Keith Dysart wrote:
Except that Richard's description sure seems to meet the
requirements of coherency. Can you offer a way for use
to know whether two signals are coherent?

If we can cause them to interfere, that is proof that
they are coherent. If their reflected waves superpose
with destructive or constructive interference present,
then they are coherent.

Secondly, I am at a complete loss to understand how you
can be arguing that when two signals of a particular
power interfere, the result is 4 times the power. This
sure seems like you're getting something from nothing.

Maybe it seems that way to you but it's because of
confusion. If the intensity of each of two coherent
waves is one watt/unit-area and total destructive
interference occurs at one point, two watts/unit-area
have seemingly disappeared. Since energy cannot disappear,
those two watts/unit-area must appear as constructive
interference somewhere else. If total constructive
interference appears somewhere else, the total intensity
is one watt/unit-area from the first wave, one watt/unit-area
from the second wave, plus the two watts/unit-area from
the total destructive interference. Adding all those
intensities up gives us four watts/unit-area at the point
of total constructive interference. The intensity at
the total destructive interference point plus the
intensity at the total constructive interference point
still averages out to two watts/unit-area, exactly the
intensities in the original waves and exactly what it
takes to satisfy the conservation of energy principle.

Your "something for nothing" doesn't exist. In the absence
of a local source, any constructive interference must be
offset by an equal magnitude of destructive interference.
--
73, Cecil http://www.w5dxp.com