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Question re GP-3 orbiting gyroscopes test of relativity

  1. May 13, 2003 #1


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    this is the website for "GravityProbe B" the test of General Relativity
    using orbiting gyroscopes

    someone told me that GP-B is already flying but
    at this site it says that shipment to Vandenburg
    for launch is scheduled for June 26, 2003.

    Does anyone know of this planned test of relativity?
    Does anyone know if launch has occurred or if not (as
    I suppose) when it is likely to take place.

    The spacecraft seems to be finished---have seen photos.
    It carries four hi-tech (cryogenic?) gyroscopes and
    extremely precise sensors to detect any tilt of axis
    due to framedragging or whatever.

    Can anyone explain this planned test of relativity
    in layman's terms. Leonard Schiff, who thought it up,
    first proposed it back in the Seventies. But it is technically
    very challenging and has taken something like 30 years
    to prepare. Apparently it tests some aspect of GR which
    has not been tested and is capable of distinguishing between
    Einsteins version of gravity and some possible alternative
  2. jcsd
  3. May 13, 2003 #2


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    As you probably know, Einstein's theory of general relativity proposes that gravity is not so much the force as a geometric shape of space itself. You have probably seen the demonstrations of a bowling ball, or some heavy sphere, rolling across the surface of the trampoline. This is a fairly good intuitive picture of what Einstein proposed was happening to space that is occupied by an object with mass.

    However, Einstein's model would be even better illustrated by showing a ball rolling across a pool of mercury. That is, GR pictures space as more fluid. Set a massive object upon it, and it curves. If that object is spinning, the space immediately around the object spinns with it. Gravity Probe-B will see if space itself is slowly rotating around our planet, being "dragged" by the spin of the earth.
  4. May 18, 2003 #3
    Hope this is not considered "thread hijacking"! ;) But, it doesn't seem like the thread is going anywhere ...

    This is about my most favorite rant .. Mach's Principle. I hate it, and don't believe it -- That the cause of inertia is the combined gravitational attraction of all objects in the observable Universe (given that the speed of gravitation is the speed of light).

    Another manisfistation of Mach's Principle is what we are to believe that when we are spinning around-and-around. That smaks of absolute motion, i.e., we don't need to observe any other object in the Universe (or Relativity) to discern that it is us making the motion, and not the Universe. Mach's Principle would have us to believe it is just as proper to say the Universe is spinning around us, and we are motionless. Blllleeeeeeeeeewww!

    Anyway, the tie in to this thread -- I propose GP-C. I doubt it is technologically feasible, but take the gyroscopes outside the solar system somewhere where it is not so much influenced by local gravitational fields. Take two of them, in different directions. If Mach's Principle is true, shouldn't both of the gyroscopes align themselves with the distribution of matter in the Universe? Shouldn't they precess to the same direction, statistically a direction different to what we (Earth) are flowing through the CMBR, if Mach's Principle is true (i.e., the distribution of matter in the Universe is not as even as the CMBR).

    Another rant against Mach's Principle. This may seem "duh", but think about it for awhile. Why are centrifugal forces manisfested only in 2 dimensions, not 3? We are to believe it is the distribution of matter in the Universe that lets us know we ourself are spinning, yet that distribution makes a difference only in 2 dimensions. You might say, "duh, you are only spinning in 2 dimensions", but spinning in 2 dimensions precludes you from spinning in 3 dimensions (neglecting precession). That 3rd dimension, and the distribution of matter in that 3rd dimension only seems to matter when you are spinning on an axis parallel to that dimension. So, how is the distribution of matter in the Universe causing me to know I'm spinning, when 1 spatial dimension is not being considered?
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