What would stationary light look like?

Question

I mean, have we ever witnessed what we understand and call as light at rest, and what would it look like at rest?

Answer 1

"What we understand and call light" includes electromagnetic radiation that propagates (i) in a vacuum or (ii) through a material medium, which includes refractive media.

In vacuum, the speed of light is fixed at $c$, and there is no such thing as stopping it or changing its speed.

On the other hand, in material media, light can propagate at different speeds $v<c$, due to the effect of the medium. (In essence this is because the wave couples to the electric charges inside the medium, and it becomes a coupled wave of electromagnetic fields and charge excitations in the medium — a 'polariton' rather than a 'photon' — but we still call it 'light'.) For regular materials, this effect can be significant (say, slowing light down by a factor of two or more), but the propagation speed is still very much in the range of hundreds of thousands of meters per second.

However, it is possible to use other 'control' light beams to change the refractive properties of media, in a process known as electromagnetically induced transparency, which produces truly remarkable effects. In this regime, it's known as slow light, and light pulses propagating through such media can be slowed down to walking speed (meters per second) or even completely stopped.

So: when you do that, what does the light "look" like? To be honest, it doesn't 'look' like anything: the light pulse gets captured as internal excitations in the layout of the electric charges inside the gas cell where you've captured it, and it doesn't propagate out until it is triggered. As such, there is no visual change to the medium when the light pulse is captured – but, of course, it's still there!