4 ms·

Photons lack mass right?

• posterboy 9y ago
  not exactly. As photons are constantly moving at the speed of light (citing the theory of relativity I don't really understand) they have no resting mass. But they have an impuls and create a force of impact (used in sun sails) from which a theoretical notion of ... mass (kinetic, perhaps?) can be derived. I'd like to know how it's derived originally, too though. edit: if E=mc² with E(photon)=f•h and f=c/λ, then m=h/(λc) in vacuum. ... I hope that's correct. What's interesting, [m]=J/[a]/m
  • gnaritas 9y ago
    Normally when discussing mass, people are referring to resting mass, not relativistic mass.
    • posterboy 9y ago
      normally, people don't discuss mass. rather they are simply heavy. most don't even really care why. and those who do, conflate the idea with weight often enough.
      • gnaritas 9y ago
        haha, very true.
• throwaway613834 9y ago
  Well I read [1] [2] that they do have gravity... which is probably a more direct answer to what you're asking. I'm not sure what counts as "mass" for a photon per se, except that I assume there's an effective mass via the energy. [1] https://www.quora.com/How-does-a-photon-exert-gravitational-pull/answer/Giordon-Stark https://www.quora.com/How-does-a-photon-exert-gravitational-... [2] https://physics.stackexchange.com/a/22878 https://physics.stackexchange.com/a/22878