From: Spike (spike66@comcast.net)
Date: Thu Jul 03 2003 - 21:00:26 MDT
I was in a hurry when I posted my one-liner.
Let me deal with this in a little more detail.
from: Jeff Davis
Subject: RE: Solar sailing vs. laws of physics ?
--- 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?...
Ja.
...Has it been
observed experimentally with stationary
versus moving mirrors?...
Ja.
...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?...
Ja.
...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?...
Oy. Jeff I hope I get to your question here by saying
what you need is to remember is momentum is
conserved, same with reflected photons as with any
other collision. You know that the momentum of a
photon is h*(nu)/c. If one collides with your
motor home it will appear from the point of view
of the driver to have the same frequency when it
leaves as when it arrives. Of course the photon
is slightly blue shifted from the drivers view
compared to the cop's view. The difference between
the frequency the cop sees and what the driver sees
is doubled, and that is the blue shift the radar gun
sees.
...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'?...
No. Assume the sail of just the right density that
the force due to light pressure exactly equals the
force due to the sun's gravity. The sail hovers,
stationary with respect to the sun, no work is done,
no frequency shift of the reflected photons.
...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...
Ja, these are all well understood physical phenomena.
Thomas Gold can access many good texts on the subject.
I looked over his site briefly and saw several areas
which look erroneous or incomplete. Solar sails are
not perfect reflectors. Standard theory has a factor
which describes what fraction of the sun's energy is
reflected.
...So, will an observer traveling with the solar sail
observe an alteration in frequency of the incoming and
outgoing radiation?...
No. The astronaut on board the solar sail sees the
same frequency incoming and outgoing. The observer
on the sun sees a redshifted spectrum if the sail is
moving away and blue shifted if the sail is inbound.
Note that in practice a solar sail would actually
orbit the sun, then orient itself at some angle, say
pi/4 to spiral away from or spiral towards the sun.
That way, there is no need to have the sail so thin
that its mass per unit area is less than the light
pressure from the sun.
Does starlight from a receding or approaching star change frequency
when reflected in an earthbound mirror?
Ja, of course. Starlight is no different from
any other kind of light.
> 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?...
The reflected photons have transferred some of their
energy to the sail. That's why they are red shifted,
assuming the sail is outbound. If the sail is inbound,
the photons are blueshifted, higher frequency, higher
energy.
...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...
Exactly so, Jeff. Whats the mystery?
...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...
No. Light pressure is verrrrry small compared to
the force generated by air inside the bulb being
heated more on the black side than on the cooler
silvered side. If you took one of those things, pulled
a sufficiently strong vacuum on it and shined a
sufficiently strong light, it *would* turn the
other way.
This would be provable with something as simple
as a vacuum bell jar. Drill a hole in the
bulb, keep the light source steady and watch the
spinner slow as the pressure in the bell jar
drops.
...Would a better solar sail result if the inward-facing
side were blackened and the outward side silvered?...
No. A solar sail doesn't have a blackened side. One
cannot afford the weight hit of such a thing, even if
it were useful for some reason, which it isn't.
Gold's Carnot cycle argument comes apart when one
realizes how very little actual energy we are extracting
from the photons. It really isn't much, and the final
resulting accumulated gravitational potential energy
isn't much. The magic of the solar sail is that the sun
is providing both the energy source and the propellant
(the photons) so the spacecraft neednt haul the stuff
itself.
Again I urge you, young man, consult the established
texts on this topic. The smart guys have figured
this stuff out. Everything's hard till ya know how to
do it, Jeff. We know how to do this one.
spike
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