1123581321 Posted February 10, 2011 Share Posted February 10, 2011 I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship.. because since energy and mass are interconnected, doesn't that mean that any particle would have to have mass regardless wether or not its physically energy or mass.. and by the way how i a photons physical particle topology held together (if thats the right wording), since its only energy.. Link to comment Share on other sites More sharing options...
alpha2cen Posted February 10, 2011 Share Posted February 10, 2011 Important things are what is mass and what is photon. We could describe well photoelectronic effect without photon mass. Link to comment Share on other sites More sharing options...
ajb Posted February 10, 2011 Share Posted February 10, 2011 I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship.. All physical particles must obey the following: [math]E^{2}-p^{2}c^{2} = m^{2}c^{4}[/math], where p is the 3-momentum and m the mass. So for a massless particle we have [math]E^{2}-p^{2}c^{2} =0[/math]. No violation of relativity here. Note we do not have a photon rest frame so we cannot set p=0. For massive particles we can chose the rest frame and the mass-shell condition becomes [math]E = mc^{2}[/math], in that frame. Link to comment Share on other sites More sharing options...
alpha2cen Posted February 14, 2011 Share Posted February 14, 2011 I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship.. because since energy and mass are interconnected, doesn't that mean that any particle would have to have mass regardless wether or not its physically energy or mass.. and by the way how i a photons physical particle topology held together (if thats the right wording), since its only energy.. Generally speaking, massless particle is DM(dark matter). And other concept is unparticle . Link to comment Share on other sites More sharing options...
ajb Posted February 14, 2011 Share Posted February 14, 2011 Generally speaking, massless particle is DM(dark matter). Dark matter must be massive otherwise it would not "clump together". It would not form halos if massless. And other concept is unparticle. Unparticles are a hypothetical so far hidden sector the the standard model that cannot be described in terms of particles. The idea is that "stuff" maybe scale invariant, but not massless. In standard QFT the particles must be massless to have this scale invariance. Loosing the notion of a particle can "cure" the insistence of zero mass. Link to comment Share on other sites More sharing options...
steevey Posted February 18, 2011 Share Posted February 18, 2011 (edited) I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship.. because since energy and mass are interconnected, doesn't that mean that any particle would have to have mass regardless wether or not its physically energy or mass.. and by the way how i a photons physical particle topology held together (if thats the right wording), since its only energy.. As far as I know, light only has mass in the sense that is has relative mass, i.e. at any moment, if it suddenly turned into matter, it would have a specific amount of mass it would turn into under the relationship E=mc^2 Edited February 18, 2011 by steevey Link to comment Share on other sites More sharing options...
Sedit Posted February 18, 2011 Share Posted February 18, 2011 I asked pretty much the same exact question about a week ago in this thread right here, http://www.scienceforums.net/topic/54756-size-of-a-black-hole/ I personally feel light does have mass but its to small for us to really care most of the time. Link to comment Share on other sites More sharing options...
ajb Posted February 18, 2011 Share Posted February 18, 2011 I personally feel light does have mass but its to small for us to really care most of the time. For sure the best one can really do is put experimental limits on the mass of a free photon. Directly from the validity of Coulombs law the photon mass must be less than [math]10^{-14}[/math]eV. Theoretically the photon mass is exactly zero as required by the gauge symmetries of Maxwell's equations. Link to comment Share on other sites More sharing options...
Oldman Posted February 20, 2011 Share Posted February 20, 2011 If a photon has mass, you can slow it down or speed it up. This you can't do. So, the mass =0. However, a photon has momentum: p = E/c. A photon's spin =1. But it has only two orientations: +1, -1. If it were to have s=0, it would not be relativistically invariant. A photon is its own antiphoton. Pretty special boson! Isn't it? I don't vision any other 'particle' like it. Do you? By the way, neutrinos have mass. Link to comment Share on other sites More sharing options...
swansont Posted February 20, 2011 Share Posted February 20, 2011 If a photon has mass, you can slow it down or speed it up. This you can't do. So, the mass =0. However, a photon has momentum: p = E/c. A photon's spin =1. But it has only two orientations: +1, -1. If it were to have s=0, it would not be relativistically invariant. A photon is its own antiphoton. Pretty special boson! Isn't it? I don't vision any other 'particle' like it. Do you? By the way, neutrinos have mass. Spin 1 can have a zero angular momentum projection. You can have an electric dipole transition with [imath]\Delta J = 0 [/imath] as long as you don't have a 0—>0 transition. Link to comment Share on other sites More sharing options...
Oldman Posted February 21, 2011 Share Posted February 21, 2011 Of course, spin 1 could have an s=0 orientation. But I had a photon on my mind. Photon is a very special boson! It does not have s=0. Link to comment Share on other sites More sharing options...
james039 Posted February 26, 2011 Share Posted February 26, 2011 i don't think that it is massless.... Link to comment Share on other sites More sharing options...
swansont Posted February 26, 2011 Share Posted February 26, 2011 i don't think that it is massless.... I refer you to post #8. Link to comment Share on other sites More sharing options...
steevey Posted February 27, 2011 Share Posted February 27, 2011 All physical particles must obey the following: [math]E^{2}-p^{2}c^{2} = m^{2}c^{4}[/math], where p is the 3-momentum and m the mass. So for a massless particle we have [math]E^{2}-p^{2}c^{2} =0[/math]. No violation of relativity here. Note we do not have a photon rest frame so we cannot set p=0. For massive particles we can chose the rest frame and the mass-shell condition becomes [math]E = mc^{2}[/math], in that frame. Can't energy also be a measurement of mass though? Or is it only ever referring to a single part of a particle such as the amount of matter OR the amount of energy and never both when describing mass? Link to comment Share on other sites More sharing options...
swansont Posted February 27, 2011 Share Posted February 27, 2011 (edited) Can't energy also be a measurement of mass though? Or is it only ever referring to a single part of a particle such as the amount of matter OR the amount of energy and never both when describing mass? Only by redefining what you mean by mass. Which renders all of the equations of relativity moot. Edited February 28, 2011 by swansont typo Link to comment Share on other sites More sharing options...
ajb Posted February 28, 2011 Share Posted February 28, 2011 Can't energy also be a measurement of mass though? Or is it only ever referring to a single part of a particle such as the amount of matter OR the amount of energy and never both when describing mass? If the particle is massive, the mass is equal to the energy in the rest frame of the particle. This is [math] E = mc^{2}[/math]. So, in this sense energy is a measure of the mass. However, energy is not a relativistic invariant, it depends on the frame employed. So, you cannot equate mass with energy absolutely, Link to comment Share on other sites More sharing options...
Oldman Posted March 8, 2011 Share Posted March 8, 2011 A photon has no (gravitational) mass. That's why no force can act on it and (de)accelerate it. BUT, ... there is usually a but, ... A photon has inertial mass = (Planck constant * frequency)/(speed **2). See my previous responses. Is that okay? Link to comment Share on other sites More sharing options...
ajb Posted March 9, 2011 Share Posted March 9, 2011 (edited) A photon has no (gravitational) mass. That's why no force can act on it and (de)accelerate it. BUT, ... there is usually a but, ... Gravity accelerates photons. Null geodesics are not straight lines, gravity does influence the motion of light. A photon has inertial mass = (Planck constant * frequency)/(speed **2). This is really just the energy or momentum of the photon. I am not sure that trying to interpret this as a mass is going to be very useful. Unless you have some idea in mind? Is that okay? Maybe you are looking for an interpretation that really is not going to be fruitful. The idea that photons have no rest mass should not unduly worry you. Having the inertial and gravitational mass of a photon being different means that the equivalence principle is violated. Which maybe the truth of nature, or it more likely means you are using an nonstandard definition of mass. Edited March 9, 2011 by ajb Link to comment Share on other sites More sharing options...
swansont Posted March 17, 2011 Share Posted March 17, 2011 ! Moderator Note The "reflections have no mass" tangent has been split off http://www.scienceforums.net/topic/55783-reflections-have-no-mass/ 1 Link to comment Share on other sites More sharing options...
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