# G2 and the Rolling Ball

• Published in 2012
In the collection
Understanding the exceptional Lie groups as the symmetry groups of simpler objects is a long-standing program in mathematics. Here, we explore one famous realization of the smallest exceptional Lie group, G2. Its Lie algebra acts locally as the symmetries of a ball rolling on a larger ball, but only when the ratio of radii is 1:3. Using the split octonions, we devise a similar, but more global, picture of G2: it acts as the symmetries of a 'spinorial ball rolling on a projective plane', again when the ratio of radii is 1:3. We explain this ratio in simple terms using the incidence geometry of G2, and show how a form of geometric quantization applied to this system gives the imaginary split octonions.

## Other information

arxivId
1205.2447
pages
28

### BibTeX entry

@article{Baez2012,
abstract = {Understanding the exceptional Lie groups as the symmetry groups of simpler objects is a long-standing program in mathematics. Here, we explore one famous realization of the smallest exceptional Lie group, G2. Its Lie algebra acts locally as the symmetries of a ball rolling on a larger ball, but only when the ratio of radii is 1:3. Using the split octonions, we devise a similar, but more global, picture of G2: it acts as the symmetries of a 'spinorial ball rolling on a projective plane', again when the ratio of radii is 1:3. We explain this ratio in simple terms using the incidence geometry of G2, and show how a form of geometric quantization applied to this system gives the imaginary split octonions.},
archivePrefix = {arXiv},
arxivId = {1205.2447},
author = {Baez, John C and Huerta, John},
eprint = {1205.2447},
month = {may},
pages = 28,
title = {G2 and the Rolling Ball},
url = {http://arxiv.org/abs/1205.2447 http://arxiv.org/pdf/1205.2447v4},
year = 2012,
primaryClass = {math.DG},
urldate = {2012-05-14},
collections = {Basically physics}
}