Monday, August 10, 2015

special relativity - How to add together non-parallel rapidities?


How to add together non-parallel rapidities?


The Lorentz transformation is essentially a hyperbolic rotation, which rotation can be described by a hyperbolic angle, which is called the rapidity. I found that this hyperbolic angle nicely and simply describe many quantities in natural units:



  • Lorentz factor: $\mathrm{cosh}\,\phi$

  • Coordinate velocity: $\mathrm{tanh}\,\phi$

  • Proper velocity: $\mathrm{sinh}\,\phi$


  • Total energy: $m\,\mathrm{cosh}\,\phi$

  • Momentum: $m\,\mathrm{sinh}\,\phi$

  • Proper acceleration: $d\phi / d\tau$ (so local accelerometers measure the change of rapidity)


Also other nice features:



  • Velocity addition formula simplifies to adding rapidities together (if they are parallel).

  • For low speeds the rapidity is the classical velocity in natural units.


I think for more than one dimensions, the rapidity can be seen as a vector quantity.



In that case my questions are:



  • What's the general rapidity addition formula?

  • And optionally: Given these nice properties why don't rapidity used more often? Does it have some bad properties that make it less useful?




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