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Solar sailing DOESN"T break laws of physics'



 
 
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  #21  
Old July 5th 03, 01:15 AM
Edward Green
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Default Solar sailing DOESN"T break laws of physics'

(Gregory L. Hansen) wrote in message ...
In article ,
Geoffrey A. Landis wrote:
Despite a recent article in New Scientist, a solar sail does not break
the laws of physics.

In an article in New Scientist recently, maverick astronomer Thomas
Gold cast doubt about solar sails:
"Thomas Gold from Cornell University in New York says the proponents
of solar sailing have forgotten about thermodynamics, the branch of
physics governing heat transfer. Solar sails are designed to be
perfect mirrors, meaning that they reflect all the photons that strike
them. Gold argues that when photons are reflected by a perfect mirror,
they do not suffer a drop in temperature. "


http://www.newscientist.com/news/news.jsp?id=ns99993895

Dang, I don't know what to say. Gold is a putz, and I thought New
Scientist was better than that. I didn't realize a solar sail was a heat
engine.


Sure it is. In the limiting case of working between, say, the sun's
outgoing radiation at 5800K, and 3K cosmic background radiation, I'd
say we just _might_ be able to get a heat engine to work. ;-)
  #22  
Old July 5th 03, 05:01 AM
Henry Spencer
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Default Solar sailing DOESN"T break laws of physics'

In article ,
Henry Spencer wrote:
Such thicknesses have the property that solar radiation pressure
approximately balances solar gravity, at any distance from the Sun.


Not quite, alas. For that you need about 0.75g/m^2 -- about 1/4 of the
mass JPL was looking at, and somewhat better than the best they expected
for near-term advanced materials.


Wups, my mistake -- multiply that number by 2, because I couldn't remember
the numeric value of solar light pressure, and had to look it up, and
forgot to check whether I was looking at the number for an absorber or a
reflector.

That puts balance possibly within reach of a near-term advanced sail,
given a low-overhead design.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |
  #23  
Old July 5th 03, 05:04 AM
Henry Spencer
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Default Solar sailing DOESN"T break laws of physics'

In article ,
Dr John Stockton wrote:
I make it, for total reflection, about 9.6E-7 kgF/m^2, or nearly one
kilogram per square kilometre; I leave conversion to archaic units as an
exercise for the reader.


Uh, "kgF" *is* an archaic unit. In modern units it is 9.126e-6 N/m^2.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |
  #24  
Old July 5th 03, 06:20 AM
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Default Solar sailing DOESN"T break laws of physics'

In article , (Mitchell) writes:
But there is energy lost from the photons. Do the math. That's where
the Doppler shift matters.


That's just not true. I'm a senior year physics undergrad at Cornell,
and I was in a group of 6 people who got to sit down and talk with
Gold about his "perfect mirror and violations of conservation of
energy/momentum" paper before it was published. Among the arguments we
attempted to refute is theory was the idea of Doppler shift. He
correctly countered that a Doppler shift does not affect the total
energy, only the power.

:-)))

Consider an elastic collision of two particles. For convenience and
simplicity we'll make it a 1D problem. Two particles coming one
towards the other, colliding and flying away. Now, lets consider it
relative to three different reference frames.

1) In the center of mass frame, both particles approach (with the
same momentum), collide and recede, still with the same momentum and
same velocity (for each one) as before the collision (remember, the
collision is elastic). Total energy is preserved and the energy of
each of the particles individually is preserved as well.

2) In the initial reference frame of A, we see prior to the collision
A stationary and B approaching, after the collision both are moving.
Total energy still preserved but A gained some energy and B lost some.

3) In the (initial) frame of B, the situation is just reversed.
Total energy still preserved but as a result of the collision A lost
some energy and B gained some.

In all (infinity of) other reference frames you'll get other,
intermediate results. Mind you, we're not talking different processes.
We're talking about *same process* as viewed from different reference
frames. The question of who gains and who loses energy in a collision
is not "inherent" to the process but a matter of accounting, depending on
the reference frame.

So, if you take an elastic collision of a photon with the mirror, in
their CM reference frame, indeed, nobody gains or loses energy, the
photon bounces back with the same energy with which it hit. But,
you're not observing from the CM reference frame but from "our" frame,
stationed on the Sun. In this frame, the photon bounces back with
different energy. And the energy difference can be represented
through the Doppler shift between the CM reference frame and our.
Note, you've to apply the shift twice, first transforming from our
frame to the CM, then back.

As I said, do the math. Will do you good.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
  #25  
Old July 5th 03, 06:39 AM
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Default Solar sailing DOESN"T break laws of physics'

In article , (Gregory L. Hansen) writes:
In article ,
David M. Palmer wrote:
In article , Steve Harris
wrote:

Come on. You're like the guy who says: when I heat up an
object, its weight doesn't change (to first order).
Therefore the equivalence of mass and energy is violated.
Duh.

There is no energy cost to move the stationary sail *to
first order.* Carnot's law is broken to exactly the degree
that you simplify the problem with approximation. But don't
confuse your approximation with violation of physical law.


There is no energy cost to move the stationary sail to first order.
There is no change in photon energy to first order

There is an energy cost to move the stationary sail to second order.
There is an equal change in photon energy to second order.

There is no breaking of Carnot's law to any order.


And yet, there is an energy cost to move a stationary sail, and there is a
change in photon energy. And no matter what thermodynamics arguments are
brought up, light pressure has been used to orient satellites, there's
nothing controversial about it. Looks like someone is going to have to
figure out where their analysis went wrong.

Whose analysis? The standard one is OK. If you mena Gold's, then
figuring what went wrong is a matter for psychiatry, not physics.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"
  #26  
Old July 5th 03, 02:25 PM
Christopher M. Jones
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Default Solar sailing DOESN"T break laws of physics'

"Dr John Stockton" wrote:
JRS: In article , seen in
news:sci.space.policy, Dale Trynor posted at Fri, 4
Jul 2003 09:54:45 :-

I do remember reading that the solar force is something like 5 pounds per
square mile if that's of much use.


I make it, for total reflection, about 9.6E-7 kgF/m^2, or nearly one
kilogram per square kilometre; I leave conversion to archaic units as an
exercise for the reader.


Both of these determinations are wrong, without the
stipulation that they are for a distance from the Sun
of 1 AU. The "solar force" scales as the inverse
square of distance from the Sun. Since Solar Sails
are meant for sailin' it doesn't make a whole lotta
sense to force the assumption of 1 AU distance into
the measurement of "solar force", as actually *using*
a Solar Sail will change that, hopefully by a lot.

  #27  
Old July 5th 03, 04:01 PM
Dr John Stockton
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Default Solar sailing DOESN"T break laws of physics'

JRS: In article , seen in
news:sci.space.policy, Henry Spencer posted at
Fri, 4 Jul 2003 18:13:32 :-
In article ,
Dr John Stockton wrote:
The JPL Halley design was
2um of Kapton, topped by 100nm of aluminum, with 12.5nm of chromium on the
back. 2-3um Kapton is commercially available.


Such thicknesses have the property that solar radiation pressure
approximately balances solar gravity, at any distance from the Sun.


Not quite, alas. For that you need about 0.75g/m^2 -- about 1/4 of the
mass JPL was looking at, and somewhat better than the best they expected
for near-term advanced materials.
...


I'll accept a factor of two for my "such ... approximately" (I was not
paying much attention to the density of the material); also up to 25% in
data and arithmetic.

However, I make it not 0.75 g/m^2 but 1.6 g/m^2 for the areal density of
a 100% reflective sail that balances with solar gravity.

Now 4 MT/s of photons over 4 pi * (150 Gm)^2 times speed of 3E8 m/s
gives a pressure about 5 microNewtons per square metre, which is 0.5
mgF/m^2, confirming a result obtained by starting with 1.4kW/m^2.

The solar gravity field here, since it is matched by w^2r, is
approximately (2 pi/3155000)^2 * 150E9 = 6E-3 m/s^2 = 6E-3 N/kg.

F = ma, so m = F/a, so the result is 5E-6/6E-3 = 0.8E-3 kg/m^2.

Thus for a reflective sail, which is twice as good, about 1.6 g/m^2 can
be supported. I think your figure omits reflection, the remaining
discrepancy being insignificant.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
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  #28  
Old July 5th 03, 05:53 PM
Steve Harris
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Default Solar sailing DOESN"T break laws of physics'


"David M. Palmer" wrote in message
...
In article , Steve

Harris
wrote:

Come on. You're like the guy who says: when I heat up an
object, its weight doesn't change (to first order).
Therefore the equivalence of mass and energy is

violated.
Duh.

There is no energy cost to move the stationary sail *to
first order.* Carnot's law is broken to exactly the

degree
that you simplify the problem with approximation. But

don't
confuse your approximation with violation of physical

law.

There is no energy cost to move the stationary sail to

first order.
There is no change in photon energy to first order

There is an energy cost to move the stationary sail to

second order.
There is an equal change in photon energy to second order.

There is no breaking of Carnot's law to any order.




If there's no change in photon energy to first order, then
obviously that's a breaking of Carnot's law to first order,
since Carnot requires an decrease in photon temperature
(photon energy) for work to be extracted. Carnot requires
two thermal baths of different temperatures for kinetic
energy to be gained.

But it's okay, because in any inertial frame where work is
being done on the sail, you see two populations of photons
(those coming and those leaving), and these two DO have two
different temperatures. That's it.

SBH


  #29  
Old July 6th 03, 11:01 AM
John Ordover
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Default Solar sailing DOESN"T break laws of physics'

Someone correct me if I'm wrong, but in general an analogy to solar
sailing would be to move a terrestrial sailboat by hitting the sail
with a whole lot of high-speed ping-pong balls.

Light is massless but not energy-less, natch, so it winds up having a
similar effect on objects to the ping-pong balls. Just as some of the
momementum of the ping-pong balls would be transfered to the sails,
some of the energy of the light is transfered to the sail. Just as
the ping-pong balls would therefore lose some energy, the photons in
the light lose some energy, but light does that all the time.

So what's the problem?
 




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