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Question about noether's theorem

  1. Apr 9, 2013 #1
    If you have purely a coordinate transformation whose Jacobian equals 1, and your Lagrangian density has no explicit coordinate dependence (just a dependence on the fields and their first derivatives), then is it true that the transformation is a symmetry transformation?

    It looks like it is, that under these conditions the action is unchanged. I just want some verification.
     
  2. jcsd
  3. Apr 10, 2013 #2

    jambaugh

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    Yes, I believe this is the gauge symmetry of unimodular gravity. The corresponding conserved quantity is the traceless part of the stress-energy tensor.
     
  4. Apr 10, 2013 #3
    Thanks.

    It seemed trivially true to me that the action is unchanged under any pure coordinate transformation so long as your Lagrangian is not coordinate dependent, but I had forgotten about the Jacobian, and the Jacobian allows you to get a non-trivial result.

    When I realized I forgot the Jacobian, then it occurred to me that if the transformation has Jacobian 1, then the trivial result would still follow. Of course translations and rotations have Jacobian 1 and the action is unchanged leading to conservation of energy and angular momentum, but it seems you can dilate each coordinate in such a way that the Jacobian is still 1.
     
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