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</o:shapelayout></xml><![endif]--></head><body bgcolor=white lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'>Grahame:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'>I think of photon momentum as something like the momentum of an ocean wave. See <a href="https://en.wikipedia.org/wiki/Wind_wave#Wave_formation"><span style='color:#002060'>wind waves</span></a> on Wikipedia and look at the gif:<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Calibri",sans-serif;color:#1F497D'><img border=0 width=384 height=209 id="Picture_x0020_1" src="cid:image001.png@01D27A19.38D4FFE0"></span><span style='font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US'><o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'>The wave <i>is</i> motion. Take away the motion and you take away the wave. Note that energy and momentum aren’t two totally different things. One is a distance measure of energy-momentum, the other is a time measure. You can’t take the momentum away from a cannonball without taking away the kinetic energy too. The same applies to a wave. Also note that when an ocean wave moves through the sea, the sea waves. And that when a seismic wave runs through the ground, the ground waves. So when an electromagnetic wave moves through space… space waves. That’s what Maxwell said. See Einstein’s <a href="http://www-history.mcs.st-andrews.ac.uk/Extras/Einstein_ether.html">Leyden Address</a>. He didn’t think of space as nothing. Also see the <a href="https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor">stress-energy-momentum tensor</a> which “describes the <a href="https://en.wikipedia.org/wiki/Density" title=Density><span style='color:#002060;text-decoration:none'>density</span></a> and <a href="https://en.wikipedia.org/wiki/Flux" title=Flux><span style='color:#002060;text-decoration:none'>flux</span></a> of <a href="https://en.wikipedia.org/wiki/Energy" title=Energy><span style='color:#002060;text-decoration:none'>energy</span></a> and <a href="https://en.wikipedia.org/wiki/Momentum" title=Momentum><span style='color:#002060;text-decoration:none'>momentum</span></a> in <a href="https://en.wikipedia.org/wiki/Spacetime" title=Spacetime><span style='color:#002060;text-decoration:none'>spacetime</span></a>”. It’s got a shear stress term:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060'><img border=0 width=236 height=143 id="Picture_x0020_2" src="cid:image002.png@01D27A1A.55D22720"></span><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'> <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'>I think it’s because space is like a ghostly gin-clear elastic. Waves run through it, and matter is made of them. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'>Regards<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'>JohnD<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:14.0pt;font-family:"Calibri",sans-serif;color:#002060;mso-fareast-language:EN-US'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'> General [mailto:general-bounces+johnduffield=btconnect.com@lists.natureoflightandparticles.org] <b>On Behalf Of </b>Dr Grahame Blackwell<br><b>Sent:</b> 29 January 2017 00:12<br><b>To:</b> Nature of Light and Particles - General Discussion <general@lists.natureoflightandparticles.org><br><b>Subject:</b> [General] On photon momentum<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>Dear All,</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>[Notably Chandra & Chip],</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US> <o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>I'm having a bit of a problem over this question of: 'How does a photon carry momentum'? (or similar words.)</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>It seems to me that in order to even beginning to address this question, one needs a clear definition of 'momentum' that's applicable to the momentum carried by a photon.</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>I may be looking in the wrong places (if so please advise), but the only definitions of momentum that I can find either refer to 'mass' or refer to some other phenomenon which in turn refers to momentum - i.e. circular references.</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>If I'm going to figure, or be persuaded, how a photon carries momentum I first need to know what momentum IS in respect of a photon (yes, I know it's E/c, that's a measure it's not a definition).</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>Of course I'm aware of the paper "Light is heavy", but I don't feel it's appropriate just to extract from that some sort of mass-equivalence of a photon. If we do, we get the result that 'm'=E/c^2, so 'm'c = E/c - gives the right result, but appears to be some sort of convoluted self-confirmation (i.e. a circular argument dressed up in fancy clothes). It certainly doesn't DEFINE a photon's momentum, just evaluates it.</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US> <o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>Does anyone have a convincing definition of momentum that's applicable to a photon? One that can be used as a firm basis for theorising?</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>(I'd be glad if colleagues didn't use this as an excuse to yet again present their own personal theory/model - I'm looking for a definition that would be agreed by all, or at least most, physicists.)</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US> <o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>Thanks in anticipation,</span><span lang=EN-US><o:p></o:p></span></p></div><div><p class=MsoNormal><span lang=EN-US style='font-size:10.0pt;font-family:"Arial",sans-serif;color:navy'>Grahame</span><span lang=EN-US><o:p></o:p></span></p></div></div></body></html>