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I have already been familiar with that exterior symmetry is the spacetime symmetry. Such kind of symmetry has been depicted by Poincare' group.

Then I am still find the concept internal space/internal symmetry ambiguous.

And I cannot understand why put Spin in exterior symmetry not internal symmetry, since there is no space-time variation for Spin.

And I have heard someone told me that exterior symmetry has something to do with U(1) group. I cannot understand it. I think exterior symmetry is associated with Poincare' group.

The last question, are the two concepts internal symmetry and intrinsic symmetry of the same meaning?

Thanks very much.

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# What is internal space/internal symmetry?

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