Spin tensor

The Noether current for translations in space is momentum, while the current for increments in time is energy. These two statements combine into one in spacetime: translations in spacetime, i.e. a displacement between two events, is generated by the four-momentum P. Conservation of four-momentum is given by the continuity equation:

${\displaystyle \partial _{\nu }T^{\mu \nu }=0\,,}$

where ${\displaystyle T^{\mu \nu }\,}$ is the stress–energy tensor, and ∂ are partial derivatives that make up the four-gradient (in non-Cartesian coordinates this must be replaced by the covariant derivative). Integrating over space:

${\displaystyle \int d^{3}xT^{\mu 0}\left({\vec {x}},t\right)=P^{\mu }}$

gives the four-momentum vector at time t.

The Noether current for a rotation about the point y is given by a tensor of 3rd order, denoted ${\displaystyle M_{y}^{\alpha \beta \mu }}$. Because of the Lie algebra relations

${\displaystyle M_{y}^{\alpha \beta \mu }(x)=M_{0}^{\alpha \beta \mu }(x)+y^{\alpha }T^{\beta \mu }(x)-y^{\beta }T^{\alpha \mu }(x)\,,}$

where the 0 subscript indicates the origin (unlike momentum, angular momentum depends on the origin), the integral:

${\displaystyle \int d^{3}xM_{0}^{\mu \nu }({\vec {x}},t)}$

gives the angular momentum tensor ${\displaystyle M^{\mu \nu }\,}$ at time t.

The spin tensor is defined at a point x to be the value of the Noether current at x of a rotation about x,

${\displaystyle S^{\alpha \beta \mu }(\mathbf {x} )\mathrel {\stackrel {\mathrm {def} }{=}} M_{x}^{\alpha \beta \mu }(\mathbf {x} )=M_{0}^{\alpha \beta \mu }(\mathbf {x} )+x^{\alpha }T^{\beta \mu }(\mathbf {x} )-x^{\beta }T^{\alpha \mu }(\mathbf {x} )}$

The continuity equation

${\displaystyle \partial _{\mu }M_{0}^{\alpha \beta \mu }=0\,,}$

implies:

${\displaystyle \partial _{\mu }S^{\alpha \beta \mu }=T^{\beta \alpha }-T^{\alpha \beta }\neq 0}$

and therefore, the stress–energy tensor is not a symmetric tensor.

The quantity S is the density of spin angular momentum (spin in this case is not only for a point-like particle, but also for an extended body), and M is the density of orbital angular momentum. The total angular momentum is always the sum of spin and orbital contributions.

The relation:

${\displaystyle T_{ij}-T_{ji}}$

gives the torque density showing the rate of conversion between the orbital angular momentum and spin.

Examples of materials with a nonzero spin density are molecular fluids, the electromagnetic field and turbulent fluids. For molecular fluids, the individual molecules may be spinning. The electromagnetic field can have circularly polarized light. For turbulent fluids, we may arbitrarily make a distinction between long wavelength phenomena and short wavelength phenomena. A long wavelength vorticity may be converted via turbulence into tinier and tinier vortices transporting the angular momentum into smaller and smaller wavelengths while simultaneously reducing the vorticity. This can be approximated by the eddy viscosity.

• Belinfante–Rosenfeld stress–energy tensor
• Poincaré group
• Lorentz group
• Relativistic angular momentum
• Center of mass (relativistic)
• Mathisson–Papapetrou–Dixon equations
• Pauli–Lubanski pseudovector

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