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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