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Light, mass and gravity

  1. Jan 20, 2010 #1
    I have read about space and time being in a form of space plane, or at least that is a way of looking at it. When something has mass it puts a dent on this plane, and the more mass that object has then greater the dent in the space plane it creates.

    Now, this theory also read that when this dent is created, light follows it along. This could be explained as if you rolled a ball down a smooth surface that at one point had a dent in it, assuming the ball had enough energy, it would overcome it, and therefor rise and fall with this 'dent'.

    I had heard that light was the same, it follows the dents. This is the creator of this theories way of explaining why light is bent by gravity even though it has no mass. Can someone inform me if I am correct/incorrect, and if I am incorrect, can you tell me the current most likely theory.

    Thanks
    -Harry
     
  2. jcsd
  3. Jan 20, 2010 #2

    bcrowell

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    The right way to understand the rubber sheet analogy is not in terms of an external force that then makes things roll down the sheet. The right way to understand it is that it redefines the geometrical concept of what a straight line is. If the rubber was initially flat, and had a graph-paper grid drawn on it, then after stretching those lines would no longer be straight (i.e., no longer as straight as they can be while remaining on the surface of the rubber). With this interpretation, light follows the *new* straight lines, which have non-Euclidean properties, e.g., these newly redefined straight lines can intersect intersect to form banana shapes, which would represent gravitational lensing of light.
     
  4. Jan 20, 2010 #3
    I take it is the increase in mass that causes these "lines" to stretch. Or at least, an object with a higher mass and therefor a higher gravitational pull.
     
  5. Jan 20, 2010 #4

    bcrowell

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    You don't need the mass to be big, or increase, or anything like that. Every object has an effect on spacetime that is in proportion to its mass. The effects basically add (although for strong fields GR is nonlinear).

    I've started a physics FAQ here http://www.lightandmatter.com/cgi-bin/meki?physics/faq , and I have the following on this topic, which may be helpful.

    The rubber-sheet analogy says that we can visualize general relativity's description of a the gravitational field of an object such as the sun by imagining that the sun is like a heavy steel ball placed on an initially flat sheet of rubber. The ball makes the rubber sag. The rubber represents curved spacetime. Many people have trouble with this analogy because they then imagine other objects, e.g., the earth, then have to roll around on the sheet as if they were being pulled "downward" by the same external "downward" force that displaced the steel ball downward.

    This leads to incorrect results. For instance, it doesn't make it clear why we should expect light rays to be deflected by the sun's field. According to Newton, light isn't affected by gravity, so why should we expect it to have any tendency to roll "downward" into the depression on the rubber sheet under the influence of the external "downward" force?

    A better way to imagine this is that we start with a graph-paper grid made of spider silk. The sun is a big beetle caught in the web. The beetle tries to free itself, but the only result of its struggle is that by the time it dies, it has distorted the nearby matrix of the spiderweb. The notion of a straight line has been redefined. Light rays and material objects will now follow paths that are straight as measured by the distorted graph-paper grid. This has non-Euclidean consequences. For instance, two distinct lines can intersect at more than one point, and if these are world-lines of light rays, we interpret the effect as gravitational lensing.

    It's also important to realize that what is being described is a distortion of spacetime, not just a distortion of space. If this were not true, then for example an object's motion would not depend on its initial velocity, since two objects starting from the same point and going in the same direction would follow the same line through space.
     
  6. Jan 20, 2010 #5
    Thanks.
    I also know that things go slower in time relative to their speed and distance from the object of gravitational pull. Is this relevent to this theory.
    And also, Im having trouble with this theory as it states time an space on what appears to be, on the way you are describing it and the way I have seen, is that this sees to be on a two dimension. And there are 3 dimensions of space and one of time. SO are there lots of these planes and does an object bend these multiple planes to form an almost sphere effect, so light is bent around it on all ways. This would show why light bends all around an object.
     
  7. Jan 20, 2010 #6
    A slightly better model than the rubber sheet model is a similarly deformed sheet of plastic. Paths of particles can be indicated by placing sticky tape in as straight a line as possible on the curved surface. This way, it does not matter whether the deformation is a well or whole model is turned upside down so that deformation is a mountain. Tape placed on either the top or lower surface of the sheet basically follows the same path. This model is purely geometrical and does not require an external force. The sticky tape paths are basically the same as the path followed by a toy car with no steering. Even though this is an improvement, it is still far from the whole story. Some even better models that demonstrate curvature of space and time were given by robphy in post #4 and A.T. in post #21 of this https://www.physicsforums.com/showthread.php?t=141447".
     
    Last edited by a moderator: Apr 24, 2017
  8. Jan 20, 2010 #7
    Just do some research on the term "Geodesic".
     
  9. Jan 21, 2010 #8
    Thank you.
     
  10. Jan 21, 2010 #9
    Not sure if you're being sarcastic, but for the record I was being serious and not teasing you. Once you grasp basic geodisics in 2 dimensions you can generalize that concept to explain how light moves at a constant speed in its own Intertial Frame, but its PATH can be distorted. Rotational Frame Dragging (not to toot my own handle) is a fine place to research extreme spacetime geodesics all the way to the point where light doesn't STOP, but rather is diverted by losing any DoF because all geodesics lead to the singularity once you reach the Event Horizon of a Black Hole. "Ergosphere" "Gravitational Lensing" are also good places for complementary research.

    If I did offend, I apologize.
     
  11. Jan 21, 2010 #10
    I wasn't being sarcastic, I had not heard the term before. You didnt do any harm, sorry if it looked that way.
     
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