Media Casualties Mount
Administration Split On Europe Invasion
Administration In Crisis Over Burgeoning Quagmire
Congress Concerned About Diversion From War On Japan
Pot, Kettle On Line Two...
Allies Seize Paris
Gore Book Sales Tank, Supporters Claim Unfair Tactics
Satan Files Lack Of Defamation Suit
Why This Blog Bores People With Space Stuff
A New Beginning
My Hit Parade
Instapundit (Glenn Reynolds)
James Lileks Bleats
Winds Of Change (Joe Katzman)
Little Green Footballs (Charles Johnson)
Eject Eject Eject (Bill Whittle)
Alan Boyle (MSNBC)
Space Politics (Jeff Foust)
Space Transport News (Clark Lindsey)
NASA Space Flight
A Voyage To Arcturus (Jay Manifold)
Dispatches From The Final Frontier (Michael Belfiore)
Personal Spaceflight (Jeff Foust)
The Flame Trench (Florida Today)
Rocket Forge (Michael Mealing)
COTS Watch (Michael Mealing)
Curmudgeon's Corner (Mark Whittington)
Tales of the Heliosphere
Out Of The Cradle
Space For Commerce (Brian Dunbar)
The Speculist (Phil Bowermaster)
Spacecraft (Chris Hall)
Space Pragmatism (Dan Schrimpsher)
Eternal Golden Braid (Fred Kiesche)
Carried Away (Dan Schmelzer)
Laughing Wolf (C. Blake Powers)
Chair Force Engineer (Air Force Procurement)
JesusPhreaks (Scott Bell)
Nanobot (Howard Lovy)
Lagniappe (Derek Lowe)
Geek Press (Paul Hsieh)
Redwood Dragon (Dave Trowbridge)
Turned Up To Eleven (Paul Orwin)
Cowlix (Wes Cowley)
Quark Soup (Dave Appell)
Assymetrical Information (Jane Galt and Mindles H. Dreck)
Marginal Revolution (Tyler Cowen et al)
Man Without Qualities (Robert Musil)
Knowledge Problem (Lynne Kiesling)
Cut On The Bias (Susanna Cornett)
The Funny Pages
Cox & Forkum
Day By Day
Happy Fun Pundit
Amish Tech Support (Lawrence Simon)
Scrapple Face (Scott Ott)
Quasipundit (Adragna & Vehrs)
England's Sword (Iain Murray)
Daily Pundit (Bill Quick)
Daimnation! (Damian Penny)
Z+ Blog (Andrew Zolli)
The Kolkata Libertarian
Midwest Conservative Journal
Protein Wisdom (Jeff Goldstein et al)
Dean's World (Dean Esmay)
Yippee-Ki-Yay (Kevin McGehee)
Spleenville (Andrea Harris)
Random Jottings (John Weidner)
On the Third Hand (Kathy Kinsley, Bellicose Woman)
Inappropriate Response (Moira Breen)
Inadvertent Comic Relief
Warblogger Watcher (Cowardly Anonymous Idiotarians)
Other Worthy Weblogs
Ain't No Bad Dude (Brian Linse)
A libertarian reads the papers
Anna Franco Review
Ben Kepple's Daily Rant
Dropscan (Shiloh Bucher)
End the War on Freedom
Insolvent Republic of Blogistan
James Reuben Haney
Mind over what matters
Page Fault Interrupt
Sand In The Gears(Anthony Woodlief)
The Blogs of War
The Fly Bottle
The Illuminated Donkey
What she really thinks
Where HipHop & Libertarianism Meet
Zem : blog
Space Policy Links
The Space Review
The Space Show
Space Frontier Foundation
Space Policy Digest BBS
USS Clueless (Steven Den Beste)
Unremitting Verse (Will Warren)
World View (Brink Lindsay)
The Last Page
More Than Zero (Andrew Hofer)
Pathetic Earthlings (Andrew Lloyd)
Spaceship Summer (Derek Lyons)
The New Space Age (Rob Wilson)
Rocketman (Mark Oakley)
Site designed by
Setback For Solar Sails?
Basically, he contends that none of the analyses of their performance has taken into account thermodynamics--that they only consider the momentum change of the photon. The sails are built to be as reflective as possible to minimize heating of the fragile sail fabric. But according to Gold, if no heat is absorbed (i.e., no temperature change in the photon), the Carnot's Rule says that no energy can be extracted.
It's been too long since my thermo courses to know if he's right or not, but he's not obviously wrong. Considering that people have been thinking about this for decades, I find it a little surprising that the physics remains unsettled. We'll find out when the first demo sails are flown shortly, and we'll see if the performance matches predictions.
[Update at 11:26 AM PDT]
There's been a lot of discussion in the comments section, but considering the source (Henry Spencer, over at sci.space.*), I'll consider this the last word for now.
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.
Well, certainly in theory both thermodynamicists and astronomers are supposed to have a good grounding in physics (and arguably, the former is a specialized form of physicist), but other than that bit of ad hominem, I've nothing to dispute.
[One more a few minutes later]
OK, Geoff Landis has also weighed in.
Unfortunately, Gold has apparently forgotten to account for a well-known physical effect: the Doppler shift.Posted by Rand Simberg at July 03, 2003 08:17 AM
The photon suffers a loss of energy, and is thus reduced in frequency (red shifted). In effect it is "cooled". Thermodynamics and Conservation of Momentum are both obeyed.
Yes, that's what I'd always thought, except that Planck's Law was a frequency shift attributed to a change in momentum, not energy. I'd never actually considered the energy, per se, just assuming that it all came out in the wash.Posted by Rand Simberg at July 3, 2003 08:56 AM
If solar sails can't work, I wonder how radiometers work? http://www.physics.purdue.edu/demo/3D/radiometer.htmlPosted by Andy Freeman at July 3, 2003 09:06 AM
Simple calculations show that the photon loses a small amount of energy to the sail, leading to a decrease in frequency of the photon. This also results in a lower total momentum for the photon after the interaction. The decrease in frequency exactly compensates for both the energy and momentum transferred to the sail. This is a very simple scattering problem, and it's frankly amazing that a physicist of Gold's caliber could make such a simple error.
Andy, the article points out that the direction a radiometer spins supports Gold.Posted by Rand Simberg at July 3, 2003 09:32 AM
According to the interview, Gold only has an issue with a sail that acts as a 'perfect mirror'. He readily admits that a comet's tail shows a momentum change from the photons, as will other non-perfect reflecting surfaces.
If the Sail is indeed a 'perfect mirror', then there should be no frequency shift of the photons. His point is still valid.
-SPosted by Stephen Kohls at July 3, 2003 10:05 AM
Even with a perfect reflection, there will still be a frequency shift due to momentum change (changing frequency a la Planck is the only way to change the momentum of a photon, since its speed can't change, and it has no rest mass).Posted by Rand Simberg at July 3, 2003 10:07 AM
JPL has done the test, at least for their material of choice. It worked.
I don't follow Gold's argument; how can lack of temperature change invalidate conservation of momentum and energy?Posted by Troy at July 3, 2003 10:21 AM
Troy, as I read that test, it indicated that they could indeed put pressure on the sail, but it wasn't clear from the story whether or not it matched up quantitatively to theoretical predictions.
One would presume so, though, because they would have had to figure out why if it hadn't.Posted by Rand Simberg at July 3, 2003 10:49 AM
I must really still be back in the buggy-whip days on this technology (what else is new?), because I thought the solar wind also includes heavier particles that can impart momentum in a way that even us buggy-whippers can wrap our minds around.
I thought that would be how a solar sail would really work; didn't know the bulk of the energy (pardon the expression) was supposed to come from photons.Posted by McGehee at July 3, 2003 11:03 AM
No, the solar wind is almost irrelevant to a light sail, and in fact degrades its performance over the long term by putting holes in it. It's propelled by photons.Posted by Rand Simberg at July 3, 2003 11:08 AM
That was my take, Rand -- if the result was wildly out-of-whack with predictions, they'd not be likely to stick their necks out by announcing a successful test. Of course, I haven't seen the results in a peer-reviewed journal, but then I don't get all the relevant journals.
I recall the Echo satellites (huge aluminized Mylar balloons) were pushed around quite a bit by solar radiation; I was under the impression that this sort of change was routinely predicted for lightweight satellites these days -- I've seen published notices that such-and-such was expected to move due to solar activity, even. Solar wind forces on these objects are orders of magnitude lower than solar radiation forces, so what else could be causing it?Posted by Troy at July 3, 2003 11:23 AM
Upon reflection, I believe solar sails will work.Posted by Sean Kiely at July 3, 2003 11:57 AM
Carnot law does not say that the temperature must go down..it says that the entropy of the universe must go up. For thermal systems, there is a temperature driven maximum efficiency. For non thermal systems, there is no such limitation, hence fuel cells, which use gibbs free energy to calculate efficiency versus carnot efficiency, have very high efficiency rates. Photons reflected from a solar cell (due to compton scattering) are at a lower energy ... Energy of photons e=hv (where h is planks constant and v is frequency) and E=mc2 (where E is energy, c is speed of light and m is mass, photons have relativistic mass = hv/c2 since momentum p is = mass x speed , thus hv=pc..h and c are constants, so if p goes down (loss of momentum, and momentum is always conserved) v (frequency) must go down, and energy was hv, thus energy went down. Since energy was transfered, the future usefulness of the reflectant photons goes down, and thus entropy of the universe goes up. The photons cannot possibly transfer more momentum than the differnce between current momentum (which was reduced by the solar cell) and the momentum due to brownian motion of the interstellar hydrogen (about 3K)with which they must be discharged. The crookes radiometer is consistent, since the mirrored surface encounters much less compton scattering it reflects most photons at a higher energy (thus on that side less momentum is transfered) than the black side which absorbs the high energy photons (visible light) and emmits lower energy photons at near infrared (it will heat up more). Thus the black side is absorbing more energy, and hence transfering more momentum, so the net moment arm pushes forward on the black side.Posted by monopticus at July 3, 2003 12:36 PM
Actually, that's not right. The Crooke radiometer is a heat engine, and doesn't work directly on a photonic propulsion principle. If you totally evacuate it, in fact, it doesn't work. It's propelled by heated air on the black side.Posted by Rand Simberg at July 3, 2003 12:59 PM
"Thus, when the sail is stationary, it can reflect photons with perfect efficiency and still gain momentum at no energy cost."
Huh? How do you gain momentum while remaining stationary?Posted by J Bowen at July 3, 2003 01:35 PM
Is Thomas Gold running for something? They say any media is good media, but good heavens. If he wants name recognition he's got it now.
This is the most absurd thing I've seen since Bill Clinton talked about the shuttle Columbia on Larry King.Posted by Transistor at July 3, 2003 02:57 PM
Sean Kiely: Upon reflection, I believe solar sails will work.
<groan!> (Never let it be said I didn't show proper appreciation for a good bad pun.
J. Bowen: "Thus, when the sail is stationary, it can reflect photons with perfect efficiency and still gain momentum at no energy cost."
Huh? How do you gain momentum while remaining stationary?
Doesn't say it "remains" stationary. Says it "is" stationary, which I read to mean, before it starts moving.Posted by McGehee at July 3, 2003 04:31 PM
Here's what I'm missing: If, at V-0 the photon emparts no energy on the sail, How does the spacecraft accelerate from adead stop?Posted by Kevin L. Connors at July 4, 2003 12:28 AM
I still don't understand how a doppler shift would occur if the sail is at rest.Posted by homer simpson at June 22, 2005 11:14 PM
Post a comment