Relationship between proper orthochronous Lorentz group $SO^+(1,3)$ and $SU(2)\times SU(2)$, or their Lie algebras

Solution 1:

$SO^+(3,1)$ is the so-called restricted Lorentz group, which is the identity component of the Lorentz group $SO(3,1)$. It is a six-dimensional real Lie group, which is not simply connected.

1. Since $SO^+(1,3)$ is not compact, but $SU(2)\times SU(2)$ is compact, the groups cannot be isomorphic as real Lie groups.

2. We have $SO^+(3,1)\simeq SL(2,\mathbb{C})/\mathbb{Z}_2\simeq SU(2)_{\mathbb{C}}/\mathbb{Z}_2$, i.e., the complexification of the restricted Loretz group satisfies $$SO^+(3,1)_{\mathbb{C}}\simeq (SU(2)_{\mathbb{C}}\times SU(2)_{\mathbb{C}})/\mathbb{Z}_2.$$

3. In the same way, $\mathfrak{so}^+(3,1)_{\mathbb{C}}\simeq \mathfrak{su}(2)_{\mathbb{C}}\oplus \mathfrak{su}(2)_{\mathbb{C}}\simeq \mathfrak{sl}_2(\mathbb{C})\oplus \mathfrak{sl}_2(\mathbb{C})$.