[ih] Speed of Light (meandering offtopic from Re: Design choices in SMTP)

[Bill Ricker]

On Tue, Feb 7, 2023 at 6:17 PM touch--- via Internet-history <
internet-history at elists.isoc.org> wrote:

> > On Feb 7, 2023, at 11:35 AM, Grant Taylor via Internet-history <
> internet-history at elists.isoc.org> wrote:
> >
> > On 2/7/23 11:37 AM, Guy Almes via Internet-history wrote:
> >> <> and the speed of light got worse.  I mostly mean this in the sense
> that it didn't get any better, while processing and transmission both got
> much faster.
>

Love it !

>> But it's also true in the literal sense that wide-area networks were
> largely built with microwave in the 1980s
> (or geosynchronous satellites),
> while they are built with fiber optic circuits now.
>

True!
Before fiber (fibre), laying cables to handle the long-lines bandwidth
required by the growing demand for telecommunications wasn't practical
except along RR rights of ways (where telegraphy had started!).
But later, conveniently after discovery of fibre, the limited number of
channels that increasingly complex baud encoding schemes could fit in each
microwave frequency "channel" was a limit on bandwidth available for still
growing telecommunications demand. While directional horns prevented
circuits going in different directions from interfering (much), one
couldn't reuse the frequencies between Manhattan and Pougkeepsie to double
the bandwidth available except by going way off path, which would interfere
with a different direction.
As most of us have learned the hard-way, the slow twist of twisted pair
doesn't prevent all cross-talk; it's not bad, but it's not good. There are
limits to how many TP circuits one can include in the same jacket, and
their bandwidth is uh limited (a lot better than we thought once, but still
limited).
The _isolation_ of fibre or coax allowed packing multiple circuits reusing
the same several frequencies (colors) of light into a small trench along
that RR right of way.
(Back of envelope SWAG, a single dark fibre in that bundle has more
bandwidth than all the transcontinental circuits in microwave-tower days,
and uses less energy?)

And the speed of light through fiber is about 2/3 the speed of light
> through air (or space).
>

True

> This floored me the first time I heard about it.
>

Indeed, after all repetitions of the relativistic "*the speed of light is a
constant*" * mantra we'd absorbed,
finding out that* different* speeds of light† are *why* lenses and prisms
are useful
is a "*can i think two thoughts*" cognitive-dissonance problem until one
works through in Physics class it and sees they're two sides of the same
coin.

*(same WRTO  different reference frames/observers, but always referring to*
freespace vacuum,* even outside gravity wells until you get to General
Relativity)
†(different WRTO different *media*)

(*In these overspecialized times, whether a future telecoms manager gets
the Physics class that works through that is a different matter.*)

Note that 2/3 (67%) is the ratio for fiber to free space, fiber is
> basically the same propagation speed as coax and most twisted pair.
> Twin-lead is actually faster - 80%.
> The only common media that comes close to air/vacuum is open-ladder wire.


Yes.
It's roughly a speed of propagation vs leakage (power efficiency & privacy)
tradeoff.

(HOWEVER the usual reasons for preferring Twin-lead or ladder-line (whether
"open" or "window)" vs Coax, fibre, or twisted pair is rarely the
phase-velocity aka speed limit.
Almost always the choice is by power limit, power efficiency/dielectric
losses at frequency, and/or impedance matching.
Which is why Ethernet and two-way radio use 50Ω coax, but Cable-TV,
long-run TV antenna leads, etc use 75Ω coax.)

Even an evacuated metal waveguide is slower than freespace air/vacuum,
because the power efficiency (defeating 1/r² law, which is a power law in
both senses) is gained by reflections off the walls, so next extra distance
travelled, zig-zag, was the pre-relativistic explanation for the delay.

(Coax or waveguide with dielectric filling gets even slower but gets more
bendable and less likely to have spark breakdown or short circuits, but
more tradeoffs vs wavelength there too.)

Air is technically slower than Physics's platonic freespace vacuum, but the
refractive index of air is so low
* n(air)=vp/c=~ 1.0003*
that it's phase-velocity
*vp  = 0.9997 × c* ,
so those are both usually negligibly different from exactly integer 1
(value at freespace vacuum),
as compared to ⅔ or 80% *c* for practical signal paths.

(At microwave frequency, the moisture in clouds changes *v*p and *n*
sufficiently that instead of scattering as light does, clouds can act as a
lens or prism.  The difference between light and microwaves is how well the
water molecules or droplets couple with the E-M wave; shorter wavelength
light treats each droplet as a lens/prism/reflector, hence rainbows and
also scattering; longer microwaves treat the cloud of droplets as an
aggregate dielectric foam 'nano material'. As that might imply, certain
varieties of common craft/buiders' foam-board can be layered to build
microwave lenses. )


NOW if speed *really* matters ... say one is a High-Speed financial trader
trying to shave milli-seconds off time to receive and react to a price
change before the other dude when arbitraging Chicago vs NY exchanges ...
dedicated microwave point to point links may still be worth the expense,
along with simplistic JFDI protocols instead of the fancy routing that we
discuss here; but the modulation/demodulation adds another layer of
router-like delay, particularly if it's an error-correcting coding. (Which
if not including TCP error detection/retransmission would be a good idea!)

So it's elephants tradeoffs all the way down.

/ Bill Ricker