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In the articles linked below, Gold from Cornell claims solar sailing won't
work essentially because it would violate the 2nd law of thermodynamics. I
have some difficulties with his argument.
Right off the bat, I wonder how his argument would change if we considered
say a baseball elastically bouncing off the sail? The gist of his complaint
seems to be that since the baseball loses no energy, the sail cannot gain
any. This argument seems flawed to me because it assumes the baseball has
negligible mass compared to the sail. In reality the sail moves away from
the baseball slightly and hence the baseball will bounce back with slightly
less speed in the lab frame. Wouldn't the same be true for a photon, ie.
wouldn't there be a Doppler redshift of the reflected light, corresponding
to an effective temperature drop?
Also am I to presume the mirror is a perfect (elastic) reflector whereas the
baseball cannot be perfectly reflected? This seems a red herring to me,
since no real mirror can be perfect. I don't see why a perfect mirror
requires any greater stretch of imagination than a perfect "trampoline."
Another red herring in my opinion is to think about sunlight as a heat
transfer and presumably (without saying so in the article explicitly) the
baseball as work transfer. So what if I shone laser light (which can be
considered work instead of heat) rather than sunlight on the sail? Doesn't
change anything that I can see.
Nor do I buy his Crooke's radiometer argument. An ideal radiometer *would*
turn in the correct (radiation pressure) direction for low-friction vanes in
an ultrahigh vacuum. Apparently done in 1901 using a fiber. [Lebedew, P.N.,
"Untersuchungen uber die Druckkrafte des Lichtes," Annalen der Physik, No.
11, pp. 433-458 (1901). ]
And what's all that at the end of the second article below that radiation
momentum is a scalar not a vector?
http://www.newscientist.com/news/news.jsp?id=ns99993895
http://www.arxiv.org/html/physics/0306050
Comments? Carl
-----
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/