Light Sails Won't Work?

http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

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In article ,
Rand Simberg wrote:
http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

Yes, as proof that Tommy Gold has reached his dotage.

A *moving* perfect mirror *does* reduce the "temperature" of photons
reflected from it -- by Doppler shift! Where does the energy lost in
Doppler shift go? Into added kinetic energy of the mirror.

(If the mirror is held stationary -- relative to the observer who is
measuring the details -- by some means, then there can be no Doppler shift.
But there is also no work done on the mirror, since work is thrust times
*distance*, and hence there is no added kinetic energy.)

Yes, Doppler shift at ordinary velocities is pretty damn small. But so is
the acceleration produced by light pressure.

Gold appears to be unaware that the physics of light pressure are well
understood and have been demonstrated many times -- in the laboratory, in
precision tracking of spacecraft, and in attitude control of spacecraft.

A particularly glaring example is Radarsat 1, which is in a dawn-dusk
sun-synchronous orbit (i.e. essentially continuous sunlight) and flies
with an essentially constant attitude. Its designers overlooked
solar-sail effects on its big solar arrays and radar antenna, which are
slightly tilted with respect to the Sun for engineering reasons. Turns
out that nearly 2/3 of Radarsat's stationkeeping fuel goes to fight
light-pressure drag -- it's trying to sail down into the atmosphere.
See "Radarsat Time Rate of Mean Semi-Major Axis Due to Drag", by Said R.
Marandi, in the AAS/GSFC 13th International Symposium on Space-Flight
Dynamics, 1998.

Note that the experts consulted for the article were a thermodynamicist
and an astronomer, neither of them a physicist. (Citing the Crookes
radiometer is just plain embarrassing -- it turns by thermal effects, not
by light pressure.)
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MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

On 3 Jul 2003 18:05:08 GMT, in a place far, far away,
(Henry Spencer) made the phosphor on my monitor
glow in such a way as to indicate that:


Note that the experts consulted for the article were a thermodynamicist
and an astronomer, neither of them a physicist. (Citing the Crookes
radiometer is just plain embarrassing -- it turns by thermal effects, not
by light pressure.)

Well, so in that sense, it sustains Gold's point. ;-)

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interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org

"Extraordinary launch vehicles require extraordinary markets..."
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In article ,
(Henry Spencer) writes:
|
| Note that the experts consulted for the article were a thermodynamicist
| and an astronomer, neither of them a physicist. (Citing the Crookes
| radiometer is just plain embarrassing -- it turns by thermal effects, not
| by light pressure.)

Well, I always thought that thermodynamics was a branch of
physics, but evidently even the term "physics" has now started
to overspecialise :-)

This seems to be a lovely example of people knowing more and more
about less and less.


Regards,
Nick Maclaren.

(Henry Spencer) wrote:

(If the mirror is held stationary -- relative to the observer who is
measuring the details -- by some means, then there can be no Doppler shift.
But there is also no work done on the mirror, since work is thrust times
*distance*, and hence there is no added kinetic energy.)

I don't understand the above statements.
The mirror will definitely still be accelerated even if an arbitrary
observer is attached to it.
Also, there'll be a Doppler shift to the photons in any
non-accelerating reference frame.

In article ,
wrote:
(If the mirror is held stationary -- relative to the observer who is
measuring the details -- by some means, then there can be no Doppler shift.
But there is also no work done on the mirror, since work is thrust times
*distance*, and hence there is no added kinetic energy.)

I don't understand the above statements.
The mirror will definitely still be accelerated even if an arbitrary
observer is attached to it.

Accelerated *with respect to who*? Not with respect to the observer, by
definition. (It is not meaningful to speak of whether it is "really"
accelerated or not; all such statements must be made relative to a
specific observer.)

I was, however, thinking of an observer in an inertial frame of reference,
and a restraint on the sail which prevents it from accelerating, e.g. an
ion engine on its back side thrusting the other way. Dealing with
non-inertial frames of reference gets messy.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

"Henry Spencer" wrote ...
In article ,
wrote:

I don't understand the above statements.
The mirror will definitely still be accelerated even if an arbitrary
observer is attached to it.

Accelerated *with respect to who*? Not with respect to the observer, by
definition. (It is not meaningful to speak of whether it is "really"
accelerated or not; all such statements must be made relative to a
specific observer.)

Er, didn't Einstein have something to say on that subject?
/Velocity/ is only meaningful relative to a particular observer, but
"is this accelerating / not accelerating" is a meaningful question.

Dealing with
non-inertial frames of reference gets messy.

Quite.

In article ,
says...
"Henry Spencer" wrote ...
In article ,
wrote:

I don't understand the above statements.
The mirror will definitely still be accelerated even if an arbitrary
observer is attached to it.

Accelerated *with respect to who*? Not with respect to the observer, by
definition. (It is not meaningful to speak of whether it is "really"
accelerated or not; all such statements must be made relative to a
specific observer.)

Er, didn't Einstein have something to say on that subject?
/Velocity/ is only meaningful relative to a particular observer, but
"is this accelerating / not accelerating" is a meaningful question.

In fact, didn't Einstein discuss how some rules change in accelerated vs.
non-accelerated systems, and that's why "is this accelerating / not
accelerating" *becomes* a meaningful question?

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In article ,
Paul Blay wrote:
... (It is not meaningful to speak of whether it is "really"
accelerated or not; all such statements must be made relative to a
specific observer.)

Er, didn't Einstein have something to say on that subject?
/Velocity/ is only meaningful relative to a particular observer, but
"is this accelerating / not accelerating" is a meaningful question.

Actually, what Einstein had to say on the subject is precisely the
opposite of what you're suggesting. In Newtonian mechanics, acceleration
is absolute: you can work in an accelerated frame of reference only by
introducing "fictitious forces" (like centrifugal force) to fudge the
details. But General Relativity removes the absoluteness of acceleration,
by telling you that those fictitious forces could be real gravitational
forces instead, and there is no way to tell the difference. Relative
acceleration between yourself and the mass of the universe produces the
same measurable effects, in GR, regardless of who you say is "really"
moving.

Dealing with
non-inertial frames of reference gets messy.

Quite.

And *that* is unchanged in General Relativity. :-)
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

JRS: In article , seen in
news:sci.space.policy, Rand Simberg
posted at Thu, 3 Jul 2003 15:45:01 :-
http://www.spacedaily.com/news/rocketscience-03zg.html

Interesting...

The originator has obviously forgotten some fairly basic physics - that
you are also unaware of - assuming that we're thinking of the same
article, which, as I am off-line, is not to hand.

Like pushing a stationary car, 100% reflection by a stationary mirror
basically transfers momentum but not energy.

Like pushing a moving car, 100% reflection by a moving mirror transfers
both momentum and energy.

As many photons as are incident are reflected, but, in the case of the
moving mirror, with a Doppler shift which alters their energy.

Thus propulsion is compatible with the standard conservation laws.

Only trust the judgement those writers on technical subjects who
remember the essentials of basic physics; trust others only to the
extent that they are faithful copyists of well-known sources. I know of
no current examples of these working for the popular media.

I believe that an accurate and acceptable parliamentary term for the
article, if it is the one I think it is, is Horlicks; but perhaps that
is not something that the USA is familiar with.



Hop : the Crookes Radiometer is propelled "backwards" because residual
gas is "reflected" from the black, hotter side with a greater gain in
energy. Some space-walker should take such a radiometer, show it
running backwards, and then open the vessel to a proper vacuum (perhaps
using a small Wake Shield) as a demonstration.

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