Originally Posted by Taoy
Whilst we're here, where does the condition {({x^1})^2 + ({x^2})^2+ ({x^3})^2}=1 come from?
In this forum we have succefully showed the double covering of SO(3) by SU(2).
SO(3) is the group of rotation in 3 dimentions.
But a rotation can be represented either by as orthogonal matrices with determinant 1 or by axis and rotation angle
or via the unit quaternions and the map 3-sphere to SO(3) or Euler angles.
Let's chose quaternions...
Every quaternion z = a + bi + cj + dk can be viewed as a sum a + u of a real number a
(called the “real part” of the quaternion) and a vector u = (b, c, d) = bi + cj + dk in R^{3} (called the “imaginary part”).
Consider now the quaternions z with modulus 1. They form a multiplicative group, acting on R^{3}.
Such quaternion can be written z = \cos(\frac{1}{2} \alpha)+\sin(\frac{1}{2} \alpha)\xi
which look like joe equation U = e^{\frac{1}{2} B} = \cos(\frac{1}{2} \theta) + b \sin(\frac{1}{2} \theta)
and \xi being a normalized vector... Does Lie group generators normalized !?
Like any linear transformation, a rotation can always be represented by a matrix. Let R be a given rotation.
Since the group SO(3) is a subgroup of O(3), it is also Orthogonal.
This orthonormality condition can be expressed in the form
R^T R = I
where R^T denotes the transpose of R.
The subgroup of orthogonal matrices with determinant +1 is called the special orthogonal group SO(3).
Because for an orthogonal matrix R: det(R^T)=det R which implies (det R)^2=1 so that det R = +1 or -1.
But The group SU(2) is isomorphic to the group of quaternions of absolute value 1, and is thus diffeomorphic to the 3-sphere.
We have here a map from SU(2) onto the 3-phere (then parametrized the coordinates by means of angles
\theta and \phi) (spherical coordinates)
Actually unit quaternions and unit 3-phere S(3) described almost the same thing (isomorphism).
Because the set of unit quaternions is closed under multiplication, S(3) takes on the structure of a group.
Moreover, since quaternionic multiplication is smooth, S(3) can be regarded as a real Lie group.
It is a nonabelian, compact Lie group of dimension 3.
A pair of unit quaternions z_l and z_r can represent any rotation in 4D space.
Given a four dimensional vector v, and pretending that it is a quaternion, we can rotate the vector v like this:z_lvz_r
By using a matrix representation of the quaternions, H, one obtains a matrix representation of S3.
One convenient choice is :
x^1 + x^2i + x^3j + x^4k = \left[\begin{array}{cc}x^1+i x^2 & x^3 + i x^4\\-x^3 + i x^4 &x^1-i x^2\end{array}\right]
which can be related of some sort !? to Garrett matrix...
T = x^i T_i = \left[\begin{array}{cc}i x^3 & i x^1 + x^2\\i x^1 - x^2 & -i x^3\end{array}\right]
Garrett, I have 2 question for you:
What is the website of your last publications (quaternions and others) ?
and
Since unit quaternions can be used to represent rotations in 3-dimensional space (up to sign),
we have a surjective homomorphism from SU(2) to the rotation group SO(3) whose kernel is { + I, − I}.
What does "whose kernel is { + I, − I}" mean ?