Monday, January 28, 2019

mathematical physics - Subtlety in the proof of 2-to-1 homomorphism between SU(2) and SO(3)


In physics, it's common to use the relations r=Rr;  and  rσ=U(rσ)U

to establish a two-to-one homomorphism between SU(2) and SO(3) where rR3, RSO(3), USU(2) and σ=(σ1,σ2,σ3) are three Pauli matrices. Both the relations of Eq.(1) represent rotation of coordinates in real three-dimensioanl space because both of them satisfy |r|2=|r|2. It's easy to see from (1) that corresponding to every 3×3 matrix RSO(3) there exist two 2×2 matrices ±USU(2) that represent the same rotation.


Question Note that the above proof of 2-to-1 homomorphism is based on fundamental representations of SO(3) and SU(2). But for any odd-dimensional representation of SU(2), if U has determinant +1, U is not a representation of SU(2) since it has determinant 1. Hence, if U is a member of an odd-dimensional representation of SU(2). U is not. Does it mean that 2-to-1 homomorphism between SU(2) and SO(3) is not true in general?





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