Study resolves century-long debate over how to describe electromagnetic momentum density in matter
(PhysOrg.com) -- Researchers from the NIST Center for Nanoscale Science and Technology and the University of British Columbia have shown that the interaction between a light pulse and a light-absorbing object, including the momentum transfer and resulting movement of the object, can be calculated for any positive index of refraction using a few, well-established physical principles combined with a new model for mass transfer from light to matter.
This work creates a foundation for understanding light absorption in metamaterials, artificially tailored materials of intense interest in nanophotonics and microwave engineering that can have negative indices of refraction, and have potential applications in high resolution imaging, lithography, optical sensing, high gain antennas, and stealth radar coatings.
Light carries momentum and can transfer momentum to matter via radiation pressure. However, for the past century, there has been an ongoing debate over the correct form of the electromagnetic momentum density in matter. In the “Minkowski formulation,” the momentum density is proportional to the index of refraction; in direct contrast, the “Abraham formulation” finds it to be inversely proportional. While light is known to carry mass, a detailed model for mass transfer from light to a medium that absorbs light had not been formulated to date.
The researchers propose a set of postulates for light-matter interaction that encompass: a) the Maxwell equations, which govern classical electromagnetic behavior; b) a generalized Lorentz force law, which describes the force felt by matter in the presence of an electromagnetic field; c) a model for electromagnetic mass density transfer to an absorbing medium; and d) the Abraham formulation of momentum density. Using both closed-form calculations and numerical simulations of the interaction between an electromagnetic pulse and a test slab, the researchers demonstrated that their postulates yield results that are consistent with conservation of energy, mass, momentum, and center-of-mass velocity at all times.
They further showed that satisfaction of the last two conservation laws unambiguously identifies the Abraham form as the true form of momentum density in a positive-index medium. In addition to the theoretical significance of these results and the implications for metamaterials, the results will enable more accurate modeling of light-matter interaction at the nanoscale and open new routes to optical control of nano-mechanical systems incorporating light absorbing materials.
More information: Revisiting the Balazs thought experiment in the presence of loss: electromagnetic-pulse-induced displacement of a positive-index slab having arbitrary complex permittivity and permeability, K. J. Chau and H. J. Lezec, Applied Physics A 105, 267-281 (2011).
Provided by
National Institute of Standards and Technology
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Dec 29, 2011
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Dec 29, 2011
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How the photons can be massless after then?
Dec 29, 2011
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Dec 29, 2011
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" How the photons can be massless after then ? "
I thought they were considered mass-less until they interact with something by exerting radiation pressure....*scratches head*...
Dec 29, 2011
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I don't think photons have ever been assumed to be massless, they simply have no "rest mass" because photons can only exist at light speed. The "rest mass" of a phton is zero, but it's mass at light speed is not zero. Gravitational lensing proves this when we observe light bends in the presence of a strong gravitational field.
Additionally, if photons were massless then black holes could not form because there is no mass for gravity to act upon to prevent the escape of photons from the highest point of gravitational attraction at the surface of the BH.
Learned this in my second semester college physics course.
Dec 29, 2011
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Dec 29, 2011
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Dec 29, 2011
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You mean no rest mass (?). It transfers energy, p=hv/c, so it must transfer an energy mass-equivalency.
Dec 29, 2011
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Only at "rest" because photons cannot exist at "rest", therefore "rest mass" is zero, however at light speed photons have mass, black holes prove this. Black holes prove this because light (or any other frequency of electro-magnetism) cannot generate enough escape velocity to free itself from the surface gravity of that body.
This also proves gravity is not a component of electro-magnetism, or the gravity of a body that prevents electro-magnetism from reaching "escape velocity" would prevent gravity from emerging from the surface of that body to create an accretion disc from which mass falls onto the surface of the BH.
Dec 29, 2011
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It's not necessary to say that 'photons have mass', in general relativity, because any form of energy in this theory is effected by and causes gravitiation,.. even pressure (which ultimately causes BH to form).
Photons have energy that is proportional to their frequency,.. they have a 'mass-equivelancy' via e=mc^2 if you like, but are massless themselves.
Dec 29, 2011
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I don't know of anyone saying that gravity is a component of electromagnetism. The error you are making I think, is that you assume that gravitation requires a constantly propagating force. The gravitation field is static,.. unless something causes it to propagate, like a orbiting neutron star binary, or some galactic collision, in which case it propagates at c. In general relativity, gravitation it is a static curvature of space-time.
Dec 29, 2011
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I think you meant "non-retardation image" above,. i.e. it follows as if gravitation is instantaneous. I have responded to this below.
http://www.physor...mic.html
Dec 29, 2011
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Dec 29, 2011
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You said the same thing I said, you simply shortened my statement, there's nothing contradictory between your synopsis of my statement & the way you correctly understood what I intended. And again, satellite tracking data proves Earth's orbit tracks the actual location of the Sun, not the 8.3 light minute retarded visible image. I make no attempt to explain what on the face appears to be superluminol phenomena as we know "information" (propagation?) cannot exceed light speed. I'll look at your link later when I have more time....
Dec 29, 2011
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Is it OK for everybody?
Dec 29, 2011
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Visualize this: If in the course of swinging the string attached ball around my head & something hits that string, the string will vibrate sending pulses along that string to the two bodies, these pulses are "information" which do not interfere with the connection previously established between the two bodies. An asteroid could be visualized as something hitting the gravitational string between Earth & Sun, it would add a brief "gravity pulse" between the two bodies then subside, generating that information at light speed, or maybe even less.
Dec 29, 2011
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Can you elucidate the difference between AWT and Wilhelm Reich's interpretation of the aether?
Dec 29, 2011
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My aether is substantially stronger model.
http://www.aether...ther.jpg
Dec 29, 2011
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This recent (March 2011) paper says it don't matter because they are "different separations of the same momentum"
http://arxiv.org/...54v1.pdf
This dissertation thinks that neither is completely correct
"While we have presented arguments in
favor of the interpretation of the Abraham momentum as the momentum of the eld,
our simplied analyses lead us to the conclusion that neither the Abraham nor the
Minkowski expressions for momentum give the recoil momentum of a particle in a
dispersive dielectric medium."
http://repository...tion.pdf
I guess it will stay unresolved for another 100 years.
Dec 29, 2011
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http://physics.ap...v22/st20
Dec 30, 2011
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HANG YOUR HEAD IN SHAME.
Jan 02, 2012
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Jan 05, 2012
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