Electromagnetic radiation transmits energy to the sail by reflection or absorption. Does it mean electromagnetic radiation also loses velocity and gain mass in the process?
$\begingroup$
$\endgroup$
0
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
Electromagnetic radiation is light and light does not have mass, nor can it lose velocity. Inside materials it might appear to lose speed, but that is only an "illusion". What electromagnetic radiation loses is its momentum when it reflects or is absorbed by material.
$\begingroup$
$\endgroup$
To some approximation, light which reflects from a sail has a momentum change due to the new direction of the light beam. To be precise, there is also a movement velocity of the sail, so there is another part of the light momentum change that is a change in frequency (due to Doppler effect).
Few light sail designs are absorptive, because that precludes strategies for steering, but absorption of light also transfers the momentum of the light to the target.
Light energy-per-photon is proportional to light frequency, and light momentum-per-photon is a vector, oriented in the direction of light travel, which is proportional to the light energy (or, equivalently, proportional to the light frequency).
Light reflected from a moving sail at an arbitrary angle changes its direction and magnitude of momentum, and that change implies an equal and opposite change in the momentum of the sail. It does not alter, however, the speed of light in a vacuum, nor does it confer any mass onto a massless photon. It CAN change the (relativistic) mass of that moving solar sail, though, to make up for energy loss(or gain) by the Doppler-shifted photon, according to conservation of mass and energy.
