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7 votes
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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 …
David Bar Moshe's user avatar
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 …
David Bar Moshe's user avatar
3 votes
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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 …
David Bar Moshe's user avatar
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 …
David Bar Moshe's user avatar
19 votes
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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 …
David Bar Moshe's user avatar
5 votes
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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 …
David Bar Moshe's user avatar
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 …
David Bar Moshe's user avatar
27 votes
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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 …
David Bar Moshe's user avatar