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| The '''[[Max Abraham|Abraham]]–[[Hermann Minkowski|Minkowski]] controversy''' is a [[physics]] debate concerning [[Electromagnetism|electromagnetic]] [[momentum]] within [[dielectric]] media. Theories have been put forward that, if the principles are proven, may allow the design of a [[reactionless drive]] which would represent a breakthrough in [[spacecraft propulsion]].
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| ==Theoretical basis==
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| Two equations exist describing momentum transfer between [[matter]] and [[electromagnetic field]]s.<ref name=Pfeifer1>{{cite journal |author=Robert N. C. Pfeifer, Timo A. Nieminen, Norman R. Heckenberg, and Halina Rubinsztein-Dunlop |url=http://www.hep.princeton.edu/~mcdonald/examples/EM/pfiefer_rmp_79_1197_07.pdf |title=Colloquium: Momentum of an electromagnetic wave in dielectric media |journal=Review of Modern Physics |volume=79 |pages=1197 |date=October–December 2007 |doi=10.1103/RevModPhys.79.1197 |issue=4 |bibcode=2007RvMP...79.1197P|arxiv = 0710.0461 }}</ref> Both seem to be supported by contradicting experimental data. The two existing equations were first suggested by [[Hermann Minkowski]] (1908)<ref>{{Citation|author=Minkowski, Hermann|year=1908|title=[[s:de:Die Grundgleichungen für die elektromagnetischen Vorgänge in bewegten Körpern|Die Grundgleichungen für die elektromagnetischen Vorgänge in bewegten Körpern]]|journal=Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse
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| |pages=53–111}}
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| :*Wikisource translation: [[s:The Fundamental Equations for Electromagnetic Processes in Moving Bodies|The Fundamental Equations for Electromagnetic Processes in Moving Bodies]]</ref>
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| and [[Max Abraham]] (1909), | |
| <ref>{{Citation|author=Abraham, Max|year=1909|title=[[s:de:Zur Elektrodynamik bewegter Körper (Abraham)|Zur Elektrodynamik bewegter Körper]]|journal=[[Rendiconti del Circolo Matematico di Palermo]]|volume=28|pages=1–28}}
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| :*Wikisource translation: [[s:On the Electrodynamics of Moving Bodies (Abraham)|On the Electrodynamics of Moving Bodies]]</ref>
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| <ref>{{Citation|author=Abraham, Max|year=1910|title=Sull'Elletrodinamica di Minkowski|journal=Rendiconti del Circolo Matematico di Palermo|volume=30|pages=33–46}}
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| :*Wikisource translation: [[s:On the Electrodynamics of Minkowski|On the Electrodynamics of Minkowski]]</ref>
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| from which the controversy name derives.
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| Both define the momentum of an electromagnetic field permeating matter. Abraham's equation suggests that in materials through which light travels more slowly, electromagnetic fields should have lower momentum, while Minkowski suggests it should have a greater momentum. “Using relativity, Feigel found that the Abraham definition accounts for the momentum of the electric and magnetic fields alone, while the Minkowski definition also takes into account the momentum of the material”.<ref>{{cite journal|url=http://physics.aps.org/story/v13/st3|title= Focus: Momentum From Nothing, Phys. Rev. Focus 13, 3 (2004).|first= Adrian |last= Cho |date=2004-01-23}}</ref> More recent work suggests that this characterization is incorrect.<ref name="Experiment">{{cite web |author=James Dacey |title=Experiment resolves century-old optics mystery |url=http://physicsworld.com/cws/article/news/37266 |publisher=physicsworld.com |date=9 Jan 2009 |accessdate=4 Mar 2010}}</ref>
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| At least one report has suggested Minkowski's formulation, if correct, would provide the physical base for a [[reactionless drive]].<ref>Hector Hugo Brito (1999). "[http://www.intalek.com/Index/Projects/Research/0994.pdf Propellantless Propulsion by Electromagnetic Inertia Manipulation: Theory and Experiment]". In ''Space Technology and Applications International Forum – 1999'', Mohamed S. El-Genk (editor). [[American Institute of Physics]]. ISBN 978-1-56396-846-4.</ref> However, an independent review from the [[United States Air Force Academy]] concluded that there would be no expected net propulsive forces, and a [[NASA]] report determined that "The signal levels are not sufficiently above the noise as to be conclusive proof of a propulsive
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| effect."<ref>Marc G. Millis (2004). "[http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2004/TM-2004-213082.html Report on Prospects for Breakthrough Propulsion From Physics]". In ''Proceedings 2004 NASA/DoD Conference on Evolvable Hardware''. [[IEEE Computer Society]]. ISBN 0-7695-2145-2.</ref>
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| The two equations for the photon momentum in a dielectric with [[refractive index]] <math>n</math> are:
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| *The Minkowski version:
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| :: <math>p_M=\frac {n h \nu}{c}</math>
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| *The Abraham version:
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| :: <math>p_A=\frac {h \nu}{n c}</math>
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| where <math>h</math> is the [[Planck constant]], <math>\nu</math> is the frequency of the light and <math>c</math> is the [[speed of light]] in vacuum.
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| A 2010 study suggested that ''both'' equations are correct, with the Abraham version being the [[kinetic momentum]] and the Minkowski version being the [[canonical momentum]], and claims to explain the contradicting experimental results using this interpretation.<ref>{{cite journal|doi=10.1103/PhysRevLett.104.070401|title=Resolution of the Abraham-Minkowski Dilemma|first=Stephen|last=Barnett|journal=Phys. Rev. Lett.|date=2010-02-07|volume=104|issue=7|page=070401 |pmid=20366861|bibcode=2010PhRvL.104g0401B}}</ref> However, a recent study shows that in the principle-of-relativity frame the Abraham momentum would break global momentum-energy conservation law in medium Einstein-box thought experiment (also known as “Balazs thought experiment”),<ref name=wang1>{{cite journal|doi=10.4236/jmp.2013.48151|title=Can the Abraham light momentum and energy in a medium constitute a Lorentz four-vector?|first=Changbiao|last=Wang|journal=Journal of Modern Physics|date=2013-08-14|volume=4|issue=8|page=1123}}</ref><ref name=wang2>{{cite journal|url=http://arxiv.org/ftp/arxiv/papers/1202/1202.2575.pdf|title=Comment on 'Resolution of the Abraham-Minkowski Dilemma' |first=Changbiao|last=Wang|date=2013-10-13|journal=http://arxiv.org/abs/1202.2575v4}}</ref> and it claims that, the justification of Minkowski momentum as the correct light momentum is completely required by the principle of relativity and the momentum-energy conservation law, which are all the fundamental postulates of physics.<ref name=wang3>{{cite journal|url=http://arxiv.org/ftp/arxiv/papers/1106/1106.1163.pdf |title= Plane wave in a moving medium and resolution of the Abraham-Minkowski debate by the special principle of relativity |first=Changbiao|last=Wang|date=2013-10-23|journal=http://arxiv.org/abs/1106.1163v77}}</ref>
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| The two equations for the electromagnetic momentum in a dielectric are:
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| *The Minkowski version:
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| ::<math>\mathbf{g}_{M} = \mathbf{D}\times\mathbf{B}</math>
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| *The Abraham version:
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| ::<math>\mathbf{g}_{A} = \frac{1}{c^2}\mathbf{E}\times\mathbf{H}</math>
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| where '''D''' is the [[electric displacement field]], '''B''' is the [[magnetic flux density]], '''E''' is the [[electric field]], and '''H''' is the [[magnetic field]]. The photon momentum is thought to be the direct result of Einstein light-quantized electromagnetic momentum.<ref name=wang1 /><ref name=wang3 />
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| Some scientists claim that the “division of the total energy-momentum tensor into electromagnetic (EM) and material components is arbitrary”.<ref name=Pfeifer1 /> In other words, the EM part and the material part in the total momentum can be arbitrarily distributed as long as the total momentum is kept the same. But some others don’t agree, and they suggested a Poynting-vector criterion. They say for EM radiation waves the [[Poynting vector]] '''E'''×'''H''' denotes EM power flow in any system of materials, and they claim that the Abraham momentum {{nowrap|1='''E'''×'''H'''/''c''<sup>2</sup>}} is “the sole electromagnetic momentum in any system of materials distributed throughout the free space”.<ref name=Mansuripur1>{{cite journal|doi=10.1103/PhysRevE.79.026608|title=Maxwell’s macroscopic equations, the energy-momentum postulates, and the Lorentz law of force |author=Masud Mansuripur and Armis Zakharian |journal=Physical Review E|date=2009-02-20|volume=79|issue=2|page=026608}}</ref>
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| Conventionally, [[Poynting vector]] '''E'''×'''H''' as EM power flow has been thought to be a well-established basic concept in textbooks.<ref>M. Born and E. Wolf, Principles of Optics (Pergamon Press, Oxford, 1986), 6th ed, Chapter XIV, p. 669.</ref><ref>R. P. Feynman, R. B. Leighton, and M. Sands, Feynman Lectures on Physics (Addison-Wesley, New York, 1964), Vol. II, Chapter 27.</ref><ref>L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media (Butterworth-Heinemann, Oxford, 1984), 2nd ed, §97.</ref><ref name=Moller1>C. Møller, The Theory of Relativity (Oxford University Press, London, 1955), Corrected sheets of the first edition 1952, VII, §76.</ref><ref>W. K. H. Panofsky and M. Phillips, Classical electricity and magnetism (Addison-Wesley, MA, 1962), 2nd ed, Chapter 10, p. 180.</ref><ref name=Stratton1>J. A. Stratton, Electromagnetic theory, (McGraw-Hill, NY, 1941), Sec. 2.19, p. 135.</ref> In view of the existence of a certain mathematical ambiguity for this conventional basic concept, some scientists suggested it to be a “postulate”,<ref name=Mansuripur1 /> while some others suggested it to be a “hypothesis”, “until a clash with new experimental evidence shall call for its revision”.<ref name=Stratton1 /> However, this basic concept is challenged in a recent study, which claims “Poynting vector may not denote the real EM power flow in an anisotropic medium”.<ref name=wang3 />
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| In addition to the Poynting-vector criterion,<ref name=Mansuripur1 /> Laue and Møller suggested an criterion of four-vector covariance imposed on the propagation velocity of EM energy in a moving medium, just like the velocity of a massive particle.<ref name=Brevik1>{{cite journal|doi=10.1016/0370-1573(79)90074-7|title=Experiments in phenomenological electrodynamics and the electromagnetic energy-momentum tensor |first=Iver|last=Brevik|journal=Physics Reports|date=May 1979|volume=52|issue=3|pages=133–201}}</ref> The Laue-Møller criterion supports Minkowski EM tensor, because the Minkowski tensor is a real four-tensor while Abraham’s is not, as indicated by Veselago and Shchavlev recently.<ref name=Veselago1>{{cite journal|doi=10.3367/UFNe.0180.201003k.0331|title= On the relativistic invariance of the Minkowski and Abraham energy-momentum tensors|author=V. G. Veselago and V. V. Shchavlev|journal= Phys. Usp.|year= 2010|volume=53|issue=3|page=317}}</ref> But some scientists disagree, criticizing that “it is widely recognized now that Abraham’s tensor is also capable of describing optical experiments,” and such a criterion of this type is only “a test of a tensor’s ''convenience'' rather than its ''correctness ''”.<ref name=Brevik1 /> Some scientists also criticized the justifications of the energy-velocity definition and the imposed four-vector covariance in Laue-Møller criterion.<ref name=Moller1 /> Regarding the energy-velocity definition which is given by [[Poynting vector]] divided by EM energy density in Laue-Møller criterion, they say “the [[Poynting vector]] does not necessarily denote the direction of real power flowing” in a moving medium.<ref name=wang3 /> Regarding the imposed four-velocity covariance, which was probably prompted by the relativistic velocity addition rule applied to illustrating Fizeau running-water experiment,<ref>W. Pauli, Theory of relativity, (Pergamon Press, London, 1958), Eq. (14), p. 18, Sec. 6.</ref> they say “one essential difference between massive particles and photons is that any massive particle has its four-velocity, while the photon (the carrier of EM energy) does not”.<ref name=wang1 />
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| Theoretically speaking, the Abraham–Minkowski controversy is focused on the issues of how to understand some basic principles and concepts in special theory of relativity and classical electrodynamics.<ref name=wang3 /> For example, when there exist dielectric materials in space,
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| *Is the principle of relativity still valid?
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| *Are the Maxwell equations, momentum-energy conservation law, and Fermat’s principle still valid in all inertial frames of reference?
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| *Does the Poynting vector always represent EM power flow in any system of materials?
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| *Does the photon have a Lorentz four-velocity like a massive particle?
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| ==Experiments==
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| The results through the years have been mixed, at best.<ref name="Experiment" /> However, a report on a 2012 experiment claims that unidirectional thrust is produced by electromagnetic fields in dielectric materials.<ref name="Applied Physics Letters">{{cite journal|title=Micronewton electromagnetic thruster |date=July 18, 2012|first=Dimitri S.H.|last=Charrier|volume=101|work=[[Applied Physics Letters]]|publisher=[[American Institute of Physics]]|url=http://apl.aip.org/resource/1/applab/v101/i3/p034104_s1?isAuthorized=no|pages=034104|doi=10.1063/1.4737940|accessdate=January 4, 2014}}</ref>
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| ==See also==
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| *[[Reactionless drive]]
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| *[[Spacecraft propulsion]]
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| *[[Stochastic electrodynamics]]
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| ==References==
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| {{reflist|2}}
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| ==External links==
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| * [http://focus.aps.org/story/v13/st3 Physical Review Focus: Momentum From Nothing]
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| * [http://focus.aps.org/story/v22/st20 Physical Review Focus: Light Bends Glass]
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| {{DEFAULTSORT:Abraham-Minkowski controversy}}
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| [[Category:Electric and magnetic fields in matter]]
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| [[Category:Scientific controversies]]
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