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7
votes
Accepted
Why helicity is proportional to the spin of particle and has two values?
Construction of the helicity formula using 3-vector notation
The zero component of the pauli Lubanski vector
$W^0 = \epsilon^{0 ijk}J_{ij}p_k = \epsilon^{ijk}J_{ij}p_k $
The angular momentum genr …
2
votes
Relativistic center of mass
This condition is due to the fact that for a free massless particle the Pauli-Lubanski vector $W=*(M\wedge P)$ must be proportional to the linear momentum (The proportionality factor being the helicit …
3
votes
Accepted
Spin state after boost
The boost matrix can be chosen in block form as (in $c=1$ units):
$$L(\mathbf{p}) = \begin{bmatrix}
\frac{E}{M}& \frac{\mathbf{p}^t}{M}\\
\frac{\mathbf{p}}{M}& 1_{(3\times3)}+\frac{\mathbf{p}\mathb …
9
votes
What is the difference between dynamical and geometric phases?
The total phase is a sum of the dynamical and geometric phases:
$$\phi = \int_0^T E(t) dt + \oint A_{\mu}(R) dR^{\mu}$$
Where $R^{\mu}$ are the coordinates of the parameter space, $E(t)$ is the instan …
19
votes
Accepted
Poincare group vs Galilean group
It is important to distinguish between three group actions that are named "Galilean":
-The Galilean transformation group of the Eucledian space (as an automorphism group).
-The Galilean transformati …
5
votes
Accepted
Unitary irreducible representations of the little group $SO(3)$
This answer is based on this article by A. Ungar.
Ungar computed the Thomas rotation formula which is almost what you need. I'll describe the general procedure, and in some cases, I'll refer you to …
3
votes
Wigner Classification via the orbit structure of the Lorentz group
The correspondence between group orbits and representations is a very general and fruitful principle which has a multitude of applications in physics.
To be precise, the correspondence is between coa …
27
votes
Accepted
Rank of the Poincare group
For semisimple groups (and the Poincaré group is not such), the number of
Casimirs (i.e., the number basic generators center of the universal
enveloping algebra) is equal to the dimension its Cartan …