A classical particle moving in an inverse square central force, like a planet in the gravitational field of the Sun, moves in orbits that do not precess. This lack of precession, special to the inverse square force, indicates the presence of extra conserved quantities beyond the obvious ones. Thanks to Noether's theorem, these indicate the presence of extra symmetries. It turns out that not only rotations in 3 dimensions, but also in 4 dimensions, act as symmetries of this system. These extra symmetries are also present in the quantum version of the problem, where they explain some surprising features of the hydrogen atom. The quest to fully understand these symmetries leads to some fascinating mathematical adventures.

You can see the slides here. For a less technical version, try this:

- John Baez, Mysteries of the periodic table.

You can see more details here:

- John Baez, Mysteries of the gravitational 2-body problem.
- John Baez, Planets in the fourth dimension.
- Greg Egan, The ellipse and the atom.

© 2019 John Baez

baez@math.removethis.ucr.andthis.edu