Solar sails - impossible?

"Robert Schneider" :

"Stanislaw Sidor" wrote in message
...
What do you think about the Thomas Gold's article:

arXivhysics/0306050 v1 5 Jun 2003
"The solar sail and the mirror"
Thomas Gold (Center for Radiophysics and Space Research, Cornell
University)

http://www.arxiv.org/html/physics/0306050

Is he right or not?

I think that he is probably right. If reflected photons still have the
same
momentum as incident photons, then a perfect mirror won't be able to gain
any momentum. However, I don't think that his ideas dismiss solar sailing
entirely. A sail that absorbs energy and radiates it preferentially in
specific direction might work.

--
(STS)

That is silly, if I throw a ball at you and it bounces off your head, does
the fact that the ball is still moving at the same speed prevent the pressure
on your head? The direction of the photon's momentum and to balance it the
momentum of the sail must also change.

Earl Colby Pottinger

--
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"Earl Colby Pottinger" wrote:
That is silly, if I throw a ball at you and it bounces off your head, does
the fact that the ball is still moving at the same speed prevent the pressure
on your head? The direction of the photon's momentum and to balance it the
momentum of the sail must also change.

Quite. If a perfect mirror were able to reflect photons
and not change their energy, it would still be forced to
generate thrust via conservation of momentum. Similarly,
if you hit someone with a ball and it bounced off and no
energy was lost, then you'd expect the target to recoil
due to conservation of momentum. You simply cannot
change momentum of an object without keeping the books
balanced. Changing the direction of photons changes
their momentum drastically, and that has to be balanced
somewhere. If Gold's theory were correct then Newton's
3rd law would be violated, and that would be quite
serious indeed, quite a lot more serious than any of the
evidence he has put forth to back up the theory that such
a violation would be possible.

However, there is also thermodynamics at play. To
simplify greatly, thermodynamics says that you can't do
work without generating an appropriate amount of
entropy. And here Gold would be correct in stating that
a perfect mirror which changes only the direction of
reflected photons but which is moved would be a violation
of the 2nd law of thermodynamics (it would be a closed
system with decreasing entropy). However, as has been
pointed out many times, the reflected photons are red
shifted, so entropy does increase in the system.

Christopher M. Jones wrote:

However, as has been
pointed out many times, the reflected photons are red
shifted, so entropy does increase in the system.


The second law doesn't require that the entropy increase,
and I don't believe it does necessarily increase here.

Paul

"Paul F. Dietz" wrote:
Christopher M. Jones wrote:

However, as has been
pointed out many times, the reflected photons are red
shifted, so entropy does increase in the system.

The second law doesn't require that the entropy increase,
and I don't believe it does necessarily increase here.

It only absolutely, positively requires that entropy not
decrease. But in reality it almost always increases. In
principle, with a perfectly flat, perfectly reflective
mirror, perhaps the entropy will be exactly balanced, but
in any real system the entropy is going to increase, due
to rough surfaces, absorption, etc.

However, I don't think that his ideas dismiss solar sailing entirely.
A sail that absorbs energy and radiates it preferentially in specific
direction might work.

Technically, that's exactly what a reflective sail does. You can think of it as
1) absorbing the photon, and then 2) re-emitting it in the appropriate
direction (in accordance with the usual laws of reflection). If you do the
math, you discover that a perfectly reflective sail gets twice the momentum of
a perfectly absorbing one. Because of the vector addition, the best "specific"
direction is with the sail normal towards the light source.