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Electromagnetic/gravitational fields

  1. Jan 3, 2008 #1
    Quick question, if the properties or electromagnetic and gravitational are basically the same is it possible to block gravity like you can block em fields using a Faraday Cage or am i basing this on bad science?
  2. jcsd
  3. Jan 3, 2008 #2
    Not with the pressent understanding.

    THe thing is, a faraday cage rely on two tings 1) the existence of charge of opposite polarity 2) the conductor in which those charges are free to flow around.

    If you place a metal sphere in an electric field, the free charges in the metal will arrange themselves until they produce a new field antiparallel to the imposed field within the sphere so the two exactly cancel. This is a faraday cage.

    On the other hand there is no known matter that is "opposite polarity" or "antimass" (not antimatter) i.e. is repelled by normal mass and there is no material in which such mass/antimass could freely arrange themselves to produce an antiparallel gravity field.

    Physicists have proposed a gravitational analog to magnetism, produced by mass in motion. But since this mass/anti-mass conducter has yet to be invented, it will most likely not be detected
    Last edited: Jan 3, 2008
  4. Jan 3, 2008 #3
    no because gravitational field do not get affected by the medium ,but electromagnetic does.
    hence any material barrier of whatsoever property cannot stop or even affect the field passing though it ,except that its mass may add to the field strength , but that,s a different thing
  5. Jan 3, 2008 #4


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  6. Jan 3, 2008 #5
  7. Jan 3, 2008 #6


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    Which part of "electromagnetic" exactly is the same with "gravitational"? Other than a similar form existing for the Gauss's law, does the fact that the divergence of B is zero have a counterpart for the classical description of gravity? Do we have a Lenz's Law equivalent for the classical gravity?

    So the premise that they are "the same" does not hold true here, and thus, the application of such similarity is faulty.

  8. Jan 3, 2008 #7
    Sorry, what i meant was there are a few similarities between them and if this could be one. I have read about gravtomagnetism in the new scientist about a year ago....i must dig it up out of the pile of them under my bed. Thanks
  9. Jan 3, 2008 #8


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    Could you list out exactly what these "similarities" are? I have listed a couple of examples where they are NOT similar. And I believe that in your question, you were only referring to the classical, newtonian gravity, no?

  10. Jan 4, 2008 #9


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    i think that if negative mass-charges existed, the maybe one could conceivably create a gravitational Faraday Cage. but even though there are positive and negative electric charges (and a Faraday Cage would not work in blocking EM fields if there wasn't both polarities of charge), there are no negative masses. just positive masses.

    it's kinda interesting to imagine a situation (in free space) where there are two balls, one of positive mass and the other of negative mass. what would (assuming the Equivalence Principle and Newtonian gravitation) the kinematics be? if these two oppositely signed masses were placed in free space, released from anything holding them, and left alone, what would their motion be? what if it were two negative masses? of course we have a pretty good idea what it would be for two positive masses.

    but David, without opposite signed charges, we couldn't make a Faraday Cage work to block EM fields. likewise, without opposite signed masses, we can't do it for gravitation.

    Zapp, i think that an incomplete (and not always accurate) concept of GEM can sorta be had with the comparative inverse-square field behavior (what makes Gauss's Law work for both EM and gravity) and the additional postulate that pertubations in the gravitational field propagate at the same speed c as do perturbations of an EM field. a set of Maxwell-like equations for either case can be constructed that are consistent with the inverse-square and finite speed properties each case has. it's not an adequate description for real physicists, but for those of us that don't do GR, it's a sorta-kinda understanding.
    Last edited: Jan 4, 2008
  11. Jan 4, 2008 #10
    Quite; I don't really understand how the classical magnetic field is supposed to come about, but I've been told that it's to do with a relativistic transform of the 'electric' field - so they're actually one and the same. Presumably if viewing an electrostatic field relativistically yields a new field, viewing a gravitational field will do the same. This had never occurred to me but is fascinating.
  12. Jan 4, 2008 #11


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    Well see, sometime when someone posts stuff on here about something, I tend to ask what is it that they actually have understood, which they used to base the question on. That is why I asked what exactly is it that the OP found to be "similar". It isn't a question on the current status of the field, it is more of a question of the current understanding of the OP that he/she used as the impetus for the question.

    All of what you said is fine and dandy, but if the OP was not aware of it, he/she certainly didn't use that knowledge as the foundation for the question. That is what I am trying to decipher. What exactly was understood or known to result in that question.

    Now everyone understands where I'm coming from?

  13. Jan 4, 2008 #12


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    As has been pointed out, the EM and gravitational field are sufficiently different for the analogy to fail when it comes to shielding.

    But I did come across this argument -

    1. Suppose gravity is carried by a massless boson.
    2. Under some conditions ( vacuum energy level changing ?) a boson can acquire mass.
    3. this will reduce the range of the graviton and weaken the force.

    So, if you can arrange a vacuum state that gives mass to gravitions ( at that place) then gravitational shielding may be achieved.

    Very speculative, but not outside the bounds I think.
    Last edited: Jan 4, 2008
  14. Jan 4, 2008 #13
    At the moment i am studying a level physics and so i do not understand all what these contributers are talking about, but i am eager to learn and i will now read up on it. What I was originally talking about was the original newtonian view of gravity ie. the inverse square law etc. Thanks

    I have read about what Mentz114 is talking about and i think it was really interesting, i think they are sending 2 platinum rods to the place where the suns and earths field cancel out to try and detect these bosons.
  15. Jan 4, 2008 #14


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    But see, this is where you need to be very careful in how you apply your logic and intuition. You'll discover this more as you go along in physics. Just because you see two animals with "tails", does not mean that they will have similar characteristics or behavior. Focusing simply on the "inverse square law" and then deducing that gravity and electromagnetism should be similar, and then going on to use that to extrapolate on something else is rather dangerous.

    I've already described where they are not similar. Classical gravity that resulted in the simple inverse square law certainly do not have the same formulation as the complete set of Maxwell equations that describe classical E&M. Do not be quick to jump onto something just because you see a small part of it being "similar". You need the bigger picture before you can do that.

  16. Jan 4, 2008 #15
    Thanks for pointing that out for me, I was under the impression that they were 2 sides of a coin.
  17. Jan 4, 2008 #16


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    this is from Wikipedia (or used to be):

    According to special relativity, electric and magnetic forces are part of a single physical phenomenon, electromagnetism; an electric force perceived by one observer will be perceived by another observer in a different frame of reference as a mixture of electric and magnetic forces. A magnetic force can be considered as simply the relativistic part of an electric force when the latter is seen by a moving observer.

    A thought experiment one can do to show this is with two identical infinite and parallel lines of charge having no motion relative to each other but moving together relative to an observer. Another observer is moving alongside the two lines of charge (at the same velocity) and observes only electrostatic repulsive force and acceleration. The first or "stationary" observer seeing the two lines (and second observer) moving past with some known velocity also observes that the "moving" observer's clock is ticking more slowly (due to time dilation) and thus observes the repulsive acceleration of the lines more slowly than that which the "moving" observer sees. The reduction of repulsive acceleration can be thought of as an attractive force, in a classical physics context, that reduces the electrostatic repulsive force and also that is increasing with increasing velocity. This pseudo-force is precisely the same as the electromagnetic force in a classical context.

    here's the quantitative version (from the talk page):

    The classical electromagnetic effect is perfectly consistent with the lone electrostatic effect but with special relativity taken into consideration.

    The simplest hypothetical experiment would be two identical parallel infinite lines of charge (with charge per unit length of [itex] \lambda \ [/itex] and some non-zero mass per unit length of [itex] \rho \ [/itex] separated by some distance [itex] R \ [/itex]. If the lineal mass density is small enough that gravitational forces can be neglected in comparison to the electrostatic forces, the static non-relativistic repulsive (outward) acceleration (at the instance of time that the lines of charge are separated by distance [itex] R \ [/itex]) for each infinite parallel line of charge would be:

    [tex] a = \frac{F_e}{m} = \frac{ \frac{1}{4 \pi \epsilon_0} \frac{2 \lambda^2}{R} }{\rho} [/tex]

    If the lines of charge are moving together past the observer at some velocity, [itex] v \ [/itex], the non-relativistic electrostatic force would appear to be unchanged and that would be the acceleration an observer traveling along with the lines of charge would observe.

    Now, if special relativity is considered, the in-motion observer's clock would be ticking at a relative *rate* (ticks per unit time or 1/time) of [itex] \sqrt{1 - v^2/c^2} [/itex] from the point-of-view of the stationary observer because of time dilation. Since acceleration is proportional to (1/time)2, the at-rest observer would observe an acceleration scaled by the square of that rate, or by [itex] {1 - v^2/c^2} \ [/itex], compared to what the moving observer sees. Then the observed outward acceleration of the two infinite lines as viewed by the stationary observer would be:

    [tex] a = \left(1 - v^2 / c^2 \right) \frac{ \frac{1}{4 \pi \epsilon_0} \frac{2 \lambda^2}{R} }{\rho} [/tex]


    [tex] a = \frac{F}{m} = \frac{ \frac{1}{4 \pi \epsilon_0} \frac{2 \lambda^2}{R} - \frac{v^2}{c^2} \frac{1}{4 \pi \epsilon_0} \frac{2 \lambda^2}{R} }{\rho} = \frac{ F_e - F_m }{\rho} [/tex]

    The first term in the numerator, [itex] F_e \ [/itex], is the electrostatic force (per unit length) outward and is reduced by the second term, [itex] F_m \ [/itex], which with a little manipulation, can be shown to be the classical magnetic force between two lines of charge (or conductors).

    The electric current, [itex] i_0 \ [/itex], in each conductor is

    [tex] i_0 = v \lambda \ [/tex]

    and [itex] \frac{1}{\epsilon_0 c^2} [/itex] is the magnetic permeability

    [tex] \mu_0 = \frac{1}{\epsilon_0 c^2} [/tex]

    because [itex] c^2 = \frac{1}{ \mu_0 \epsilon_0 } [/itex]

    so you get for the 2nd force term:

    [tex] F_m = \frac{v^2}{c^2} \frac{1}{4 \pi \epsilon_0} \frac{2 \lambda^2}{R} = \frac{\mu_0}{4 \pi} \frac{2 i_0^2}{R} [/tex]

    which is precisely what the classical E&M textbooks say is the magnetic force (per unit length) between two parallel conductors, separated by [itex] R \ [/itex], with identical current [itex] i_0 \ [/itex].
  18. Jan 4, 2008 #17


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    The linearized Einstein equations (i.e. those applying for "weak" gravitational fields) can actually be put into a form which is equivalent to the full set of Maxwell equations. There is also a close analog of the Lorentz force law; specifically [itex]\mathbf{a} = - \mathbf{E}_g - 4 \mathbf{v} \times \mathbf{B}_g[/itex]. Of course, the interpretations of the various terms here would take some time to explain. There's a discussion of this in most textbooks on general relativity.

    There is also a paper which may be relevant here: WH Press, "On gravitational conductors, waveguides, and circuits" General Relativity and Gravitation, vol 11, pgs 105-109 (1979). It talks about how gravitational conductors could exist, what that would mean, and so on. This requires the existence of a type of matter which has never been observed, so the idea is probably best considered as a fun exercise more than anything else. In any case, the paper shows that a "gravitational conductor" must be a material with an exceedingly large shear modulus or shear viscosity. To give an idea of how odd such a material would be, something like steel would have to be stiffened up by a factor of at least [itex]10^{10}[/itex].

    In almost all astrophysical contexts, these types of effects are exceedingly small. And as others mentioned, simple screening effects are ruled out from the lack of negative masses roaming around.
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