Robert100 said:
I have a question about the wording of the Wikipedia article on gravitoelectromagnetism (GEM). Their article states:
Despite the electromagnetism in gravitoelectromagnetism, and despite the similarity of the GEM force law to the Lorentz force law, gravitomagnetism should not be confused with any of the following:
* claims to have constructed anti-gravity devices,
* Eugene Podkletnov's claims to have constructed gravity-shielding devices and gravitational reflection beams,
* the so-called Electric Universe "theory", which claims to identify gravity as a form of electromagnetism.
These and other claims are considered pseudophysics by mainstream science. Gravitoelectromagnetism, on the other hand, is firmly part of our gold standard theory of gravitation, general relativity, has testable predictions, and is in the final stages of being directly tested.
I understand fully that the article is trying to explain the distinction between real science and pseudoscience, but I am wondering if the text is worded too strongly. If GEM is real, then it truly is a gravitational force, and if it can act (in some cases) as gravitational repulsion, then would it not count as "anti-gravity"?
However, I also want to make clear that the wording of a phrase doesn't always capture the intent of what the phrase really means. Just because we could define "anti-gravity" in this way doesn't mean that physicists actually do so. I'm under the impression that physicists usually avoid the term "anti-gravity"; they'd only use that term if a new force was discovered that actually was distinct from gravity and counter to it. (As such, that would make the phrasing of the Wikipedia article correct and useful.)
there is, BTW, a little dispute at Wikipedia about the Gravitomagnetism article. it needs a real expert to go there and fix it up. it also should have the primary name changed to Gravitoelectromagnetism (with Gravitomagnetism redirecting to it), but i lost that fight a couple of months ago because not enough physicists came to set the "popularists" straight about the terminology. but i think those references to pseudophysics are correct.
first of all, GEM is not the "first principle" in gravitation. GR is (to our present state of knowledge). but, given certain conditions, that is reasonably flat space-time and reasonably slow speeds relative to whatever observer, the Einstein GR equation can be manipulated to form equations that look just like Maxwell's Equations with charge density replaced by mass density and with 1/(4 \pi \epsilon_0) replaced by -G. those are th GEM equations and they're only an approximation to reality unlike Maxwell's which is presently believed to be pretty much exactly reality (dunno if unification theories would say that).
just as the (electro)magnetic field is not really a "new" or different action from the electrostatic field (it is a manifestation of the electrostatic field where the effect of relativity to what the "stationary" observer sees is accounted for), in GEM, the gravitomagnetic field is the same as the gravitostatic field (but with the consequences of relativity considered).
as with the electromagnetic field, any "repulsive" gravitomagnetic field does not exceed the attractive gravitostatic field (the regular old Newtonian "gravity" field). it only
reduces the gravitostatic field, but that is really just a model. there is
only the gravitostatic field, but because of relativity, an observer believes he/she sees the acceleration due to gravity being reduced (due to time-dilation) and attributes that reduction of acceleration to a
repulsive gravitomagnetic force that acts in the opposite direction to the gravitostatic force. but there is no net repulsion, no net anti-gravity.
there is no net anti-gravity. we would need negative mass charges to make anti-gravity (or to construct gravity shielding) and, because of the equivalence of inertial mass to gravitational mass, a negative mass will repel everything (negative or positive masses) and a positive mass attracts everything (negative or positive mass). so imagine what would happen in free space if you had two masses of equal magnitude and opposite sign separated by some distance? (it's a little paradox, which is why i don't think we really ever see globs of negative mass around in the universe. perhaps in conceptual particles, but not little piles of in on the street.)
both the electromagnetic and gravitomagnetic forces are pseudo-forces to account (without reference to SR) for an observed reduction in electrostatic or gravitostatic forces. (and, to be really correct, so is the gravitostatic force also a pseudo-force, although Newton might not agree.)
that's my spin on it.
