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Ian McDonald asked the Naked Scientists: If light has no mass:(a) How can light be subject to gravity?(b) How can the equation E=mc2 be correct? If we let E be the energy of a photon (or a billion, gazillion photons), and m be the mass of that/those photons (i.e. zero), then E=0 x c2, or E=0. So if photons have no mass, it would seem they can have no energy either, which is patently incorrect.What do you think?

Ian, you know that light has a energy Every time you go out in the sun you feel that energy warm you up. So no matter what equations you consider you have to admit to that.

Ian McDonald asked the Naked Scientists: If light has no mass:(a) How can light be subject to gravity?

(b) How can the equation E = mc^{2} be correct?

If you choose to use the one called relativistic mass, E=mc^{2} holds, and light has a non-zero value of m so that it can have energy.The more common choice, and the one that's used when saying light is massless, is the invariant mass. If you use the invariant mass, the proper equation is E^{2}=m^{2}c^{4}+(pc)^{2}, where m=0 for light, and p is momentum, which is non-zero.

The second equation involves momentum which is the objects mass x velocity and you said the mass is 0.

Please could someone explain why the second equation is right? The second equation involves momentum which is the objects mass x velocity and you said the mass is 0. Therefore you are saying E^{2} is still 0 and so is E ∴ contradicting everything you just said. E=mc^{2} and E^{2}=m^{2}c^{4} + (pc)^{2} don't work when equating energy of light. Don't use E=mc^{2} and E^{2}=m^{2}c^{4} + (pc)^{2} when finding energy of light, use E=hf and f=hc / λ, where h is plancks constant, f is frequency of the photon and lamda is the wavelength of the photon to work energy out from its wavelength, NOT MASS.

Quote from: freewillyThe second equation involves momentum which is the objects mass x velocity and you said the mass is 0.The photon carries momentum - this is the mechanism behind the solar sail, with its deployment recently tested in space by the Planetary Society. ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN

Quote from: evan_au on 30/10/2015 23:39:03 Quote from: freewillyThe second equation involves momentum which is the objects mass x velocity and you said the mass is 0. The photon carries momentum - this is the mechanism behind the solar sail, with its deployment recently tested in space by the Planetary Society. ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN--lightarrow

Quote from: freewillyThe second equation involves momentum which is the objects mass x velocity and you said the mass is 0. The photon carries momentum - this is the mechanism behind the solar sail, with its deployment recently tested in space by the Planetary Society. ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN--lightarrow