# Lorentz Transformations boost?

Define $B( \theta, \vec{n} ) \in SL( 2 , \mathbb{C} )$ by

$B( \theta , \vec{n}) = \cosh { \frac{1}{2} \theta} + \vec{\sigma} \cdot \vec{n} \sinh{ \frac{1}{2} \theta}$ where $\vec{n}^2 =1$

Show that this corresponds to a Lorentz boost with velocity $\vec{v}=\tanh{ \theta} \vec{n}$. Show that

$( 1 + \frac{1}{2} \vec{sigma} \cdot \delta \vec{v}) B(\theta, \vec{n}) = B( \theta' , \vec{n}' ) R$

where, to 1st order in $\delta \vec{v}$,

$\theta' = \theta + \delta \vec{v} \cdot \vec{n}$, $\vec{n}'=\vec{n} \coth{\theta} (\delta \vec{v} - \vec{n} \vec{n} \cdot \delta \vec{v})$

and $R$ is an infinitesimal rotation given by

$R= 1 + \tanh{\frac{1}{2} \theta} \frac{1}{2}i ( \delta \vec{v} \times \vec{n} ) \cdot \vec{\sigma} = 1 + \frac{\gamma}{\gamma + 1} \frac{1}{2} i ( \delta \vec{v} \times \vec{v} ) \cdot \vec{\sigma}$
and $\gamma = ( 1 - \vec{v}^2) ^{-\frac{1}{2}}$

Show that $\vec{v}' = \vec{v} + \delta \vec{v} - \vec{v} \vec{v} \cdot \delta \vec{v}$

Note that $\vec{\sigma} \cdot \vec{a} \vec{\sigma} \cdot \vec{b} = \vec{a} \cdot \vec{b} 1 + i \vec{\sigma} \cdot ( \vec{a} \times \vec{b} )$

I don't understand how to go about the first bit here - what do I need to do in order to show this is a Lorentz boost?

Thanks!

## Answers and Replies

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fzero
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I don't understand how to go about the first bit here - what do I need to do in order to show this is a Lorentz boost?

Thanks!
$$B( \theta, \vec{n} )$$ is an element of a representation of $$SL( 2 , \mathbb{C} )$$. What vector space does the representation act on? Is there a map from Minkowski space to this vector space that you know of?

$$B( \theta, \vec{n} )$$ is an element of a representation of $$SL( 2 , \mathbb{C} )$$. What vector space does the representation act on? Is there a map from Minkowski space to this vector space that you know of?
well $SL(2, \mathbb{C})$ is just matrices so surely they can act on any vector in $\mathbb{R}^2$?