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Problems with beaming

Timothy van der Linden writes:
 > To Lee,
 > >There are two problems really. First of all; there is the inverse square
 > >law, the intensity (and therefore the power) of radiation decreases as the
 > >inverse square of the distance. I have not seen anyone's calculations here
 > >take into account the amount of power that must be generated HERE in order
 > >to provide a reasonable amount of power THERE. 
 > Yes, but lasers keep a tight beam and thus their diametre (cross-section)
 > does not increase with distance and the energy-density (per surface) stays
 > the same.
 > So I don't see a problem here.

Actually, practical lasers do diverge with distance.  Coherency does not
mean unidirectionality.

For example, lasers that work by reflecting light between two mirrors
produce a primary beam where an integral number of wavelengths N falls
between the mirrors, and a secondary annular beam where N+1 wavelengths
fall between the two mirrors, with a slight angle between the primary
beam and the secondary annular beam.  The angle isn't noticeable in
typical lasers and laboratory distances, since it's on the order of
milliradians; shining the laser over a long enough distance would show a
bright spot surrounded by a ring, rather than just a spot (assuming your
mirrors are that good, anyway).