From: Jeff Davis (jrd1415@yahoo.com)
Date: Thu Jul 03 2003 - 04:30:16 MDT
--- Spike <spike66@comcast.net> wrote:
>
> Subject: Solar sailing vs. laws of physics ?
>
> The reflected photon would have the same
> frequency as the pre-reflection photon
> only if the light source and the reflector
> are stationary with respect to each other.
Is this actually the case? Has it been
observed experimentally with stationary
versus moving mirrors? Certainly we've
heard of radiation that has been Doppler-shifted
red or blue from radiation sources moving
towards or away--respectively--from the
observer. Is a moving reflector equivalent to a
moving emitter? What are the differences between
these two radiation-'manifesting' processes? And what
are the differences between the reflection(1) we
associate with mirrors (metallic and dielectric), and
the reflection(2) we associate with, say, the moon,
which seems more rightly a case of absorption and
re-emission?
More fun.
Consider the solar sail when first unfurled. If it
were in a conventional stable orbit, with no radial
velocity relative to the sun, would there then be no
force arising from 'radiation pressure'?
That doesn't seems reasonable. I've just finished
reading a few web pages on radiation pressure. It
seems a fairly established phenomenon, with both
heavyweight theoretical
foundation--Maxwell--http://www.wikipedia.org/wiki/Radiation_pressure
and experimental confirmation.
http://www.wikipedia.org/wiki/Nichols_radiometer
A Nichols radiometer is the apparatus used by Nichols
and Hull in 1901 for the measurement of radiation
pressure.
And nowhere is motion of the illuminated area
mentioned as a factor.
So, will an observer traveling with the solar sail
observe an alteration in frequency of the incoming and
outgoing radiation? Does starlight from a receding or
approaching star change frequency when reflected in an
earthbound mirror?
> If the reflector is moving away from the
> light source, there is clearly gravitational
> potential energy coming from somewhere.
Yesiree. Everything suggests a pressure, a force over
an area, a transfer of and increase in momentum, and a
consequent inescapable increase in energy, some
combination of potential:out of the gravity well, and
kenetic--increase in velocity. The question remains,
where's the energy coming from?
It seems logical to look to the incident radiation for
the source of the energy. I would be looking for less
energy coming 'out' in the reflected radiation, than
went in, originally. And I would expect the degree to
which the reflection is 'perfect', as with a
finely-polished mirror, to correlate with low
absorption/heat generation, and high ratio conversion
to velocity parallel to the radiation.
> The reflected photons are red shifted by the
> receding reflecting surface. Photon's
> momentum is transferred to the sail, no
> energy is lost, no conundrum or paradox, no
> physically illegal procedures.
>
> spike
>
Also, from the original article, we have this quote:
"There may also be evidence to support Gold's theory,
in the form of a quirky device called a Crookes
radiometer. It consists of four paddles attached to
the arms of a rotor, inside a vacuum jar. Each paddle
is silvered on one side and coated with a black
absorber on the other. When placed in sunlight, the
rotor spins. If the theory of solar sailing is right,
the rotor should spin with the reflecting silver
surfaces moving away from the light. But it actually
spins the other way..."
Now, I've seen one of these gadgets, and the little
vanes do indeed turn away from the blackened side and
in the direction of the silvered side. Clearly the
'radiation pressure' on the silvered side, with a high
degree of reflection, is no match for the competing
force generated, presumably, from the absorption,
heating, and reemission from the blackened side.
Hmmmm.
Would a better solar sail result if the inward-facing
side were blackened and the outward side silvered?
Best, Jeff Davis
"Everything's hard till you know how to do it."
Ray Charles
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