Gravitomagnetism

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This article is about the gravitational analog of electromagnetism as a whole. For specifically the gravitational analog of magnetism, see frame-dragging.

Gravitomagnetism (sometimes Gravitoelectromagnetism, abbreviated GEM), refers to a set of formal analogies between Maxwell's field equations and an approximation to the Einstein field equations for general relativity, valid under certain conditions. For instance, the most common version of GEM is valid only far from isolated sources, and for slowly moving test particles.

Contents

[edit] Background

This approximate reformulation of gravitation as described by general relativity makes a "fictitious force" appear in a frame of reference different from a moving, gravitating body. By analogy with electromagnetism, this fictitious force is called the gravitomagnetic force, since it arises in the same way that a moving electric charge creates a magnetic field, the analogous "fictitious force" in special relativity. The main consequence of the gravetomagnetic force, or acceleration, is that a free-falling object near a massive rotating object will itself rotate. This prediction, often loosely referred to as a gravitomagnetic effect, is among the last basic predictions of general relativity yet to be directly tested. A group at Stanford University is currently analyzing data from the first direct test of GEM, the Gravity B satellite experiment. Frame-dragging is often mentioned as a gravitomagnetic effect, but the Lense-Thirring effect (precession) may be a more appropriate example.

[edit] Equations

According to general relativity, the gravitational field produced by a rotating object (or any rotating mass-energy) is formally analogous to the magnetic field in classical electromagnetism. Starting from the basic equation of general relativity, the Einstein field equation, and assuming a weak gravitational field or reasonably flat spacetime, Mashhoon, Gronwald, and Lichtenegger,[1] and Clark and Tucker[2] have derived the following gravitational analogs to Maxwell's equations for electromagnetism, called the "GEM equations":


\nabla \cdot \mathbf{E} = -4 \pi G \rho \
\nabla \cdot \mathbf{B} = 0 \
\nabla \times \mathbf{E} = -\frac{1}{c} \frac{\partial \mathbf{B} } {\partial t} \
\nabla \times \mathbf{B} = \frac{1}{c} \left( -4 \pi G \mathbf{J} + \frac{\partial \mathbf{E}} {\partial t} \right) = \frac{1}{c} \left( -4 \pi G \rho \mathbf{v}_{\rho} + \frac{\partial \mathbf{E}} {\partial t} \right) \

where:

For a test particle whose mass m is "small," the net (Lorentz) force acting on it due to a GEM field is described by the following GEM analog to the Lorentz force equation:

\mathbf{F}_{m} = m \left( \mathbf{E} + \frac{\mathbf{v}_{m}} {c} \times 2 \mathbf{B} \right).

where:

In the literature, all instances of B in the GEM equations are multiplied by 1/2, a factor absent from Maxwell's equations. This factor vanishes if B in the GEM version of the Lorentz force equation is multiplied by 2, as shown above. The factors 2 and 1/2 arise because the effective gravitomagnetic charge is twice the static gravitational (gravitoelectric) charge, a remnant of the spin-2 character of the gravitational field. For a pure spin-1 field such as the genuine electromagnetic field, the magnetic charge equals the electric charge.

[edit] Comparison with electromagnetism

The above GEM equations are very similar to Maxwell's equations in free space. Assuming cgs units, these equations are:

\nabla \cdot \mathbf{E} = 4\pi\rho
\nabla \cdot \mathbf{B} = 0
\nabla \times \mathbf{E} = -\frac{1}{c} \frac{\partial \mathbf{B}} {\partial t}
\nabla \times \mathbf{B} = \frac{1}{c} \left( \frac{\partial \mathbf{E}} {\partial t} + 4\pi \mathbf{J} \right)

Adopting Planck units eliminates G and c from both sets of equations by normalizing these constants to 1. The two sets of equations are now identical but for the minus sign preceding 4π in the GEM equations. These two minus signs stem from an essential difference between gravity and electromagnetism: electrostatic charges of identical sign repel each other, while two like signed (positive) masses attract each other. Hence the GEM equations are simply Maxwell's equations with mass (or mass density) substituting for charge (or charge density), and -G replacing the Coulomb force constant 1/(4πε0). The following Table summarizes the results thus far:

Common Structure of the Maxwell and

GEM Equations Given Planck units.
\nabla \cdot \mathbf{E} = \iota 4\pi\rho

\nabla \cdot \mathbf{B} = 0

\nabla \times \mathbf{E} = -\partial \mathbf{B}/ \partial t

\nabla \times \mathbf{B} = \iota 4\pi\mathbf{J} + \partial \mathbf{E}/ \partial t

ι = 1 (Maxwell) or -1 (GEM).

The factor of 4π remains in both the GEM and Maxwell's equations because G and 1/(4πε0) are normalized to 1, and not 4πG and ε0.

[edit] Higher-order effects

Some higher-order gravitomagnetic effects can reproduce effects reminiscent of the interactions of more conventional polarized charges. For instance, if two wheels are spun on a common axis, the mutual gravitational attraction between the two wheels arguably ought to be greater if they spin in opposite directions than in the same direction. This can be expressed as an attractive or repulsive gravitomagnetic component.

Gravitomagnetic arguments also predict that a flexible or fluid toroidal mass undergoing minor axis rotation ("smoke ring" rotation) will tend to pull matter preferentially in through one throat and expel it from the other (a case of rotational frame dragging, acting through the throat). In theory, this configuration might be used for accelerating objects (through the throat) without such objects experiencing conventional g forces. It is not yet clear whether this suggestion is of any practical value.

Consider a toroidal mass with two degrees of rotation (both major axis and minor-axis spin, both turning inside out and revolving). This represents a "special case" in which gravitomagnetic effects generate a chiral corkscrew-like gravitational field around the object. The reaction forces to dragging at the inner and outer equators would normally be expected to be equal and opposite in magnitude and direction respectively in the simpler case involving only minor-axis spin. When both rotations are applied simultaneously, these two sets of reaction forces can be said to occur at different depths in a radial Coriolis field that extends across the rotating torus, making it more difficult to establish that cancellation is complete.

Modelling this complex behaviour as a curved spacetime problem has yet to be done, and is believed very difficult.

[edit] Gravitomagnetic field of Earth

Bg,Earth = 10-14 rad.s-1[3]

See the Gravity Probe B experiment.


[edit] Fringe physics

Incomplete understanding of the meaning of the similarity of the gravitomagnetic formulas, above, and Maxwell's equations for (real) electricity and magnetism have given rise to fringe physics which mainstream physicists find both tedious and alarming. [citation needed]

Use of the gravitomagnetic analogy for a simplified form of the Einstein field equations, on the other hand, is firmly part of General Relativity. It is an approximation to the current standard theory of gravitation, and has testable predictions, which are in the final stages of being directly tested by the Gravity Probe B experiment.

Despite the use of the word magnetism in gravitomagnetism, and despite the similarity of the GEM force laws to the (real) electromagnetic force law, gravitomagnetism should not be confused with any of the following, which are rejected by mainstream scientists:

[edit] See also

[edit] External links

[edit] References

  1. ^ http://arxiv.org/PS_cache/gr-qc/pdf/9912/9912027.pdf
  2. ^ http://www.iop.org/EJ/article/0264-9381/17/19/311/q01911.pdf
  3. ^ "Experimental Detection of the Gravitomagnetic London Moment" by Martin Tajmar, Florin Plesescu, Klaus Marhold & Clovis J. de Matos
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