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Light always wants to travel in a straight line

  1. May 5, 2005 #1
    this was posted before, but I was told to put it in this section...

    Light always wants to travel in a straight line, however, the space is curved around massive objects such as black holes, so it would seem as if the light was being bent as it traveled around the black hole. The space around the earth is curved.. so satellies travelling along parallel paths can actually collide with eath other right?
  2. jcsd
  3. May 5, 2005 #2
    i think your right, there's this thing i read where you can see a star which is actually behind another bodies which should block it, but because space curves, the light of the star still reaches us.
  4. May 5, 2005 #3


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    All earth satellites travel in elliptic (usually close to circular) orbits with the earth's center at a focus. Therefore these orbits will always lie in planes that intersect each other. The orbits will cross if they happen to be at the same distance from the center of the earth.
  5. May 5, 2005 #4


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    1] The space in Earth's vicinity is only veeeerrrry slightly curved; Earth is not very massive. A beam of light passing Earth will be deflected by an amount almost to small to detect with the finest of instruments.
    2] Satellites have paths that form 'great circles' around the Earth; there are no satellites that travel parallel to each other. Yes, they will collide (eventually) *IF* they are at the same altitude. It's pretty hard to do though. Space - even the space around Earth - is pretty big.
  6. May 5, 2005 #5
    yes, if they are at the same altitude, they will collide eventually. But can the collide if they are on parallel paths? Are you saying that the curvature is so small that they will not collide?
  7. May 5, 2005 #6


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    What does it mean for two elliptical orbits to be "parallel"? Also, are you talking about the paths of their orbits through space, or the path of their worldlines through spacetime? General relativity doesn't say objects take the shortest path through curved space, it says they take the path through curved spacetime with the greatest proper time (time as measured by a clock that follows that path through spacetime).
  8. May 6, 2005 #7
  9. May 6, 2005 #8


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    But it's not possible for orbits to be parallel like that. If you draw the orbits as ellipses, each ellipse should have one focus as the center of the earth, while the parallel ellipses you drew don't share either focus.

    edit: Actually, I'm confused by what you mean when you posted that picture. There are four sets of ellipses there, in two different planes, one ellipse in each plane being drawn with a solid line and a smaller ellipse drawn with a dotted line inside. I thought you meant "parallel" like ellipses in different planes are parallel, but then you said something about dotted vs. solid lines, so maybe you want me to ignore the extra plane and just consider a dotted ellipse inside a solid one? Either way, it would be better if you could find a picture without the extraneous stuff...
    Last edited: May 6, 2005
  10. May 6, 2005 #9
    No. It wants to move on a geodesic. That's a worldline of extremal length. It is spacetime that is curved around a black hole, not simply space. The spatial portion of the worldline of light does bend.

    No. They can't move on parallel paths if they are moving freely.

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