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Fine Structure Constant

  1. May 3, 2007 #1
    Scientists are doing experiments to find any change in the Fine Structure constant in the past.

    It is dependent on the following constants.

    C (Velocity of Light)
    e (Charge of the electron)
    E0 (Permittivity of free space)
    h (plancks constant)

    Could you pls let me know which of the above constants would have been different in the past which inturn would have made fine structure constant different ?
  2. jcsd
  3. May 3, 2007 #2


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    I worked on varying alpha for my honours thesis some years ago. I had the same question as you at the time. As it turns out the answer is somewhat subtle. It turns out that it you construct a theory in which the speed of light varies then it can be shown to be equivalent to a theory in which say the electron charge varies but c stays constant. It sounds odd and I must admit I do not fully understand the details of this, but it has been demonstrated that this is the case (theoretically of course!).

    The moral of the story is that it turns out the only thing that matters is changes in dimensionless constants, such as the fine structure constant. A change in a dimensionfull constant (c,e etc) can always be reconstructed in ambiguous ways, since units are defined by humans. But changes in dimensionless ratios indicates a real variation in physics.

    Sorry I can't be any more specific as to the whys, it was always a question I though about when I was working on this and never fully grasped. I will see if I can find some references for you.
  4. May 4, 2007 #3


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    It's very difficult to devise a test where you can detect a change in a dimensionless constant. Any such variation messes with whatever yardstick you use to measure it.
  5. May 4, 2007 #4
    John Barrow seems to have done plenty of work in the field of varying constants. I've read quite a lot of work investigating a possible change in the speed of light (though it must be pointed out that if there is a change in alpha it now looks to be extremely small).

    Here are some interesting papers on varying constants:

    http://arxiv.org/PS_cache/astro-ph/pdf/0511/0511440v1.pdf (Barrow, J.D.)
    http://arxiv.org/PS_cache/gr-qc/pdf/0209/0209080v1.pdf (Mota, David.F.,Barrow, J.D)

    And those specific to varying alpha:

    http://arxiv.org/PS_cache/astro-ph/pdf/0309/0309273v2.pdf (Barrow,J.D)
    http://arxiv.org/PS_cache/astro-ph/pdf/0202/0202374v1.pdf (Maguiejo,J. Barrow, J.D, Sandvik, H.)

    The cosmological implications of a varying alpha are also worth looking at.
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