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'Real' spacetime in GR?

  1. Apr 26, 2010 #1
    The wikipedia article titled "Geodesics & General Relativity" opens with:

    "the path of a planet orbiting around a star is the projection of a geodesic of the curved 4-D spacetime geometry around the star onto 3-D space."

    Uhhhh.... "Real" spacetime has four dimensions - 3 space; 1 time. To map the precession of Mercury in "real" spacetime, GR practitioners MUST be projecting GR's "curved" spacetime onto "real" spacetime.

    No?
     
  2. jcsd
  3. Apr 26, 2010 #2

    Dale

    Staff: Mentor

    What do you mean by '"real" spacetime'? I have never heard such a term.
     
  4. Apr 26, 2010 #3

    bapowell

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    Science Advisor

    All they're saying here is that the planet traces out a path in 4D spacetime. The path of the planet -- the spatial route taken by the planet -- is the projection of this 4D geodesic onto the 3D (spatial) hypersurface.
     
  5. Apr 26, 2010 #4
    I likewise never heard the term until I searched for words to understand Wikipedia. Maybe they are just wrong. Curved 4D spacetime maybe is not mapped anywhere because we are actually living in it?
     
  6. Apr 27, 2010 #5

    Nabeshin

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    Science Advisor

    Right, but we perceive events as a sequence of 3-dimensional slices of 4-dimension spacetime, strung together by observations at different time values. So the 3-d slices we perceive are the projection of the full four-dimensional worldline into the three spatial dimensions.
     
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