Monday, July 04, 2011

Nothing is faster than light ....

Trawl through discussions of the NBN and other fibre projects (New Zealand's for example) and you'll soon stumble upon a justification for the NBN that "nothing is faster than light".

Without entering into the subject of apparent cases where this isn't true, it is particularly interesting in the discussion of fibre versus wireless.

See, both happen to be electromagnetic radiation and we know that both visible light and radio waves travel at the same speed in a vacuum. But any medium they travel through slows them down. The question is "by how much". Here's where it gets tricky. The refractive index is the measure of how fast light travels with respect to a medium it is travelling through, and is defined as n=c/v that is the ratio of the speed of light in a vacuum and the speed of light in the specific material.

One well known consequence of this is the fact that light gets bent by glass - hence how we make lenses. But we also know that different frequencies get bent differently - hence how you get a rainbow from a prism or indeed from droplets of water.

In general, the higher the frequency the lower the speed. So when refractive indexes are listed it is usually at a specific frequency (the sodium D line).

The refractive index of air is 1.000293 so light in the atmosphere and radio waves don't get slowed down much.

Fibre optic cable is actually a solid core of silica wrapped in a transparent cladding with a lower index of refraction. This is what makes the light reflect within the fibre. The refractive index of the core is typically 1.62 while that of the cladding is 1.52.

That is the light in a fibre cable is travelling at a shade under 190,000 kilometres per second compared to the 300,000 kilometeres per second for light in a vacuum,which is pretty much what radio waves achieve in air. Radio is a far more complex propogation than simply line of sight effects.

Finally, of course, the speed with which a copper cable carries information is also based on the field speeds not of an actual electron propogating. Propagation speed is affected by insulation, such that in an unshielded copper conductor range 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the speed of light.

So it is true that nothing is faster than light in a vacuum, but electric current in twisted pairs and radiowaves through the air both travel faster than light through silica.

It is not the speed of the transmission medium that counts - that produces latency - it is the bandwidth that is significant.

Novae Meridianae Demetae Dexter delenda est

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