Letter to the Editors of New Scientist Re: Solar Sailing Breaks Laws of Physics
Subject: Re: Solar Sailing 'breaks laws of physics'
My website was linked to in the recent story "Solar sailing 'breaks laws of physics'" without requesting comment from me before publishing. I disagree with the conclusions drawn in the article.
Light pressure does work. It is derived directly from Maxwell's equations of electromagnetism. It has been measured in laboratories since 1900, and has played a major role in space missions since the beginning of spaceflight. The Echo space balloons, which were used to to reflect radio signals, were driven out of orbit by light pressure. The Mariner 10 mission to Mercury used light pressure on the solar arrays to steer itself when it ran low on propellant. US weather satellites have reflective booms to balance solar pressure on their solar arrays. The Iridium global telephone satellites receive an orbit boost at certain times of the year because of light pressure. Solar sails merely build on this experience.
Dr. Gold claims that solar sails are governed by Carnot's rule for heat engines. Carnot's rule cannot be applied to an open system like a solar sail, because they receive light from the sun and reflect it back out into the universe. Treating sails as closed loop heat engines ignores the rest of the universe.
[ Editor's note: After a number of responses, I feel there is a need to clarify this point. Dr. Gold neglects the frequency shift of sunlight reflected by a sail due to relativity. This frequency shift accounts for the orbital energy (kinetic + potential) loss or gain of the sail. A sail sprialling away from the sun gains energy, so the sunlight loses energy by shifting down in frequency. A sail sprialling towards the sun loses energy, so the sunlight gains energy by shifting up in frequency. For a sail with constant orbital energy the sunlight does not change in frequency. ]
Dr. Gold claims that a black sail would absorb sunlight and the momentum associated with it until it reached thermal equilibrium, when it would stop absorbing sunlight. Objects do not stop absorbing light once they reach equilibrium. Equilibrium means that the temperature of the sail has stopped changing, and the radiated power equals the absorbed power. The sail will continue to absorb light after equilibrium is reached, and radiate infrared light out the front and back. The thrust from the radiated light cancels itself out, but the sail still receives thrust from the absorbed light.
Dr. Gold brings up the Crooke's Radiometer as an example of a failed test of light pressure. He is correct, but 130 years late in pointing it out. Maxwell is also credited with solving this problem:
Light pressure can be measured in the lab with the correct setup. The first experiment in 1900 used a thin fiber with vanes attached to it. Light struck one of the vanes, and the fiber twisted measurably. Today, powerful lasers and microwaves are used to test light pressure on the ground. Spacecraft provide a ready laboratory in space.
Dr. Gold's final point is that light can't have pressure because it is a scalar quantity while Newtonian momentum is a vector. Light pressure is a vector. Maxwell's equations are vector equations. The velocity of a photon (an individual packet of light) is a vector equal to its direction multiplied by the speed of light. So, there is no conflict.