Why is it not possible to shield gravity?

Discussion in 'Physics & Math' started by pluto2, Dec 3, 2013.

  1. pluto2 Registered Senior Member

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    Why is it not possible to shield gravity?

    Is this because it would violate the equivalence principle of general relativity?

    Wikipedia says that gravitational shielding is considered to be a violation of the equivalence principle and therefore inconsistent with both Newtonian theory and Albert Einstein's general relativity.

    http://en.wikipedia.org/wiki/Gravitational_shielding
     
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  3. Trapped Banned Banned

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    What would it mean or involve to shield gravity? Do you tackle the question of inertia which is an intrinsic property of matter? Do you somehow shield yourself from a physical field, yet it is generally believed that gravity isn't even a real field. So where does that lead us? How do you shield against spacetime curvature when you need to sit in spacetime to begin with? This isn't like switching on a magnetic field by the manipulation of magnets.
     
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  5. andy1033 Truth Seeker Valued Senior Member

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    Probably because gravity is multi dimensional, and effects everything in terms of matter. We only have a limited view of reality, and to try and work out whats beyond, people are only really guessing.

    They have no idea what gravity is, its not what you learned at school, thats just simple rubbish.

    I believe the earth and sun communicate with each other, but has mankind worked this out, no of course not. There is far more to the universe than man has or will work out, as science is limited.
     
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  7. pmb Banned Banned

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    Since there are negative charges we can shield a region of space from electromagnetic fields so I assume a similar thing can happen with gravity. I.e. I believe that the reason for not being able to sheild gravitational fields is due to the absense of negative active gravitational mass in the universe which could counter act the gravitational forces.
     
  8. billvon Valued Senior Member

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    Because it is not a thing that's transmitted - it is a distortion of space-time.
     
  9. Trapped Banned Banned

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    There is a region of psuedo-negative energy created by the Casimir Effect, but apart from that, there is no evidence that negative matter exists... though it would act as the perfect shield against positive gravity.
     
  10. pmb Banned Banned

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    That's not true. Gravity is not a distortion in spacetime. Gravitational tidal gradients are the same thing as spacetime curvature. That's all. The effects can be cancelled if sources which act opposite exist. E.g. if you have matter with a negative emergy density then that can be used to generate a gravitational field which opposes matter wich positive energu density.
     
  11. paddoboy Valued Senior Member

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    As someone has already said, gravity according to our best theory, is the curvature of space/time in the presence of mass/energy.
    Space/time is the 4 dimensional background on which everything including SR/GR is built and/or calculated.
    It would be impossible to shield from.

    If somehow one day in the future, we are able to utilise or manipulate whatever is causing space/time to expand, [DE/CC] we may achieve a perception of "turning off gravity". A highly speculative notion.
     
  12. billvon Valued Senior Member

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    OK. Call it curved spacetime, distorted spacetime, a nonlinear region of spacetime - the important concept is that gravity is not like electromagnetic radiation propagating through space, it is a fundamental change to the structure of space.

    Heck, if you have matter with a POSITIVE energy density it can cause an effect that looks like you are counteracting gravity. Put a massive plate atop some pillars, and if the material is massive and dense enough, someone beneath it at the right distance will not feel any effects from gravity* - they would float weighless, the gravity from the plate counteracting the gravity from the earth. (Naturally there are a lot of practical problems with that, but it would work if the materials and processes were available.)

    (* - depending on the densities involved however they might well feel tidal effects.)
     
  13. paddoboy Valued Senior Member

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    While the first sentence is true, we do understand how gravity works, and are able to predict and forecast its actions quite correctly. We also know it is related to the geometry of space/time in the presence of mass/energy.

    The first part of your second sentence sounds a bit woo woo. With the second part, science is not limited as such, it is just the knowledge mankind has gathered so far that is limited. But that will improve with time.
     
  14. pluto2 Registered Senior Member

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    Humans are not Gods. Sure, some people are very strong and capable but I doubt that human beings will ever be able to effect the fundamental forces of nature.
     
  15. paddoboy Valued Senior Member

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    21,703
    Do you hold the same thoughts with manipulating space/time, ala Star Trek to obtain a perception of FTL??
    I would put that in the same category. It only requires energy...Lots of it.

    How do you think Captain Cook and his crew, a mere 250 years ago, would view society today?
    TV, Phones, etc....
    A great scientist/Author once said, Äny sufficiently advanced civilisation, would appear as magic to us"
     
  16. billvon Valued Senior Member

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    You realize we 'effect' the fundamental forces of nature today in nuclear reactors and particle accelerators, right? We even manipulate electromagnetic fields to drive our cars and trains, and to transmit information over hundreds of miles. It's not unreasonable to think that, someday, we might be able to control gravity in a similar way.
     
  17. pmb Banned Banned

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    I know that, of course. I.e. I agree but I don't think that's what the OP had in mind when he was asking about shielding gravity. What is that supposed to mean? All definitions I see assume one knows what "sheild' means and that's not clear to me. I'm using the analogy of what shielding is in electrodynamics myself. I.e. if you place metal plates on all walls, the floor and the ceiling of a room then that room would be "Sheilded" from EM waves.

    Of course all this could be wrong as well since we don't have a solid definition of what it means to "shield." Any definition of shielding won't rule out that its accomnplished by creating an opposing gravitational field and that's what I did except from the same location using opposite sources, i.e. like using negative charges at the same location as the positive charges are which created the field, The negative charges then act to "sheild" the electric field created by positive charges.

    Then there is the kind which seems to block the passage of gravitons. I don't believe sheilding devices exist although there are claims to that effect such as http://electrogravityphysics.com/gravity-shielding-finland/
     
  18. billvon Valued Senior Member

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    I've heard this question asked before and I usually take it as "how do you stop gravity from acting on something?" They sometimes compare it to wrapping something in aluminum foil to shield it from EM radiation (which is generally effective.) I think the idea comes from the thought that a gravity "field" must be a lot like an electromagnetic field since they are both fields.
     
  19. OnlyMe Valued Senior Member

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    It would seem th OP is in line with your assessment. The following is the first sentence from the Wiki reference in the OP
    The term gravitational shielding refers to a hypothetical process of shielding an object from the influence of a gravitational field.
     
  20. Janus58 Valued Senior Member

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    When a lot of people think of shielding gravity, I believe that they are thinking along the lines of something like Wells' "Cavorite" from First Men in the Moon. I seriously doubt that this type of shield is possible.

    Something along the lines of passive magnetic shielding is also brought up. However, this type of shielding works by redirecting the lines of force around the shielded area rather than blocking it. The lines are concentrated into the shielding material. Even if possible, this seems to me would lead to the shield's weight to increase to compensate for the decrease in weight for whatever is being shielded. IOW, you wouldn't be able to encase an object in this shield and have the combination be weightless.

    The other possibility is to have your shield in place, and place an object over it. However, I would think that this would be an unstable position as the shield would only be effective on that gravity coming from directly below and unless the object stayed in the exact center, it would drift towards the edge like a ball rolling off the peak of a hill.

    Then there's the conservation of energy issue. In moving your object from outside the shielded area to the center, you are in effect moving it from one gravitational potential to another. IOW, it should take as much energy to do so as it would to lift that object to an infinite distance from the Earth or accelerate it up to escape velocity. In addition, as that object drifts away from the center it will gain that energy back as kinetic energy accelerating as it does so, and when it hits the edge of the shielded region would be moving at escape velocity.

    Any type of active shielding would have the same energy conservation issue; to render any object weightless, it would have to consume at least as much energy as it would to take to get that object up to escape velocity.
     
  21. Grumpy Curmudgeon of Lucidity Valued Senior Member

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    Until we learn to manipulate spacetime dimensions I see no way to manipulate gravity as it is an intrinsic property of mass in spacetime. How you erase the warpage of spacetime, even locally, I haven't a clue.

    Grumpy

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  22. pmb Banned Banned

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    I made an error in my post when I said that electric shielding was possible due to the existance of negative charges but now realize that's not true. It's possible because charges in conductors move due to the electric field and they arrange so as to not allow fields inside of them by having them cancel out.
     
  23. Dinosaur Rational Skeptic Valued Senior Member

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    The following ignores the concept that the Relativity POV relating to space-time is a model, not a reality.
    The above is an excellent model, providing a lot of equations pertinent to modern physics.

    Note, however, that it deals with the position of a particle being specified by (x, y, z, t) & the path of a particle as a static curve in 4D space.

    If you accept the model as reality, your POV includes the belief that the universe is a static 4D entity. Id Est: There is no motion & the concepts of past/present/future are illusions.

    I prefer not to accept the above implications of this model, although many of its other consequences seem to provide very useful insights.
     

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