Keith Dysart wrote:
That is why I pose the question, hoping for someone to
describe the mechanism that the energy for the flow that
is happening now can be borrowed from the future.

Destructive interference would have to happen first.

But what happens if the generator is turned off before
the future arrives? Where did the extra energy come from
then?

There is no extra energy. Constructive interference is
impossible without that supply of energy.

Actually, I have a pretty good grasp of what is happening
in free space, and it is all available to you by extension
from the behaviours of the one dimensional transmission
line. But there is little point in going there until the
transmission line is understood.

It is the exact opposite. There is no point in inventing
new laws of physics for transmission lines if those new
laws don't work in free space.

So please present an example of EM waves reflecting off
of other EM waves in free space. Do you really think the
energy in the standing wave beam of a laser is reversing
direction and momentum every cycle?
--
73, Cecil http://www.w5dxp.com