Before you reply with one of the obvious answers, let me explain what I mean:
I know you can slow light in various media, that doesn’t count for the purposes of this thread
I don’t mean detecting a photon and knowing it travelled at c (which much of our technology relies upon…it’s not like it’s in doubt).
I mean…let’s say you have an extremely powerful pulse of EM radiation (e.g. the radio waves of a pulsar) go through a very thin cloud of material that scatters that radiation. Then we would see a “wake”, travelling across the cloud at c.
So we need a combination of a very powerful EM source, that’s far enough away, that pulses in some way, near to a medium that scatters it an appropriate amount.
Is there anywhere like that? Sorry if it’s a dumb question, it’s strangely hard to google.
I don’t know of any specific examples of being able to observe the propagation of light at a vast distance away in space, although the right circumstances for observing such a thing must surely have occurred at some point. However, here is a different approach that really does show the propagation of a pulse of laser light. It doesn’t involve any significant slowing, though technically the light traveling through the milky water in the Coke bottle has been slowed a bit, but the reflections that can be seen emerging from the bottle are pretty much back to traveling at c again. It’s a fascinating apparatus that, through repeated scans, effectively has a frame rate of a trillion FPS:
That’s the one I thought of, too. It’s really not what it looks like, though: It’s not a bunch of frames of a single pulse; each frame is of a different pulse of light, and the timing of the camera is just precisely adjusted such that each frame captures the next pulse at a point slightly further on.
Incidentally, it’s possible for a light echo to appear to travel faster than c.
Imagine a pulse of light far away from some source, so that it’s nearly planar. And imagine that it intersects a slab of gas at a shallow angle relative to the light. You’ll see the line of intersection illuminate, but it’ll move faster than c.
Of course, this is no different than how the spot of a laser beam, or even just a shadow of something, can move faster than light. There’s no physical object moving, and so no top speed. It just means you have to be careful with your conclusions.
This can also happen when things are illuminated at the same time, but you see things close to you first (since the signal also goes at c) and you make invalid conclusions about the speed of things. That can happen to physical objects, but again there’s no violation since you’re really seeing events that happened at different times.
My favourite light echo example is Hubble’s Variable Nebula. It has shadows moving faster-than-light, because of the effect mentioned by Dr Strangelove above.
