gets moved in the opposite direction, towards the light source.

So it is not only failing to quantitatively satisfy the principle that centre of mass transport should be conserved in an isolated system, but is making a qualitatively opposite prediction that the glass moves in the opposite direction.

Note, however, that the principle being broken, uniform motion of center of mass, is not at all one of the big principles of physics, especially not with the extra step of converting the photon energy to mass.

I had not previously heard of the principle, and don't think it is anywhere near the weight class of things like momentum conservation.

A variation on this experiment has been done.

An optic fiber has a high-power flash of laser light exit from the end facet.

As the laser flash leaves the glass into air, its momentum increases, according to Abraham, which requires a reaction force against the fiber itself, propelling it backwards.

So that when the light leaves the fiber springs back our video.

Link in text the spring back is evidence of the Abraham version of the momentum.

One problem with this experiment is that light, by any definition, has very little momentum, so they had to put fairly high powers through a very small fiber.

This means that other things, thermal expansion, static electricity etc., are pushing on the fiber as well, and we need to put some trust in estimates of how strong these competing effects are relative to the direct change in the light momentum.

Also, if Minkowski was right the fiber would get tugged down, which, given some elasticity in the fiber, might look somewhat similar to it springing back.

A clean-cut proof of Minkowski's result.

There is a nice, simple, way of showing that Minkowski's result must be right.

Get a mirror, put it in a liquid.

Bounce a beam of light off the mirror and measure the radiation pressure on said mirror as the light bounces off.