# Hyperbolic Geometry in special relativity

## Main Question or Discussion Point

Hi, I am new to the study of special relativity but think I understand it pretty well from the common circular geometrical framework. How important is it that I also understand it from the hyperbolic perspective and what would I gain over my current circular understanding?

## Answers and Replies

Related Special and General Relativity News on Phys.org
robphy
Science Advisor
Homework Helper
Gold Member
common circular geometrical framework
What common(?) circular geometrical framework are you specifically referring to?

Sorry, I mean circular as far as the standard non-hyperbolic unit-circle analyses that do not use hyperbolic trig functions in their explanations of the science.

Alright, let me put it another way, my basic understanding of special relativity relies mainly on the popular conceptions of implementing the lorentze transformation in regards to relative motion. In my experience, I have not until recently been advised that I need to supplant this framework with one that analyzes and implements hyperbolics in this pursuit. Is this absolutley necessary, or can I just stick with my colloquial interpretation?

bcrowell
Staff Emeritus
Science Advisor
Gold Member
Like the others who have posted in this thread, I don't understand what the OP is referring to. StationZero, you're using terms like hyperbolic geometry in nonstandard ways, which makes it harder to figure out what you mean. You seem to have in mind two different ways of presenting SR, and you're asking whether they're both OK, or whether one is better than the other. Can you point us to some online sources that present these so that we can tell what you're referring to?

robphy
Science Advisor
Homework Helper
Gold Member
So, it seems to me that you are saying that
you are familiar with standard presentations in terms of relative-[dimensionless] velocities ($\beta$) and time-dilation factors ($\gamma$)
and are wondering about
presentations involving "rapidity" ($\theta$), where $\beta=\tanh\theta$ and $\gamma=\cosh\theta$. (Note that, in his preface, Tevian Dray is describes this as "hyperbola geometry" [read as: "Minkowskian spacetime geometry, where the hyperbola plays the role of the locus of events equidistant from a given event, together with the Parallel Postulate"], which is not the same as "hyperbolic geometry", which violates the Parallel Postulate. The latter appears in special relativity when considering the space of velocities.)

In my opinion, the rapidity ("Minkowskian angle") approach allows one to more clearly draw analogies to your Euclidean intuition... and possibly make special relativity less mysterious. It will reveal the geometrical origin of the numerous "formulas" of special relativity.

bcrowell
Staff Emeritus
Science Advisor
Gold Member
These are just two different but equivalent ways of describing SR.

Hi, I am new to the study of special relativity but think I understand it pretty well from the common circular geometrical framework. How important is it that I also understand it from the hyperbolic perspective and what would I gain over my current circular understanding?
You'll see that what are usually called "Lorentz boosts" are really just the analogue for rotations compared to the usual Euclidean framework.

In general, Euclidean rotations are generated by some object $i$ that squares to $-1$. Taking $e^{i \theta}$ generates (circular) sine and cosine through power series. $e^{i \theta} = \cos \theta + i \sin \theta$.

"Rotations" involving a timelike and spacelike direction in Minkowski space are generated by some object $\epsilon$ such that $\epsilon^2 = +1$ yet $\epsilon \neq 1$. A similar power series argument sowing that $e^{\epsilon \phi} = \cosh \phi + \epsilon \sinh \phi$ generates the hyperbolic trig functions. Just as $e^{i \theta}$ is at the heart of rotations in the 2d Euclidean plane, so is $e^{\epsilon \phi}$ at the heart of "rotations" in the 1+1d plane.