Solar sails in the silly season
Jul. 10th, 2003 06:23 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
mmcirvinDavid Appell links to some good material (and I added a comment) about Thomas Gold's erroneous claim that solar sails violate the laws of thermodynamics, and the New Scientist treatment of same.
Appell mentions Lou Friedman's dismissal of Gold's claim. Actually, reading Friedman's article on the subject, I think that his list of experimental evidence is good but his theoretical rebuttal isn't quite right. You can't violate the Carnot limit using quantum mechanics, or else we'd surely already have cars that run on ambient heat and use their exhaust for air conditioning. Nor is the "quantum mechanical particle nature of light" crucial, though it helps when thinking about the momentum of light.
Gold's thermodynamics error is simpler than that. Because sunlight has something like a blackbody spectrum, Gold treats it as if it were nothing but a heat bath, and derives his result from the absence of a cold sink. No temperature differential, no work to be extracted. But this is incorrect; unlike an ideal heat bath, sunlight is streaming out from the sun in a particular direction, and that makes all the difference. As several people on the sci.physics thread said, if you want to model the system (imperfectly) as a heat engine, the heat bath is the Sun itself, and the cold sink is the rest of the universe.
What Gold's really proven is that if you put a solar sail in a mirrored cavity filled with a thermal bath of radiation bouncing around in all directions, so that only heat energy can be transferred to it, it will not work. But that is no surprise, and irrelevant to the use of solar sails in space.
The remarks at the bottom of Gold's preprint (and Soter's comments) dismissing the very concept of photon momentum are truly flabbergasting to me-- as others mentioned on sci.physics, it's as if they don't think anyone has ever measured the Compton Effect. Not only am I pretty sure it has been measured, I'm pretty sure that I've measured it personally, in a laboratory course back in school. It's a standard exercise.
I regret to say that I'm not terribly surprised at something like this breaking in New Scientist. In my opinion, they have a long-standing tendency to report sensational-sounding stories with more credulity than is sometimes warranted. They do report many legit stories, but when I hear New Scientist cited as the source for something wild, I tend to withhold judgment until I can find corroboration elsewhere. Actually, that's a good rule for popular science magazines in general, but for New Scientist more than some.
The whole business reminds me not so much of the Jayson Blair affair as of a somewhat earlier incident involving the New York Times.
(Finally, an aside: As usual in discussions of radiation pressure, the Crookes radiometer came up. The phenomenon of the radiometer is actually pretty subtle, and half-correct explanations abound that are wrong in the details. Appell linked to a page that is actually a mirror of this excellent explanation of the radiometer from the Usenet Physics FAQ.)
Appell mentions Lou Friedman's dismissal of Gold's claim. Actually, reading Friedman's article on the subject, I think that his list of experimental evidence is good but his theoretical rebuttal isn't quite right. You can't violate the Carnot limit using quantum mechanics, or else we'd surely already have cars that run on ambient heat and use their exhaust for air conditioning. Nor is the "quantum mechanical particle nature of light" crucial, though it helps when thinking about the momentum of light.
Gold's thermodynamics error is simpler than that. Because sunlight has something like a blackbody spectrum, Gold treats it as if it were nothing but a heat bath, and derives his result from the absence of a cold sink. No temperature differential, no work to be extracted. But this is incorrect; unlike an ideal heat bath, sunlight is streaming out from the sun in a particular direction, and that makes all the difference. As several people on the sci.physics thread said, if you want to model the system (imperfectly) as a heat engine, the heat bath is the Sun itself, and the cold sink is the rest of the universe.
What Gold's really proven is that if you put a solar sail in a mirrored cavity filled with a thermal bath of radiation bouncing around in all directions, so that only heat energy can be transferred to it, it will not work. But that is no surprise, and irrelevant to the use of solar sails in space.
The remarks at the bottom of Gold's preprint (and Soter's comments) dismissing the very concept of photon momentum are truly flabbergasting to me-- as others mentioned on sci.physics, it's as if they don't think anyone has ever measured the Compton Effect. Not only am I pretty sure it has been measured, I'm pretty sure that I've measured it personally, in a laboratory course back in school. It's a standard exercise.
I regret to say that I'm not terribly surprised at something like this breaking in New Scientist. In my opinion, they have a long-standing tendency to report sensational-sounding stories with more credulity than is sometimes warranted. They do report many legit stories, but when I hear New Scientist cited as the source for something wild, I tend to withhold judgment until I can find corroboration elsewhere. Actually, that's a good rule for popular science magazines in general, but for New Scientist more than some.
The whole business reminds me not so much of the Jayson Blair affair as of a somewhat earlier incident involving the New York Times.
(Finally, an aside: As usual in discussions of radiation pressure, the Crookes radiometer came up. The phenomenon of the radiometer is actually pretty subtle, and half-correct explanations abound that are wrong in the details. Appell linked to a page that is actually a mirror of this excellent explanation of the radiometer from the Usenet Physics FAQ.)
