The subject of Gravitoelectromagnetism (GEM) deals with the effects that moving masses have on the shape of spacetime. Textbooks often prefer to talk about gravitomagnetism (GM) which is a slightly shorter word, and is less likely to be mistaken for something that involves actual electric fields (which GEM/GM doesn't).
"EM" vs "GEM"
The GEM name is based on an analogy with electromagnetism (EM) – where the subject of EM deals (in part) with the effects due to a moving electrical charge, GEM effects are the analogous behaviours caused by a moving gravitational charge (a moving mass).
The analogy is not exact (electrical charges can be positive or negative, gravitational charges can only be positive), but since both types of field are bound by the same general constraints of classical field theory, and both have a finite propagation speed (defined as "[math]c[/math]" for EM, and generally assumed to nominally be "[math]c[/math]" for GEM), the two classes of problem can't help but share some basic behaviours.
|“||Frame dragging is a direct manifestation of gravitomagnetism. Its consequences have found important astrophysical application in, for example, models of relativistic jets ...||”|
|— National Research Council, Review of Gravity Probe B 1995|
GEM effects cause a deflection of light and a deflection of the paths of objects passing through a region, and can be modelled as spacetime distortion effects, or as the result of nontraditional gravitational fields.
We can define three main classes of GEM effect:
- Rotational GEM – creates a radial attraction at right angles to the rotation axis, and also a rotational dragging around the rotation axis, in the direction of rotation.
These two effects are "Machian", and appear in C20th GR.
- Accelerational GEM – creates a dragging effect around the accelerated body, in the direction of forced acceleration.
This effect is "Machian", and appears in C20th GR.
- Velocity-based GEM – creates a dragging effect around a moving body, in the direction of motion.
This effect is derivable from the rotational effect or from general gravitational arguments, but is at odds with special relativity. Its status under C20th GR is problematic.
Multiple paths to GEM
- GEM effects considered as the result of "smudging"
- If the properties of physics, including the properties of spacetime and the properties of bodies can be described using field theory, then the condition that classical field theory has "no sharp edges" means that our descriptions of how matter interacts with spacetime and with other matter end up with a certain degree of "blurring". If the mass of a moving particle, idealised as a point, is not allowed to be described as a point, but has to be smudged out into the surrounding region, then the mass becomes a field whose strength dies away with distance, and since a field carrying the property of mass is a gravitational field (or an inertial field), smudging or blurring turns a description of particulate matter physics into a description in which each particle has its own gravity-well.
- If we now consider a situation in which particles are moving, the smudging of a particle's rotational momentum, accelerational forces or linear momentum produces a field description in terms of rotational GEM, accelerational GEM, and velocity-based GEM.
- GEM effects considered as the result of statistical mechanics
- If two bodies with relative acceleration, rotation or velocity are placed in a particulate medium, the intermediate particles will acquire the "imprint" of those bodies by collision, and then by colliding with each other, create an interaction between the two bodies at a distance ("indirect collision"). The smoothed and averaged statistical behaviour of these interactions can then be modelled in an abstract way without knowing the positions or velocities of the intermediate particles as a field, which then gives GEM behaviour.
- GEM analogues under aether theory
- While GEM is not derived as an "aether theory" effect, GEM classes typically have easily-visualisable aether-theory counterparts – for instance, the rotational GEM effects are broadly similar to the effects expected from a dragged-aether theory. This is partly because aether models tend to have a "statistical mechanical" component, and partly because their dragging effects are usually expressable in idealised form as fields, making them subject to the same "classical field theory" limitations and restrictions as GEM fields ("smudging").
- GEM as the result of Quantum Mechanics
- The statistical route to GEM seems to mesh well with quantum theory. if we fire photons or other small particles at a moving target particle, the target's position will have a certain degree of uncertainty, meaning that our pattern of potential "hits" will be scattered over the region, and can be idealised as a probability field, with the attributes of mass and momentum of the original particle. This then takes us back to the earlier description of the moving particle having effective static and GEM gravitational field components, which describe its rest properties and state of motion. We can also statistically model the interaction of bodies via virtual particles and arrive at the same basic patterns of behaviour as before.
Status of GEM effects
The status of GEM effects under C20th General relativity is somewhat elusive – the general principle of relativity requires that the rotational and accelerational effects must be real, and general gravitational arguments and extrapolations from GEM-r then seem to say that the velocity-dependent effects also have to exist, too. A full logically-consistent description of GEM seems to require all three classes of effect.
However, special relativity is derived from the assumption that there are no distortion effects associated with relative motion, so the "SR" side of GR1960 requires GEM-v not to exist. The other two effects were described by Einstein as appearing in general relativity, but were found in 1960 to also conflict with SR. The apparent absence of a full peer-reviewed study of GEM/GM effects seems to be down to implicit and explicit conflicts with special relativity.
Not to be confused with:
- Electrogravity ... GEM/GM describes gravitational field-effects that mimic some EM field behaviours, "electrogravity" is a concept that deals with possible interactions between the two types of field.
- Albert Einstein, Is There a Gravitational Effect Which Is Analogous to Electrodynamic Induction? Vierteljahrsschrift für gerichtliche Medizin und öffentliches Sanitätswesen 44 (1912): 37-40 – pdf
- Herbert Pfister and Markus King, Inertia and Gravitation: The Fundamental Nature and Structure of Space-Time 4: Mach's Principle, Dragging phenomena, and Gravitomagnetism
- It has been accepted since around 1960 that the GPoR and special relativity are mutually incompatible (Schild) – the logical inconsistency of C20th GR, even after 1960, can be expressed in terms of the 1960 theory's inability to properly process questions related to GEM.
- C20th textbook theory appears to deal with the subject of GEM and its associated contradictions ("Gravity and the GPoR requires GEM to exist" / "SR requires GEM not to exist") in the manner of a disassociative identity disorder ("DID") – by compartmentalising information and descriptions that would otherwise conflict, and "blanking" subjects that would reveal logical inconsistencies.
- If we embrace full-range GEM (which seems necessary for compatibility with QM and with a range of other principles), the result seems to be a Cliffordian universe, a relativistic acoustic metric, and a fully-general general theory of relativity.
This category has only the following subcategory.
Pages in category "Gravitoelectromagnetism (GEM)"
The following 6 pages are in this category, out of 6 total.