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Thread: Is there a preferred reference frame?

  1. #1 Is there a preferred reference frame? 
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    Special relativity tells us that all inertial frames are equally valid. And when it comes to acceleration the acceleration is measured with respect to any inertial frame. My question is, that given at any point in space there is only one truely inertial frame (the one that is in free-fall under local gravity conditions), does this form a preferred frame of reference?
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  2. #2  
    Senior Member AlexG's Avatar
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    And when it comes to acceleration the acceleration is measured with respect to any inertial frame
    No. Acceleration is not relative, it's absolute.
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    Mmmm, absolute with respect to what? You have to be accelerating with respect to something.
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    Senior Member AlexG's Avatar
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    Acceleration takes place with respect to everything. That's what absolute means. SR does not deal with acceleration, that's the realm of GR.
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  5. #5  
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    Acceleration is with respect to EVERY inertial frame. However for any point in space there is ONLY ONE true inertial frame. That is the frame that has no gravity, the frame that is in free-fall.
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  6. #6  
    KJW
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    Quote Originally Posted by Jilan View Post
    (the one that is in free-fall under local gravity conditions)
    This does not uniquely specify an inertial frame of reference. Consider two objects dropped from different heights. At a given height, both objects will be in free-fall, yet have different velocities.
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    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  7. #7  
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    Quote Originally Posted by Jilan View Post
    Mmmm, absolute with respect to what? You have to be accelerating with respect to something.
    You measure acceleration with an accelerometer. Its readings do not depend on the identification of a specific frame. If you are in a rocket and you throttle up, you will feel an acceleration, yes? Would your sensation change if the reference frame changed from Mars to Ceti Alpha V?
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    Quote Originally Posted by AlexG View Post
    SR does not deal with acceleration, that's the realm of GR.
    Actually, that isn't correct. SR is perfectly able to describe accelerated frames as well, it is just that those frames are not related via Lorentz transformations :

    Acceleration in Special Relativity
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  9. #9  
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    Quote Originally Posted by tk421 View Post
    You measure acceleration with an accelerometer. Its readings do not depend on the identification of a specific frame. If you are in a rocket and you throttle up, you will feel an acceleration, yes? Would your sensation change if the reference frame changed from Mars to Ceti Alpha V?
    At maximum thrust I guess it would feel just the same, but depending on the mass of Ceti Alpha you may not actually be going anywhere!
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    Quote Originally Posted by Markus Hanke View Post
    Actually, that isn't correct. SR is perfectly able to describe accelerated frames as well, it is just that those frames are not related via Lorentz transformations :

    Acceleration in Special Relativity
    I like this. "In General Relativity all motion is relative"
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  11. #11  
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    Quote Originally Posted by Jilan View Post
    At maximum thrust I guess it would feel just the same, but depending on the mass of Ceti Alpha you may not actually be going anywhere!
    You miss the point entirely, it would seem (perhaps you're joking, but I can't separate that from a lack of understanding). Your accelerometer reading does not in any way depend on declaring that your reference frame is here or Ceti Alpha.
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  12. #12  
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    Quote Originally Posted by Jilan View Post
    Special relativity tells us that all inertial frames are equally valid. And when it comes to acceleration the acceleration is measured with respect to any inertial frame. My question is, that given at any point in space there is only one truely inertial frame (the one that is in free-fall under local gravity conditions), does this form a preferred frame of reference?

    For every point in space, there are a number of trajectories for freely falling particles that will pass through this point. All the freely falling frames that can be attached to these particles seem to be Lorentz transformations of each other at the given point at the same time.

    I think you may have wanted to ask about falling frames at a space-time event, instead.
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