Question about noether's theorem

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SUMMARY

The discussion centers on the conditions under which a coordinate transformation with a Jacobian of 1 qualifies as a symmetry transformation in the context of unimodular gravity. It is established that if the Lagrangian density is independent of coordinates and only depends on fields and their first derivatives, the action remains invariant under such transformations. This invariance leads to the conservation of the traceless part of the stress-energy tensor, confirming the transformation as a gauge symmetry of unimodular gravity.

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The discussion is beneficial for theoretical physicists, particularly those focused on gauge theories, Lagrangian mechanics, and the study of unimodular gravity. It is also relevant for students and researchers interested in the foundational aspects of symmetry in physics.

geoduck
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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.
 
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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.
 
jambaugh said:
Yes, I believe this is the gauge symmetry of unimodular gravity. The corresponding conserved quantity is the traceless part of the stress-energy tensor.

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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