Dārziņš - Komentāri

There's a kind of a duality here. Photons are supposed to be massless, because otherwise they wouldn't be able to reach lightspeed (according to Newtons Law F=ma, if it applies, because to accelerate a photon to lightspeed would require imense forces).

On the other hand it's proven they have some kind of mass because scientist through several experiments have been able to see a photon transfer momentum to other particles

http://www.physicsforums.com/archive/index.php/t-29768.html