This is interesting (and possibly dismaying for solar sail fans). Thomas Gold says that they won’t work, at least not the way people have had in mind.

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.

(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.)

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.

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 [sic] 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 [sic] 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.