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0 pointsjacobolus4y ago0 comments

Gyrovectors are a generally poor representation for rotations compared to quaternions (or the like).

In a spherical context, a “spherical gyrovector” can represent any rotation of the sphere whose axis is on the equator, with the representation being the point where the north pole gets sent. This gets you 2 out of 3 degrees of freedom for spherical rotations. Then you can represent an arbitrary rotation of the sphere as the composition of a “gyrovector” and a rotation about the north pole. But the details here are tricky and unintuitive and a lot of the symmetries of spherical rotation are not reflected in the representation.

The deficits of this system are a bit less obvious in a context (hyperbolic space) that students are less familiar with. But if you represent the hyperbolic plane as a paraboloid in pseudo-Euclidean space (akin to representing a sphere as a surface embedded in Euclidean space), a tool similar to unit quaternions is also a more convenient and natural representation for hyperbolic rotation.

* * *

Geometric algebra as a language makes it easy and natural to understand and describe the meaning and relationships between various rotation representations, and is much better for this purpose than e.g. matrices.

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Qem4y ago

Can you recommend good geometric algebra books starting at a elementary level, say, not assuming starting knowledge much beyond high school mathematics? I did some preliminary research, but I had the impression the intended audience for most books in this area is people that already master the conventional approach but are open to see the subject under a new light, so a lot of previous knowledge is assumed.

jacobolusOP4y ago

What I have repeatedly found with GA is that I can solve some problem I have using some other brute-forceish tools with a few pages of tricky error-prone scratch-work that balloons out to a complicated mess before simplifying back down at the end, and then afterward think about it a bit and come up with 2–6 lines of simple GA identities showing the same thing in a much higher-level coordinate-free way, and with most of the steps geometrically interpretable, rather than just opaque calculation. But coming up with the simple version at the beginning is hard.

The tricky part about it is that there are a lot of useful identities that can be written down, and properly learning a decent number of them and figuring out which ones to apply in which situation takes probably years practice, ideally with some guidance/support from someone who knows more than you. (I do not feel like I have mastered the subject.) The same thing happens using whatever other formalism, with the difference that many identities that are pretty short to write down in GA are much more complicated to write down, so people don’t even try to use them.

I’m not sure if there’s really a good beginner source, but I haven’t ever really sat down and tried to go comprehensively through the exercises in any books pitched at a relatively elementary level. You could try Alan MacDonald’s book Linear and Geometric Algebra which is designed as an intro undergraduate textbook. If you want to also learn some mechanics, you could try Hestenes’s book New Foundations for Classical Mechanics.

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