What happens when light stands still?

Answer

Hello Martin,

Those are two questions I think 😉

1) I wonder what exactly happens to the light when it is stationary.

First of all, it is important to understand that this is not about light in a vacuum. As you know, the propagation speed of light in a vacuum is always 299,792,458 m/s, so you cannot slow down or stop light in a vacuum.

What it is about is the propagation of light in very special materials (here vapors of alkali metals). These materials are chosen in such a way that there is a very strong resonance. The energy of the light wave is then briefly held in the resonance of the material, which causes the delay.

There is a certain resemblance to the propagation of light in glass: energy is also exchanged between the light wave and glass. As a result, the speed of light in glass is 1.5 times slower. But in the experiment you refer to, this was done with a special quantum mechanical effect to get a much stronger delay.

Did you know that we in Brussels also conduct research into the slowing down of light in so-called metamaterials (these are materials made up of very small electrical circuits instead of atoms)?

So it makes little sense to talk about “rest mass” in these cases because there are not only the photons, but also the resonances in the material. It is the resonances in the material with which the photons interact that provide an apparent rest mass. The photons themselves remain rest massless.

2) Does that mean that when light is stationary, it is actually stationary relative to us but still has an absolute speed?

Indeed. Velocity is always relative to an observer (or an inertial frame). You can’t talk about “absolute speed”.

Greetings,

Philippe
Physicist Free University of Brussels & Iowa State University