Originally posted by: MrDudeMan
yeah i was thinking about those little shiny things in sealed boxes that move when you shine a flashlight on them or put them in the window. how does that work if a photon has no mass?
Yes those "toys" work on the principle that conservation of momentum dictates that the imparted momentum for a reflected photon is
twice the magnetude of the incident photon momentum (one times to stop it and a further one times to reverse it) while the imparted momentum is only equal to
one times the incident photon for an absorbed photon. Thus even if the same light levels are incident on both sides of the "sail" there is still a net tortional force.
Force and momentum are not the same thing but are closely related. Force is the time rate of change of momentum, so if you multiple the momentum per photon by the number of photons striking per second (which is proportional to the
intensiity of the light) then you get units of momentum imparted per second which is force.
BTW, an interesting side-note is that most of the cheap varieties of those "toys" as found in novelty shops actually work more on thermal convection rather than photon momentum as they're supposed to. If not fully evaccuated then air heats differentially on each side of the sail (black side heating more) and the resulting convection away from the heated region causes an air pressure differential enough to turn the sail slowly. So alas the cheap ones are actually more of a conventional wind sail than a solar sail.
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