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Light Sails Won't Work?



 
 
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  #11  
Old July 6th 03, 04:25 AM
Alex R. Blackwell
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Default Light Sails Won't Work?

Rand Simberg wrote:

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

Interesting...


For those who are interested, the New Scientist press release (which is
merely repeated on the SpaceDaily website) refers to Gold's recent
preprint, "The solar sail and the mirror," which appeared on the LANL
preprint server. See http://xxx.lanl.gov/abs/physics/0306050

--


Alex R. Blackwell
University of Hawaii

  #12  
Old July 6th 03, 04:25 AM
Parallax
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Default Light Sails Won't Work?

Herb Schaltegger wrote in message ...
In article ,
(Henry Spencer) wrote:

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

Interesting...


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

(snipped good stuff)


There's a very entertaining thread over on Slashdot about this story
today - 459 posts so far (of course once you filter out the comments
mod'd down to 0 or -1, the number is a slightly more reasonable 365 or
so.)

http://science.slashdot.org/article....9&mode=thread&
tid=134&tid=160


I am glad somebody pointed out the thermal reasons why a Crookes
Radiometer works. I always thought that the light pressure was too
feeble to cause it to spin and even then couldnt figger out why it
should spin even if the light pressure was greater

  #13  
Old July 6th 03, 05:25 AM
Henry Spencer
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Default Light Sails Won't Work?

In article ,
Christopher A. Bohn wrote:
Something I've been wondering about is the effect of the "heavier"
particles in the solar wind -- will the impacts of protons and alpha
particles have a significant effect, when compared to the acceleration
realized by the reflection of photons?


In principle, the solar wind -- which is just the charged particles, it
does *not* include the light -- would have some small effect.

However, when looked at closely, the solar wind carries 100-1000 times
less momentum, per square kilometer, than the light does. The particles
are heavier, yes, but they are very widely spaced and their speed is
relatively modest. So they don't add noticeably to the thrust of a
solar sail.

The one real option for exploiting the solar wind is that it is a plasma,
and hence can interact with electric and magnetic fields, which the light
cannot. A substantial magnetic field, in particular, can interact with
the solar wind over a *much* larger area than that actually occupied by
any physical piece of hardware (even an ultrathin sail). Propulsion
systems using that approach have been proposed, although even the best of
them is nowhere near an in-space test right now.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

  #14  
Old July 7th 03, 02:35 AM
Christopher A. Bohn
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Default Light Sails Won't Work?

Good afternoon,

On 6 Jul 2003, Henry Spencer wrote:

Christopher A. Bohn wrote:
Something I've been wondering about is the effect of the "heavier"
particles in the solar wind -- will the impacts of protons and alpha
particles have a significant effect, when compared to the acceleration
realized by the reflection of photons?


In principle, the solar wind -- which is just the charged particles, it
does *not* include the light --


Right -- minor parsing ambiguity -- I didn't mean to suggest the photons
were part of the solar wind, but rather I wanted to emphasize why I was
wondering about the solar wind's effects.

would have some small effect.

However, when looked at closely, the solar wind carries 100-1000 times
less momentum, per square kilometer, than the light does. The particles
are heavier, yes, but they are very widely spaced and their speed is
relatively modest. So they don't add noticeably to the thrust of a
solar sail.

[...]

Thank you very much.

Take care,
cb

--
Christopher A. Bohn ____________|____________
http://www.cis.ohio-state.edu/~bohn/ ' ** ** " (o) " ** ** '
"Si Vis Pacem, Para Bellum"
- Appius Claudius (the Blind), Roman Senate

  #15  
Old July 7th 03, 12:25 PM
Paul Blay
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Default Light Sails Won't Work?

"Dr John Stockton" wrote ...
news:sci.space.policy, Rand Simberg
On 3 Jul 2003 23:15:01 GMT, in a place far, far away, Dr John Stockton

The originator has obviously forgotten some fairly basic physics - that
you are also unaware of


"Interesting" doesn't necessarily mean that I agree with it.


But you did not say that you disagreed with it.


Everything not forbidden is compulsory.

  #17  
Old July 9th 03, 02:15 PM
Henry Spencer
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Default Light Sails Won't Work?

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! |

  #18  
Old July 9th 03, 02:25 PM
Paul Blay
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Default Light Sails Won't Work?

"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.

  #20  
Old July 9th 03, 04:20 PM
Henry Spencer
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Default Light Sails Won't Work?

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! |

 




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