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A Gravitational law of induction

  1. Aug 25, 2016 #1
    By reading on the internet I ve found various websites talking about the relation between gravitation and electromagnetism, and in particular, under: - "Small" Gravitational fields -"small" changes in energy with respect to time, The linearized enstein' s field equation becomes pretty much similar like Maxwell's equations of EM. What I wanted to ask, was: For the gravitational law of induction (equivalent of faraday' s law), if there s a time-varying magnetic field, is there such a limit for the frequency of the magnetic field under which the relation between gravity law of induction and faraday' s law does no longer apply? If so, how would the equation become, I mean, how would the system "Gravitational field -Magnetic field" behave? Thank you
     
  2. jcsd
  3. Aug 25, 2016 #2

    PeterDonis

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    Please give specific references. And be prepared to be told that they aren't valid ones, unless they are textbooks or peer-reviewed papers. There is lots of unreliable information on "various websites".
     
  4. Aug 25, 2016 #3

    pervect

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    You might look at https://en.wikipedia.org/wiki/Gravitoelectromagnetism

    Note that there are gravitational analogues to electric and magnetic fields in the GEM approximation, but one should not confuse the electromagnetic versions of the electric and magnetic field with their GEM "gravitolelectric" and "gravitomagnetic" counterpaerts. The gravitoelectric field is similar to the coulomb field of a charge, but it's due to mass*, not due to charge. Similarly the gravitomagnetic field is due to "mass currents".

    As far as frequency limits go - I'm not sure. I do believe you need low velocities of the matter to use the GEM approximation. THis may or may not impose some sort of frequency limit.

    *In the full theory, gravity is not just due to mass, but due to the stress-energy tensor, which includes momentum and pressure. As I recall, in GEM the pressure terms must be negligible, and the momentum terms are handled by the ##B_g## field, so I believe it's correct to say that the gravitoelectric field is due to mass. But I could be mistake, it's not something I routinely work with.
     
  5. Aug 26, 2016 #4
    https://www.google.it/url?sa=t&sour...tjrtPJZ5GNA1-B5AQ&sig2=ZoRFLeAZxbIYK9R3-_UsoQ


    https://www.google.it/url?sa=t&sour...AzSuj2Wo-A5JhjZ6g&sig2=KGDBkm1DZiCyEHp8RojnpQ
    Here are two references. About reliability of sources, as Peter Donis said, wikipedia considers ECE theory ( which one paper i linked referred to), as "pseudoscience", because of the lack of mathematical rigor in this theory.
    But what made me think about the possibility of its predictions were the experiments with superconductors referred in the second paper, which seem to show that the phenomenon actually exists in nature, even if it needs a " weak field" approximation.
     
  6. Aug 26, 2016 #5

    pervect

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    The Mashroon paper looks good, the Evans paper - not so much. Perhaps one of the moderators can quantify this rather general observation with some detailed citation analysis - though certainly having Wiki mention that a paper is pseudoscience is not a good sign.

    Meanwhile, note that the Mashroon paper is about standard GR, and that Mashroon mean something different by "Faraday's law of induction" than Evan's does. The Evans paper is definitely NOT about standard GR. I don't believe the experimental results that Evans cites have ever been replicated. Mashroon is drawing an analogy between purely gravitational fields in GEM and electromagnetism, while Evans is claiming something rather different.
     
  7. Aug 26, 2016 #6
    I understand...just let s wait if are there other physicsts who have read about this..
     
  8. Aug 26, 2016 #7

    robphy

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    Note that these google links somehow appear to retrieve PDFs
    from arxiv.org (for the Mashhoon paper) and
    from Evans' own website (for the Evans paper).
     
  9. Aug 26, 2016 #8
    Yes, I know...I ve read some articles on arxiv as well..
     
  10. Aug 26, 2016 #9

    robphy

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    It's not the arxiv link that I am worried about.
    Why not post a link directly to arxiv?

    It's best to be direct (and transparent).
    I'm bothered that this Google link seems to redirect to a download.
     
  11. Aug 26, 2016 #10

    PeterDonis

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    This is my understanding as well.
     
  12. Sep 2, 2016 #11
    https://www.google.it/url?sa=t&sour...5M5fEjYTwmApQdzgA&sig2=naJ4jFeAVFGuamUo4rtMuQ
    Just to continue the talk of before, I found this one on google, and it talks about the possibility, for a superconductor, to spin around his axis, and therefore, create a relatively strong geomagnetic field, ( maybe not so strong, but several orders of magnitude higher than expected). Anyone knows if this effect has ever been replied for higher frequencies? ( The device here spins at around 6500 rpm, which would be about 108 hz). I m just curious about what could happen at higher frequencies, with these phenomena.
     
  13. Sep 2, 2016 #12

    robphy

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    Please do a little research and link directly to the article:
    http://arxiv.org/abs/gr-qc/0203033
    ... not a google redirect to it. It's a courtesy to others so they know what to expect.
    In fact, some details would be appreciated...
    "Gravitomagnetic Field of a Rotating Superconductor and of a Rotating Superfluid"
    M. Tajmar, C.J. de Matos
     
  14. Sep 2, 2016 #13
    Sorry about it, I tried to paste it, but didn t know how to write the link the way you did..i only knew how to do with the "google" stuff. Anyway, what do you mean with "some details" ?
     
  15. Sep 2, 2016 #14

    robphy

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    Here's what I did:
    when the pdf file shows up, I copied its title and googled the title
    (when Acrobat prompts me if I want to open it, it tells me the only the first part of URL... so I have a hint where it is).
    Then I found the URL and pasted that.

    Some details would be a title and an author, and maybe a date, maybe a part of the abstract.
    (If you can't find a good URL, you could just ask folks to google the title of the paper you provided.)

    With all of the dangers of clicking arbitrary links these days on the internet,
    folks (like me) are more likely to follow a link when they have a sense of what they will get back as a result.
    If you just say "Click here" or "Click [cryptic link]", many folks will not follow it [even if they might have been interested in the topic].
     
  16. Sep 2, 2016 #15
    I will do it next time then :)
     
  17. Sep 2, 2016 #16

    A.T.

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    This forum redirects all links anyway. This is the URL of your "direct" link above:

    Code (Text):
    https://api.viglink.com/api/click?format=go&jsonp=vglnk_147285253521311&key=6afc78eea2339e9c047ab6748b0d37e7&libId=ismadfw6010009we000DL2k31qo1&loc=https%3A%2F%2Fwww.physicsforums.com%2Fthreads%2Fgravitational-law-of-induction.883082%2F&v=1&out=http%3A%2F%2Farxiv.org%2Fabs%2Fgr-qc%2F0203033&ref=https%3A%2F%2Fwww.physicsforums.com%2Fthreads%2Fgravitational-law-of-induction.883082%2F&title=Gravitational%20law%20of%20induction%20%7C%20Physics%20Forums%20-%20The%20Fusion%20of%20Science%20and%20Community&txt=http%3A%2F%2Farxiv.org%2Fabs%2Fgr-qc%2F0203033
     
  18. Sep 2, 2016 #17

    robphy

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    Yes, I have known about the annoyance that I can't merely "copy the link location" to my clipboard... instead, I have to select it, then copy to my clipboard [as plain text of the unredirected URL].

    I trust the redirect from the Physicsforums website (or else I wouldn't be here)...
    and, if I want, I can copy the unredirected URL as plain text and paste it myself into my browser or into an email.

    With the already google-redirected link, I can't [unless I get a script to decode the embedded link].

    The point is to be more transparent.

    By the way, if I print out the thread, the redirected (and likely truncated) URL on my printout would be worthless. :mad:
     
  19. Sep 23, 2016 #18
    Wave equations in electromagnetism admit always advanced and delayed mathematical solutions which propagate one in the past and one in the future. Usually the advanced solution which propagates towards the past is not considered by classical physicists.
    Is the situation similar in General Relativity for what concerns Gravitational waves ? Do the mathematics allow gravitational waves propagating towards the past ?
     
  20. Sep 23, 2016 #19
    that's an interesting question as well :)
     
  21. Sep 24, 2016 #20
    I think that in a "linearized approximation" of GR which has many common points with electromagnetism theory, the two types of wave solution should exist. But I don't know if we are able to understand what happen in the full non-linear version of GR.
     
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