SO(3) as a quotient group of SU(2)?

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    Group quotient So(3)
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Discussion Overview

The discussion revolves around the relationship between the groups SU(2) and SO(3), specifically exploring whether SO(3) can be considered a quotient group of SU(2) with respect to the subgroup {I, -I}. The scope includes theoretical aspects of group theory and manifold representations.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that SO(3) can be interpreted as a quotient group of SU(2) due to the two-to-one homomorphism from SU(2) to SO(3).
  • It is noted that {I, -I} is an abelian and normal subgroup of SU(2), which supports the idea of forming a quotient group.
  • Another participant argues that while SU(2)/{I, -I} is a quotient group, it has not been conclusively shown to be isomorphic to SO(3).
  • One participant mentions that the 3-sphere (S^3) is isomorphic to SU(2) and that the real projective space derived from S^3 under antipodal identification is isomorphic to SO(3), suggesting a relationship between the two groups.
  • There is a reference to the adjoint representation of SU(2) being identified with the fundamental representation of SO(3), which may imply a deeper connection.

Areas of Agreement / Disagreement

Participants express differing views on whether SO(3) is isomorphic to SU(2)/{I, -I}. While some support the quotient group interpretation, others challenge the assertion of isomorphism, indicating that the discussion remains unresolved.

Contextual Notes

There are unresolved aspects regarding the specific conditions under which the isomorphism holds, as well as the implications of the manifold representations mentioned.

wdlang
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we know there is a two to one homomorphism from SU(2) to SO(3)

suppose u is an element in SU(2)

then u and -u map into the same element in SO(3)

the question is, maybe SO(3) is a quotient group of SU(2)? with respect to the subgroup {I,-I}?
 
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Yes, SO(3) can be interpreted as a quotient group of SU(2).

{I,-I} is an abelian subgroup and it is a normal subgroup of SU(2). A quotient group G/H is always trivially related to a group G and a normal subgroup H of G.

Thus, indeed SO(3) equals (or is at least isomorphic to) SU(2) after dividing out by {I,-I}.

Another way to see it:

The 3-sphere (S^3) can be considered as a 'manifold' isomorphic to SU(2) and the real projective space of dimension 3, which is a quotient of S^3 under 'antipodal identification' is isomorphic to SO(3). Both are orientable manifolds.
 
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{I,-I} is an abelian subgroup and it is a normal subgroup of SU(2). A quotient group G/H is always trivially related to a group G and a normal subgroup H of G.

Thus, indeed SO(3) equals (or is at least isomorphic to) SU(2) after dividing out by {I,-I}.
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yes, you have shown that SU(2)/{I,-I} is a quotient group

but i think you did not show that it is isomorphic to SO(3).
 
Well, you didn't ask to show it.

I wrote: "The 3-sphere (S^3) can be considered as a 'manifold' isomorphic to SU(2) and the real projective space of dimension 3, which is a quotient of S^3 under 'antipodal identification' is isomorphic to SO(3). Both are orientable manifolds."

Working out the defining relations of any matrix in the fundamental representation of SU(2) will give us that SU(2) can be regarded as a 3-sphere. Doing the same for SO(3) also gives us that SO(3) can be regarded as a 3-sphere, however, an antipodal pair of points will give us the same element of SO(3).

Another way to see it: The adjoint representation of SU(2) can be identified with the fundamental representation of SO(3).
 

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