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Solar sailing DOESN"T break laws of physics'
Despite a recent article in New Scientist, a solar sail does not break
the laws of physics. In an article in New Scientist recently, maverick astronomer Thomas Gold cast doubt about solar sails: "Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about thermodynamics, the branch of physics governing heat transfer. Solar sails are designed to be perfect mirrors, meaning that they reflect all the photons that strike them. Gold argues that when photons are reflected by a perfect mirror, they do not suffer a drop in temperature. " Unfortunately, Gold has apparently forgotten to account for a well-known physical effect: the Doppler shift. It's worth saying that the photon pressure on a spacecraft is not theoretical; its effect on spacecraft is measurable, and it has been observed and measured to great precision routinely in space. Photon pressure-- the solar sail effect-- has already been used for an operational space mission; it was for spacecraft attitude control on the Pioneer Venus-Mercury mission. The Crookes radiometer does not operate on photon pressure, and the explanation for how it operates has been known for over a century. The energy transfer to a solar sail can be accounted for from the Doppler shift of reflected photons; even when the reflectivity is 100%, a photon looses energy when reflecting from a moving sail. This effect exactly corresponds to the energy increase of the sail. No sophisticated physics is needed to analyze this effect, it is a problem suitable for a homework assignment for a college undergraduate. When the sail is moving, then the reflected photons are Doppler shifted, and leave the sail with lower energy than they arrived. This loss of energy exactly equals the energy imparted to the sail, a fact which can be trivially verified by using Newton's laws, the Doppler formula, and the Einstein equation for photon momentum p=E/c If the sail is not moving, there is no Doppler shift. However, note that since energy is proportional to momentum squared, the derivative of energy with respect to momentum is zero for a non-moving sail. Thus, when the sail is stationary, it can reflect photons with perfect efficiency and still gain momentum at no energy cost. For completeness, note that if the sail is moving *toward* the light source, then the phtons are Doppler shifted to *higher* energy by the reflection. This implies that the sail must lose energy-- which is correct; when the sail moves toward the light source, it slows down. -- Geoffrey A. Landis http://www.sff.net/people/geoffrey.landis |
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