DETECTING THE `MAGNETIC’ COMPONENT OF THE GRAVITATIONAL-WAVE FORCE
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DETECTING THE `MAGNETIC’ COMPONENT OF THE GRAVITATIONAL-WAVE FORCE
L P Grishchuk (Cardiff U and Moscow U)
[Based on paper with D. Baskaran, Class. Quant. Grav. 21, 4041 (2004)]
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Motion of a charge in the field of an electromagnetic wave
Electromagnetic (Lorentz) force:
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A weak gravitational wave:
In the frame based on principal axes:
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Coordinate transformation to a local inertial coordinate system (the closest thing to a global Lorentzian coordinate system):
Trajectories of the nearby free particles, including `magnetic’ oscillations back and forth in the direction of the wave propagation, the z-direction.
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Familiar picture of deformations of a disk consisting of free particles. Zero-order approximation in the wavenumber k; `magnetic’ contribution ignored:
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Deformations of the disk with the `magnetic’ contribution included:
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Gravitational-wave force from the geodesic deviation equation:
The second term is the `magnetic’ force, proportional to the particle’s velocity:
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Variation of the distance between the central particle (corner mirror of interferometer) and a nearby particle (end mirror of interferometer).`Magnetic’ contribution (terms proportional to the wavenumber k) is included:
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Astrophysical example: a pair of stars on a circular orbit in a plane orthogonal to the line of sight.
Correct response of the interferometer, including its `magnetic’ part:
Response based on the `electric’ contribution only (incorrect):
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Conclusions
In the LIGO interferometers, the `magnetic’ component of the g.w. force, proportional to (kl), provides a correction to the interferometer’s response at the level of 5 percent in the frequency band of 600 Hz, and up to 10 percent in the frequency band of 1200 Hz.
Data analysis based on the `electric’ contribution only can significantly affect the determination of the parameters of the g.w. source.
`Magnetic’ contribution is measurable and must be taken into account in the accurate observations of periodic and quasi-periodic astrophysical sources by advanced interferometers.