NSA tapping undersea fibers?

[Matt Crawford]

Trusting that Perry will declare this OT before too much longer ...

> > To lift the midpoint of a cable 1000 units long by 5 units requires
> > only 0.067 units of slack, or the ability to stretch by 0.0067%.
> > (This takes into account the catenary shape of the lifted cable.)
> 
> Finish your example please...
> 
> You know gravity, calculate the force along the axis of the cable and then
> compare to it's tensile strength. Include the weight of the cable as well
> as the gravitic effects.

Neither the University of Chicago's graduate program in theoretical
physics nor Noah Webster's successors introduced to me that that
science-fictiony word "gravitic", but let's assume it's completely
redundant with "include the weight of the cable."

I don't know squat about an undersea cable, except that it's heavily
armored against damage.  Let's suppose it weighs no more than would a
4-cm diameter solid steel cable.  That, in water, would be something
like 6 kg per meter.

Assuming there was only the minimum required amount of slack required
for the hypothetical lifting (or equivalently, that the cable was
being stretched just enough to reach the surface), and supposing that
the units of length in the example are kilometers, then the tension
in the cable turns out to be pretty close to a uniform 1500 kN, which
also turns out not to be far above the typical tensile strength for
the assumed cross-section of steel, and well within the strength of
special-purpose items like conveyor belts in coal mines.



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