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Why can we shield some forces but not others?

  1. Jan 19, 2009 #1
    I'm wondering why we can shield ourselves from electromagnetic and nuclear radiations but not gravity.
    Are there any mathematical suggestions/insights/explanations? (I'd hope for a written explanation/interpretation rather than a long string of formulas!!)

    I'm pretty sure no one knows the physical reasons for this but am wondering if the equations of general relativity compared with maxwell's equations, for instance, offer any suggestions or insights. There are similarities between electromagnetic and gravitational field formulations but differences as well...are there hints in those differences? Seems interesting that the three forces (strong,weak, electromagnetic) we have unified mathematically can all be shielded yet gravity cannot. alpha,beta,gammas rays, photons,etc. can all be shielded...."gravitons" apparently cannot.
     
  2. jcsd
  3. Jan 19, 2009 #2

    D H

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    You can't shield against fictitious forces for the simple reason that they aren't real. Gravity is a fictitious force.
     
  4. Jan 19, 2009 #3

    Dale

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    One very important difference between the equations are that Maxwell's equations are linear and therefore obey the principle of superposition (this is how most shielding actually happens) while the EFE are non-linear and do not allow superposition. Another important difference is that there appears to be no negative gravitational charge.

    I don't know if there are other reasons, perhaps more fundamental, but that is what popped into my mind on reading your question.
     
  5. Jan 19, 2009 #4

    robphy

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    One reason we can be shielded from electric fields is that electric charge comes in two types: positive and negative.

    In addition, note that electromagnetism and gravity are long-range forces... whereas nuclear forces are not.
     
  6. Jan 19, 2009 #5
    Gravity is not a radiation, either a wave type or particulate, if that's the right word, so can't really be blocked by something made of matter. I suppose you could nullify its effect by getting another earth type body hovering just above your head.
     
  7. Jan 19, 2009 #6
    If you want to read more online about how electromagentic fields are sheilded google "Faraday cage". I think thats what you are thinking off.
    Jeff

    Added:
    Does a Faraday cage actually stop an electric field? The way I understand it, any magnetic field causes the charges to rearrange within the conductor making the cage. The way the charges rearrange they make another field that exactly cancels the original field or EMP or whatever. Like sound canceling headphones
     
    Last edited: Jan 19, 2009
  8. Jan 19, 2009 #7
    Ah, interesting point: So I CAN shield myself from gravity by freefalling?? That's equivalent to an inertial frame and gravity effectively disappears???
     
  9. Jan 19, 2009 #8
    Dalespam posted:
    How does superposition relate to shielding? Now that I think about it I don't know exactly how shielding works.

    Is there positive gravitational charge? What is that?? But there IS negative gravity via negative pressure, the effect of dark energy....the cosmological constant.....could such repulsive gravity be harnessed to offset positive gravitational attraction?? never thought of that!!! Brian Greene has a one liner to that effect in FABRIC OF THE COSMOS: to the effect that negative pressure and repulsive gravity overcomes the attractive gravity of mass and energy at cosmological distances....all we would need is an amplifier!! (just joking.)

    I just posted about negative pressure/repulsive gravity that at another current thread:
    https://www.physicsforums.com/showthread.php?t=285886
     
  10. Jan 20, 2009 #9
    According to Wikipedia at
    the mechanisms for RF shielding and magnetic shielding are different:
    I'm not sure I understand just what's at work here, but it would seem the term "shielding" has multiple meanings...that it is effected via different mechanisms.
    So unless someone else posts with further insights, I'm leaving this discussion with the idea that gravitational "shielding" , cancellation, of some sort might be theoretically possible based on my post # 8:

     
  11. Jan 20, 2009 #10

    Mentz114

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    This is not a totally serious comment.
    There are theoretical problems raised by the notion of 'gravitational shielding'. In Jules Verne's book 'Journey to the Moon' (?) he proposes Cavorite, a substance that shields gravity. A disc of Cavorite held horizontally ( parallel to the ground) will shield everything above it from the earth's gravity. So a conical column of the atmoshere will be pressureless and blast outwards. How does one make or store this stuff ? If it occured naturally in the earth, it would jeopardize the geological integrity and the planet might disintegrate along the 'gravitational fault line'.

    Obviously this Cavorite is not a practical idea. Where can I get some dilithium crystals ?
     
  12. Jan 21, 2009 #11
    Also... as Arthur C. Clarke puts it. Cavorite, or anything that could nullify a gravitational force would allow for the creation of free energy, or put another way, a perpetual motion machine.
     
  13. Jan 21, 2009 #12

    Dale

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    Let's use the specific example of E-field shielding by a conductor. If a positive charge is positioned to the left of the origin there will be a field pointing to the right at the origin. If the origin is surrounded by a hollow conductor (the charge on the outside) then the conductor will have negative charge on the left and positive charge on the right. The configuration of charge on the conductor would result in a field pointing to the left. By superposition the two fields add and cancel each other out. This is essentially how shielding works generally with EM.

    Mass is gravitational charge. We don't know of any particles with negative mass.
     
  14. Jan 21, 2009 #13
    Dale...Ok I get that...magnetic shielding is more obscure.

    ...not really: if it were dispersed like most minerals all it would do is weaken the gravitational field of other elements.

    no: "..... negative pressure and repulsive gravity overcomes the attractive gravity of mass and energy at cosmological distances...."

    It's just that we don't yet know how to harness such dark energy.....
     
  15. Jan 24, 2009 #14

    Dale

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    It is really pretty much the same. For example, if you have a passively shielded MRI system what that means is that you have the big magnet of the MRI and then the room surrounds it with several tons of iron. As you slowly ramp the magnet up to field the iron shielding becomes magnetized. Where the MRI field points north the iron shielding field points south. If you rapidly quench the MRI then you can see that the shielding material has an "equal and opposite" magnetic field which is how it canceled out the MRI's field.
     
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