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Light Sails Won't Work?
http://www.spacedaily.com/news/rocketscience-03zg.html
Interesting... -- simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole) interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org "Extraordinary launch vehicles require extraordinary markets..." Swap the first . and @ and throw out the ".trash" to email me. Here's my email address for autospammers: |
#2
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Light Sails Won't Work?
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.) -- MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer first ground-station pass 1651, all nominal! | |
#3
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Light Sails Won't Work?
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. ;-) -- simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole) interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org "Extraordinary launch vehicles require extraordinary markets..." Swap the first . and @ and throw out the ".trash" to email me. Here's my email address for autospammers: |
#4
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Light Sails Won't Work?
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#5
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Light Sails Won't Work?
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#6
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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! | |
#7
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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. |
#9
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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! | |
#10
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Light Sails Won't Work?
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. -- © 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. No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News. |
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