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Why light curves around objects of mass. 3 answers. 3 questions.

  1. May 7, 2010 #1
    Lately I have been going nuts trying to find a definitive answer to this relatively simple question. Why does light curve around bodies of mass? After a bit of digging I have found 3 answers.

    1. Light follows the curve of space-time.
    2. The mass of a photon is attracted by gravity of the object.
    3. Light is refracted by the change in the density of space-time.

    ...and of course I have then 3 questions :

    1. If light follows a curve spatially does not that imply that curved space-time refers to volumetric space, instead of (or perhaps including) gravitational forces and time dilation? In other words, this answer seems to imply curved Space rather than curved space-time. If this is the case we can move the discussion https://www.physicsforums.com/showthread.php?t=400147".

    2. There have been many discussions about the mass of a photon, and it is my understanding that the mass is so small that only under 'relativistic' forces does it come into play (ie Black Holes). However we see curved light paths around the Sun during eclipse. Does the mass of a photon play a greater role in less force scenarios?

    3. If indeed the principles of refraction are at play, would not the light be refracted both entering the density field and exiting, and thus the path of light would make a slight 'S' shape?(see diagram 1). Reason : Light passing a body of mass experiences greater and greater density in the gravity/time field, and thus its refraction curve would increase. Until it reaches the 'escape' point where the fields begin to decrease from there on out and thus would begin to refract in the opposite way through them. http://commons.wikimedia.org/wiki/File:Refraction_varies_by_frequency.gif" [Broken].

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 7, 2010 #2


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    Curved space is contained within curved space-time. However, we know that mass curves the entire 4-dimensional manifold, both space and time. The effect of photon deflection is due to the spatial curvature part, but gravitational time dilation for example arises out of the time part curvature.

    Mass of the photon is identically equal to zero. Not very small.
    I've never heard of this "refraction through spacetime". Certainly this is not how the problem is viewed in General Relativity. Explanation #1 is the correct one.
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  4. May 7, 2010 #3
    Well then, do you mind checking out <<link deleted>> and telling me if he is full of it. If he is, I was fooled hook line and sinker.

    Last edited by a moderator: May 7, 2010
  5. May 7, 2010 #4


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    I'm very hesitant to accept any of what this guy is saying. First off, his claims are so grandiose (explain DE and DM!) that it definitely requires scrutiny.

    One thing I thought of while browsing, perhaps someone more familiar with the ins and outs of optics could explain: Could such a "refraction" explanation ever explain photons orbiting a massive object? Seems to me it could not.
  6. May 7, 2010 #5


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    Link to theory removed. I see no sign that it has been published anywhere except on the author's web site and as a presentation at a conference (probably a contributed talk or poster session, which aren't peer-reviewed).
  7. May 7, 2010 #6
    Thanks everybody. These are exactly the kind of responses I've been looking for.
    I will move ahead with the answer number 1 "Light follows the curve of space time" and "mass curves the entire 4-dimensional manifold, both space and time".

    I will put some new questions and diagrams into the https://www.physicsforums.com/showthread.php?t=400147" discussion based on what was said here.

    And this time I'll try not to be fooled by pretty websites with lots of equations. :-p

    Last edited by a moderator: Apr 25, 2017
  8. May 8, 2010 #7
    The refraction analogy to gravitation has been around for a long ling time. It is a valid analogy because refraction occurs precisely because portions of a wavefront encounter a slower medium and bend the wavefront. The diagram an reasoning showing an "s" bend is incorrect because it's founded on the idea of an abrupt change in the index of refraction. In fact gradual index changes also bend light and just as gradually as a gravity. (look up GRIN lenses to see how these are made and used). Where the analogy between gravity an refractive index fails is in the bending of space which refraction cannot induce.
  9. May 8, 2010 #8


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    Whether or not this "gravitational refraction" idea is viable, the above diagram does not illustrate it; the diagram is flawed.

    Even if light did follow gravity as if refracted it would not follow path B is illustrated; there is no S-curve, and A and B will not emerge parallel.

    You can prove to youself that path B is wrong simply reversing the direction of the light beams. (simply reverse the arrows in the diagram). Light paths are reversible. Look at the linked prism diagram again; you will see that you could remove the arrows, the light rays could be going in either direction; it makes no difference.

    If the diagram were accurate, you would have a light ray following path (minus)B, but it is now falling straight into a gravitational well without being deflected, and then inexplicably deflecting inward as it is leaving the well.

    In essence, for the diagram to be correct, the laws of physics would have to behave differently depending on whether light is traveling to the left or to the right. :bugeye:
    Last edited: May 9, 2010
  10. May 10, 2010 #9


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    1. In GR, light's path in spacetime is a geodesic. Geodesics are paths particles take when there is no force acting on them. So (1) is true.

    2. In static circumstances, it is possible to mathematically split spacetime unambiguously into 3D "space" and 1D "time".
    The "time" part of spacetime curvature describes Newtonian gravitation (at least for weak fields). Its contribution is therefore identical to a Newtonian result (2). This contributes 1/2 of the total deflection. Note that the difficulty of Newtonian physics - whether a photon has mass or not - is irrelevant in GR.
    The "space" part is curved also, this contributes the other half of the observed effect.

    3. You can choose http://en.wikipedia.org/wiki/Schwar....29_formulations_of_the_Schwarzschild_metric".
    Last edited by a moderator: Apr 25, 2017
  11. May 10, 2010 #10
    I think particle needs to be without mass to travel at the speed of light like gauge bosons, gluons, photons and i am not sure about neutrinos. But they are at quantum level. So the photons path curves is because the object with mass curves the space-time and the photom follows the curved space. Also if gravitons exist they should travel at the speed of light (i don't think so). So trough the famous equation E=mc^2 you can't have mass and travel at the speed of light...the more the mass the less the maximum speed. Of course in theory objects with mass can travel at the speed of light trough warping the space time with huge energy also known as warp drive.
  12. May 11, 2010 #11
    Although my questions on this topic have been answered I do like thought experiments. And thus I would like to discus the path of light as it passes a body of mass in the incorrect model of refraction. Below was my reasoning for the refraction diagram.

    First all let us look at Snells Law as it applies to various densities increasing and then decreasing. Here we see the 'S' shape appear. In this model the path of light is reversible.


    Lets move a little closer to the 3D space time refraction model. Here we see the densities in concentric spheres. (In "reality" the density spheres are not separate and exists as a single increasing gradient of density).


    Watching light enter this spherical environment we can see that angle 'a' is not equal to angle 'b'. This is because the path bends toward the increasing density on the way in and away from the increase on the way out. Thus light paths are not reversible. Furthermore, the light spends more time going through density on the way in than it does on the way out resulting in the final direction not being parallel with the entry direction.

    Do not the spherical properties combined with the parabolic increase of density account for these peculiarities?
  13. May 15, 2010 #12


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    Your diagram definitely clear things up; it is now consistent with refraction. Note that the light path is indeed still reversible. If you change the direction of the arrows, it would work... IF light followed your refractive hypothesis.

    But I think your example is contrived. If you add a few more light rays, I think you'll find that this hypothesis diverges from observed measurements.
  14. May 16, 2010 #13


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    No, it isn't. Watch the angles.
    If the model were consistent with refraction, it would work.
  15. May 16, 2010 #14


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    Dang! I looked at it several times! Yeah, it still doesn't work in the reverse direction.
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