From Eric Kvaalen
Photons do have mass, according to E = mc2. What they do not have is a positive “rest mass”, and this means they cannot stand still.
There is certainly a lot of free energy available in sunlight, because it comes from only one direction. The sunlight has a “temperature” of more than 5000 kelvin, whereas the microwave background radiation coming from other directions has a temperature of about 2 K. A solar sail would not work in a uniform, thermodynamically equilibrated radiation field.
And the reason a Crookes radiometer works has to do with the gas that remains in the “vacuum”. In 1901 a similar radiometer was made, with a better vacuum, which turned in the way predicted by the theory of solar sails.
Paris, France
