Why are Pauli Matrices Invariant under Rotation?

In summary, the Pauli Matrices remain invariant under a rotation because the rotation operator is defined in terms of the exponential of a Pauli matrix and an arbitrary unit vector, allowing for rotation of both the spinor indices and the vector index. This can be seen in Sakurai 3.2.44.
  • #1
shehry1
44
0

Homework Statement


Can anyone tell me why Pauli Matrices remain invariant under a rotation.


Homework Equations


Probably the rotation operator in the form of the exponential of a pauli matrix having an arbitrary unit vector as its input. It may also be written as:
I*Cos(x/2) - i* (pauli matrix).(unit vector) * Sin(x/2) where x is the angle of rotation.

See Sakurai 3.2.44


The Attempt at a Solution

 
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  • #2
shehry1 said:

Homework Statement


Can anyone tell me why Pauli Matrices remain invariant under a rotation.


Homework Equations


Probably the rotation operator in the form of the exponential of a pauli matrix having an arbitrary unit vector as its input. It may also be written as:
I*Cos(x/2) - i* (pauli matrix).(unit vector) * Sin(x/2) where x is the angle of rotation.

See Sakurai 3.2.44


The Attempt at a Solution


Pauli Matrices are just matrices... they are just arrays of numbers. They don't rotate.
 
  • #3
It's because you need to rotate both the spinor indices AND the vector index; let
U = I*Cos(x/2) - i* (pauli matrix).(unit vector) * Sin(x/2) where x is the angle of rotation, and let R_ij be the corresponding matrix that would rotate a vector by the angle x about the unit vector. Then

sigma_i = R_ij (U sigma_j U^dagger)

where j is summed and the spinor indices are suppressed.
 

FAQ: Why are Pauli Matrices Invariant under Rotation?

What are Pauli matrices and how are they related to rotation?

Pauli matrices are a set of three 2x2 matrices named after physicist Wolfgang Pauli. They are used in quantum mechanics to represent the spin of particles. Under rotation, these matrices can be used to describe the change in spin orientation of a particle.

How do Pauli matrices transform under rotation?

Under rotation, Pauli matrices transform according to the rotation matrix. This means that the new matrix is equal to the product of the rotation matrix and the original Pauli matrix.

What is the significance of Pauli matrices under rotation?

Pauli matrices are important in quantum mechanics because they represent the fundamental property of spin. Under rotation, they allow us to describe how the spin of a particle changes.

How do Pauli matrices relate to quaternions?

Pauli matrices are closely related to quaternions, which are mathematical objects used to describe rotations in three-dimensional space. The three Pauli matrices can be used to create a set of quaternion units, which can then be used to represent rotations.

Can Pauli matrices be used to describe rotations in higher dimensions?

Yes, Pauli matrices can be extended to higher dimensions. In 3D space, we use three Pauli matrices, but in higher dimensions, we would use more matrices to represent rotations. However, the same principles for transformation and relation to quaternions still hold.

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