Does light wrap around black holes?

Discussion in 'Physics & Math' started by Jennifer Murphy, Sep 20, 2014.

  1. Jennifer Murphy Registered Senior Member

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    Light is bent by gravity. If the gravity is strong enough, like near a black hole, couldn't the light be bent 180° and be sent back where it came from? And can't that be used to detect black holes?
     
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  3. origin Heading towards oblivion Valued Senior Member

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    The short answer is yes and no.

    You could shoot a laser at the closest blackhole canditdate and 15,000 years later you could try to pick out the 1 or 2 photons that may return. See the problem?
     
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  5. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Good answer, but lets point out that gravitational lens effect on more distant stars (galaxies really) gives immediate detection.
     
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  7. Jennifer Murphy Registered Senior Member

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    I wasn't suggesting that I go out in the backyard with my PowerPoint laser pointer and search for black holes. There are plenty of very bright light sources close to black holes, such as at the center of most galaxies. It seems to me that those light sources should be visible twice: from the light coming directly from them and from the light being bent around the black hole. No?
     
  8. origin Heading towards oblivion Valued Senior Member

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    I would think not. The number of photons that would be at exactly the right trajectory to orbit at approximately 180 degrees would be very small and after traveling thousands of light years there would be essentially no photons that would hit the earth.
     
  9. brucep Valued Senior Member

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    The following is a nice explanation for the first great test of GR. The prediction Eddington confirmed.

    dphi =4M/R
    http://www.mathpages.com/rr/s8-09/8-09.htm

    You can predict the path of light through curved spacetime using GR. For your query this prediction describes the path. Take a look at some of the stuff describing lensing experiments. For the Schwarzschild geometry there's a photon sphere at r=3M where light can orbit the black hole.
     
  10. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I agree, except have no idea what light that could be as no detectable light leaves a black hole. - At least not before its final dying flash.
     
  11. river

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    the thing is , the black hole doesn't have a 180degree of influence on the light
     
  12. danshawen Valued Senior Member

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    180 degrees, as in a U shaped trajectory would mean that the light accomplished 1/2 orbit. Here's the problem: an orbit sustained by something massless like a photon (and which does not perturb the gravitational field) about a black hole must be a conic section. Ellipses are possible, and so are circles, parabolas, even hyperbolas. A 'U' shape just isn't in there, although it might be possible to approximate it if the BH were part of a binary system, which is common.

    Interferometric observations are regularly used to determine multiple images of the same distant object subject to gravitational lensing. Read Evalyn Gates' book: 'Einstein's Telescope". Presumably, such techniques could conceivably be applied to the BH - binary star situation also. That is to say, the visible star that is the BH companion emits light as it circles the black hole. At some point, light leaving the binary companion is bound to be bent by its invisible companion so that there will appear to be multiple images of the star. Possibly something rarer than an eclipse, but I'm sure it must happen.

    A means for detecting BHs isn't really a problem. Saggitarius A is possibly the best known BH in the Milky Way, and it it is orbited by several visible stars. All we get from it by way of radiation are radio waves. It's a BH for sure, possibly even a "super massive" one.
     
  13. lpetrich Registered Senior Member

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    That's just plain wrong. One can get closed-form solutions if one uses something called elliptic functions, however. One only gets conic sections only in:
    • Degenerate cases
      • Circular orbits
      • Radial orbits
    • The fast-flyby limit
    • The Newtonian limit
     
  14. danshawen Valued Senior Member

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    3,951
    Yes, you are right. I also forgot to stipulate that I was talking about spherical masses (or BHs) of uniform density. One could also conceivably slice a spherical BH mass perfectly in half and get very close to a half orbit that was perfectly u-shaped.
     
  15. Mr. G reality.sys Valued Senior Member

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    Space/Time is bent by gravity. Light goes straight through space/time.
     
  16. paddoboy Valued Senior Member

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    Spacetime is certainly bent/warped/curved in the presence of mass/energy, which exhibits a phenomenon we call gravity. Light follows that same curved shape which we refer to as travelling in geodesics.
    In fact, due to the momentum of light at a constant "c", it also contributs to the same bending/warping/curving of spacetime, albeit at a very tiny infinitesimal amount.
     
  17. Mr. G reality.sys Valued Senior Member

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    You're introducing unnecessary complexity.

    Good effort.
     
  18. Trippy ALEA IACTA EST Staff Member

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    This is flat out wrong. There is only one special case where the trajectory or the light is U shaped - when the black hole and the earth are moving in the same direction at the same speed. Even then it's only true when observed from the earth's reference frame.

    In ALL other cases the trajectory is parabolic in all reference frames (although the exact sbape of the parabola may vary)
     
  19. paddoboy Valued Senior Member

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    27,543

    Nup, not correct.....
    Spacetime is bent/warped by mass which then exhibits the property we call gravity:
    Light/photons follow geodesics in that spacetime.
    Geodesics of course are simply a path that is as straight as possible, and following the curvature of that medium of spacetime.
    Other examples of geodesics are straight lines on plane surfaces, and "great circles" on spheres.
    Hope that helps.
     
    Last edited: Nov 21, 2014
  20. theorist-constant12345 Banned Banned

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    I am no expert in the knowledge of black holes, but in my opinion, EM radiation, ''light'', flows around a black hole, and propagates . Distorting time and space.
    In a black hole, relative to us, it is timeless and there is no possible timing of the nothing of inside the hole.<nothing, on the basis we can not see inside a black hole>.

    Similar to the observation in this link -



    Please do not take my opinion has fact.
     
  21. theorist-constant12345 Banned Banned

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    2,660
    Added- In this video link, the surrounding volume of 3 dimensional space, <which is the empty space surrounding the body of water>, has zero G, the water droplet has mass, that propagates the light, and distorts space time.
     
  22. paddoboy Valued Senior Member

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    27,543
    Light follows geodesic paths in spacetime.
    It does not distort spacetime to any noticable degree. The BH's mass distorts the spacetime. Light follows those distortions.
    Within the vicinity of a BH, those paths will see light apparently circling a BH at the Photon sphere or 1.5 Schwarzchild radius.




    Nothing you have Imagined has been taken as fact by anyone, especially your belief that light is an illusion and dark and shadows are real, which BTW seems to now contradict what you are now claiming about light.
    You appear to be making up hypothesis as you go.
     
  23. theorist-constant12345 Banned Banned

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    2,660
    Seemingly that I have and tried to explain the same idea for several years, I would hardy say ''made up''. The rational thoughts are logical and critical thought about the reality in a formal fundamental representation.

    You can clearly observe in the total internal reflection on the nasa observation video, that the propagation of light on the ''outer'', is dark, showing you the propagation ''maximum'', is equal magnitude of EM propagation to darkness.
    ''Lights stop pass band'' is the spectral magnitude of black in this instant.
    The EM radiation propagation around any spherical mass, is of isotropic nature, the transparentness , only countable by refractive values and propagation. Being observed, by the exact perpendicular being equal to observation.
     
    Last edited: Nov 21, 2014

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