Im confused slightly...
Its interesting that NASA Documentarys say the commands take 20 minutes to reach the Mars rovers because the signals travel at the speed of light...In the next breath i hear nothing can travel at the speed of light except light..
You're not the only one who's confused. If you want to win a quick Nobel Prize Synch, a tidy up of the Theory of Everything is still very much MIA.
Light is an Electromagnetic Wave. People who talk about 'photon packets' are mostly just trying to make the maths easier. Waves though have a continuous anologue shape and the maths soon gets pretty tough.
Especially so as a rising / falling electric field, in a wave, induces a magnetic field perpendicular to itself. This in turn affects the original electric field, and they almost dance together, oscillating in an electro-magnetic wave.
And that's just one wave! We live in a whole sea of ripples crossing every which way and interfering with each other .. it's such a complex system that Quantum Mechanics resolves out the smallest known dimension, the Planck Length (1.6 × 10-35 metres) which is described as being a bit like 'foam', maybe being something like the 'choppiness of the sea'.
Ordinary waves are in themselves quite interesting.
In an ocean the water doesn't actually move; each molecule describes a circle as a wave passes through. So even light doesn't really travel at the speed of light in a way, because nothing actually goes anywhere only the 'little shockwaves' do. Shockwaves can move faster than the medium they travel in for this reason, or put another way, all you need is for the molecule next door to 'notice' that it's neighbour moved, this is 'knowledge' so can move very quickly. It doesn't have a physical constraint other than 'how closely the molecules in line are watching each other'. Some theory may say that this has something to do with the Planck Length.
As for the 'speed of radio', Radio uses the same electro-magnetic force interaction but with a longer wave-length. Length as measured in metres.
Visible Light has a high frequency, around 10E+15 cycles per second. In that second the electro-magnetic wave can travel 300 million metres, 'lightspeed'; each 'wave', one cycle, has a length, by dividing up, of between four and seven hundred nano-metres. (1 nm = 10E-9 metres).
FM Radio uses electromagnetic frequency (decodable by a receiver) of about 100 million Hz, so visible light vibrates another 10 million more times each second than an FM radio signal does, FM wavelength is around one metre.
Deep Space radio may perhaps use Long Wave; the quantum foam doesn't interfere with it as much and the wave can penetrate further into deep space without dissipating. (Low pitch sound-waves also travel further, for similar reasons). Long Wave radio wavelengths are measurable in kilometres but are still electromagnetic in nature, and still being a shockwave, 'the signal' still travels at the speed of light.
As for what does the wave travel in, isn't space a vacuum? This is most likely the 'dark matter'/'dark energy' conundrum for which, reader, if you do win a Nobel Prize, maybe you'll remember me if I got you thinking. ;p
