ecoli Posted June 7, 2005 Share Posted June 7, 2005 I have a question about photons. How can they have momentum but be massless. If they are massless, them they should have no momentum, and vice versa, right? p=mv. Or am I just applying classical mechanics laws to something I shouldn't? Link to comment Share on other sites More sharing options...
5614 Posted June 7, 2005 Share Posted June 7, 2005 Because with photons we are not talking classical momentum, you can work out momentum using p = hf/c and I think there's another one, possible h/f or something, I can't remember off by heart. Link to comment Share on other sites More sharing options...
ecoli Posted June 7, 2005 Author Share Posted June 7, 2005 How does a photon have momentum? just because it's traveling fast? Link to comment Share on other sites More sharing options...
5614 Posted June 7, 2005 Share Posted June 7, 2005 Any moving object has a momentum, that's logic... however you are using a classical equation for a non-classical particle (a photon). We know that (for a photon): e^2 = (pc)^2 + (mc^2)^2 but for a photon we have no mass, this results in: e^2 = (pc)^2 or for easier viewing but losing technicality e=pc which is rearranged as p=e/c but now we must define e, so we use the equation e=hf so we get p=hf/c Link to comment Share on other sites More sharing options...
5614 Posted June 7, 2005 Share Posted June 7, 2005 Oh yeah, I looked it up... about when I mentioned the other formula, it is [math]p = \frac{h}{\lambda}[/math] Link to comment Share on other sites More sharing options...
Tom Mattson Posted June 7, 2005 Share Posted June 7, 2005 p=mv. Or am I just applying classical mechanics laws to something I shouldn't? Your reasoning does indeed go south for this reason. As we learn from quantum mechanics, [math]p= h / \lambda[/math], regardless of whether a particle has mass or not. It's funny you bring this up because just minutes ago I was reading over a beautiful presentation of just this very thing in a free e-book: http://www.geocities.com/zcphysicsms/chap3.htm Link to comment Share on other sites More sharing options...
rajama Posted June 17, 2005 Share Posted June 17, 2005 It's funny you bring this up because just minutes ago I was reading over a beautiful presentation of just this very thing in a free e-book: http://www.geocities.com/zcphysicsms/chap3.htm Grandfather paradox - nice... Link to comment Share on other sites More sharing options...
Hyd Posted June 23, 2005 Share Posted June 23, 2005 If you use higher dimensional theory, is light not consider a vibration in the 4th Spatial Dimension (aka The 5th Dimension)? If you consider higher dimensional theory, look up when the kaluza-klein theory (which had flaws of course, but lead to the string theory). Link to comment Share on other sites More sharing options...
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