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What happens to the shape of an accelerating mass

  1. Aug 19, 2012 #1
    Hi:

    When a mass is accelerated, what happens to its length and volume? I know that when the acceleration ends, its length is governed by the equation l = l0 x sqrt (1 - v^2/C^2), but what about during the acceleration?
     
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  3. Aug 19, 2012 #2

    bcrowell

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    Time dilation and length contraction only depend on velocity, not acceleration. Some people call this the "clock postulate" for time dilation.
     
  4. Aug 20, 2012 #3

    HallsofIvy

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    The "shape" at any instant during acceleration is given by the velocity of the object at that instant.
     
  5. Aug 20, 2012 #4
    Thanks guys. Taking the case of a neutron moving with velocity v, would its radius in the direction of v shorten while its radius perpendicular to v increases? It seems if this is so, then the neutron would assume an ellipsoid shape, more specifically an oblate spheroid shape, until it would become a disk at some point. Is this correct?
     
  6. Aug 20, 2012 #5

    bcrowell

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    The radius perpendicular to the direction of motion is unchanged.
     
  7. Aug 20, 2012 #6

    PAllen

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    Yes, such appearance as a disk has been dubbed a 'pancake', and, I believe Richard Feynman was the first to describe that the collision data for energetic electrons and nucleons could be explained on the assumption that the nucleon appeared as a pancake to the electron. This modelling assumption was part of his famous parton research which explained key behavior of the quark model of nucleons. Without modelling the nucleon as a pancake, the collision data could not be explained.
     
    Last edited: Aug 20, 2012
  8. Aug 21, 2012 #7
    Thanks Ben. How then does the mass increase without the perpendicular radius increasing? I am assuming the neutron is the most dense unit of mass possible.
     
  9. Aug 21, 2012 #8
    That is extremely interesting to me, but I am not surprised by it. My own thoughts have led me to this conclusion (that of pancaking mass) which would max out at C
     
  10. Aug 21, 2012 #9

    bcrowell

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    Most physicists these days don't use the relativistic mass convention that you're using, but anyway, the density of mass-energy is higher as seen in the frame where the neutron is moving.

    No, that's not true.
     
  11. Aug 23, 2012 #10
    Hi Ben. Tks for replying. Is the mass expansion and length contraction real then. I thought it was, from the observer's viewpoint. And what is more dense than a neutron?
     
  12. Aug 23, 2012 #11

    PAllen

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    "from an observer's point of view" does not mean "not real". Everything is measured from some observer's point of view. There isn't a "chosen" observer who's measurements are called "reality".

    A quark is more dense than a neutron. There is no limit to the density observed for a moving body, as it approaches ever close to c relative to the observer. However, this does not mean a black hole might be created:

    http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html
     
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