# Tag Info

### What is the advantage of using spherical tensor over cartesian tensor?

The Cartesian tensor is just $3\times 3=9$ values tied to an arbitrary coordinate system, and they get scrambled by rotations. While as you point out, spherical tensors are irreducible, and that means:...
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### What is the advantage of using spherical tensor over cartesian tensor?

The Coulomb potential is spherically symmetric. As a consequence, the wave functions of electrons orbiting a nucleus contain spherical harmonics. Therefore it is better to work in a spherical basis.
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### Confusion on the definition of dielectric permittivity

Dielectric permittivity quantifies how much do the dipoles inside a material want to get polarised ie how much do they want to align with an applied electric field such that the induced electric ...
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### Does the creation operators for photons with different polarization commute?

The commutation relations for the quantized EM field are \left[a_\mathbf{p}^s, a_\mathbf{p'}^{s'\dagger}\right] = \delta^{ss'}\delta^{(3)}(\mathbf{p} - \mathbf{p}')\,, \quad \left[a_\mathbf{p}^s, a_\...
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