# Lie group Homomorphism $SU(2) \to SO(3)$

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The Lie algebra of $\mathfrak{so(3)}$ and $\mathfrak{su(2)}$ are respectively

$$[L_i,L_j] = i\epsilon_{ij}^{\;\;k}L_k$$ $$[\frac{\sigma_i}{2},\frac{\sigma_j}{2}] = i\epsilon_{ij}^{\;\;k}\frac{\sigma_k}{2}$$

And of course, there is an isomorphism between these two algebras, $$\Lambda : \mathfrak{su(2)} \rightarrow \mathfrak{s0(3)}$$ such that $\Lambda(\frac{\sigma_i}{2}) =L_i$

Now is it possible, using $\Lambda$, to construct a group homomorphism between $SU(2)$ and $SO(3)$?

I was checking up on Lie group homomorphism, and in Wikipedia, there is a beautiful image

In this image's language, how are $\phi$ and $\phi_*$ related to each other (just like the algebra and group elements are).

Note : I know there is a one-to-two homomorphism between these two groups which can be directly found using the group elements. I am not looking for this.

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user user35952
In this language, $\phi_\star$ is essentially the differential of $\phi$ (at identity), sometimes also called the pushforward (this is what the star in the index means). en.wikipedia.org/wiki/Pushforward_(differential) This question is probably more appropriate at math.SE.

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user Heidar

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First notice that the generators are $-i\sigma_k/2$ and $-iL_k$, since the groups are real Lie groups and thus the structure tensor must be real.

The answer to your question is positive. In principle it is enough to take the exponential of the Lie algebra isomorphism and a surjective Lie group homomorphism arises this way $\phi : SU(2)\to SO(3)$: $$\phi\left(\exp\left\{-\sum_k t^k i\sigma_k/2\right\}\right) =\exp\left\{-\sum_k t^k iL_k\right\}\:.$$ The point is that one should be sure that the argument in the left-hand side covers the whole group. For the considered case, this is true because $SU(2)$ is compact.

If you instead consider no compact Lie groups, like $SL(2,\mathbb C)$, the exponential does not cover the group. However it is possible to prove that products of exponential do. In that case a product of two exponentials is sufficient, in practice decomposing an element of $SL(2,\mathbb C)$ by means of the polar decomposition, mathematically speaking, or as a (unique) product of a rotation and a boost physically speaking.

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user V. Moretti

answered Apr 1, 2014 by (2,075 points)
edited Apr 2, 2014
Thanks !! But I wish to know what will be the functional form of this $\phi$. I am pretty new to this subject, can you please be more explicit on that part ?

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user user35952
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So I take it you are clearly aware that the big A Adjoint representation is the homomorphism you're after in this case, so you're seeking a more general method.

Also, I'm assuming you know that the homomorphism of Lie algebras can only lift to a group homomorphism if the homomorphism's domain is simply connected, in which case there is a unique group homomorphism with the given algebra homomorphism as its Lie map. In this case, we're in the clear because $SU(2)$ is simply connected. Page 73 through 76 of:

can then help you. Knapp gives you two methods of systematically constructing the simply connected Lie group: the first leaves you with differential equations for the left / right invariant vector fields, the second I believe is the same as V. Moretti's Answer.

A final "method" is to used Ado's theorem, which assures us that we can always realise a Lie algebra as a matrix Lie algebra; there is even an explicit software algorithm for this:

W. A. De Graaf, "Constructing Faithful Matrix Representations of Lie Algebras"

but if you can understand this algorithm, you are doing better than I (this paper has so far defeated me). Once you have a matrix algebra, you can used the matrix exponential to construct a neighbourhood of the identity, indeed the whole group if the latter is compact; as in V. Moretti's Answer the Lie algebra does not exponentiate to the whole group for noncompact groups (as far as I am aware, the problem of exactly what in a noncompact Lie group can be realised as an exponential of a Lie algebra element is to some extent still an open problem).

So, once you have the Lie group, you can in principle construct the universal cover with homotopy classes and carve out the discrete centre $\mathcal{Z}_d$ of the universal cover. Your original group will have as its fundamental group the quotient group of $\mathcal{Z}_d$ and one of its (normal) subgroups.

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user WetSavannaAnimal aka Rod Vance
answered Apr 1, 2014 by (485 points)
De Graaf is a colleague of mine! Some doors next to mine.

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user V. Moretti
@V.Moretti Tell him I still tackle his paper from time to time but still leave off badly bruised! No slight intended on his technical writing abilities: an actual implementation of an Ado theorem algorithm is not an easy thing to convey, I'm likely not the sharpest audience and he's (de Graaf) has certainly done some very beautiful work.

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user WetSavannaAnimal aka Rod Vance
OK, I will tell him all that you wrote. Bye

This post imported from StackExchange Physics at 2014-04-01 17:33 (UCT), posted by SE-user V. Moretti

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