Universal Properties of Electromagnetic Pulses
We survey the existing known universal properties of electromagnetic pulses,and discuss their consequences.The established universal properties are(i)the time invariance of the total electromagnetic energy U,momentum Pz and angular momentum Jz of the pulse,and(ii)the inequality cPz < U.(Pulse propagation is along the z-direction.)In both(i)and(ii),the theorems follow directly from Maxwells equations.The conservation of energy,momentum and angular momentum is no surprise,but the inequality cPz < U implies that all localized electromagnetic pulses have a zero-momentum frame(not a rest frame,waves are never at rest).The above is of course in contradistinction to Einsteins light quantum,for which the momentum P is purely in one direction,and cP = U.The implication seems to be that we cannot form a model of the photon by any pulse wave-function satisfying Maxwells equations.If the momentum P and energy U formed a four-vector(cP,U),U2-c2P2 would be a Lorentz invariant.This holds for point particles,but not universally for wavepackets.We show however that u2-c2p2 is a Lorentz invariant,non-negative at all space-time points(u and p are the energy and momentum densities).We also discuss the helicity of electromagnetic pulses,and the counter-intuitive relation between the helicity and angular momentum of certain exactly calculable examples.
John Lekner
MacDiarmid Institute and School of Chemistry and Physics Victoria University,Wellington,New Zealand
国际会议
Progress in Electromagnetics Research Symposium 2013(2013年电磁学研究新进展学术研讨会)
台北
英文
458-463
2013-03-01（万方平台首次上网日期，不代表论文的发表时间）