**Two electrons**with equal spin in space.- Place the center of mass in the origin.
- A state is given by the vector to one of the electrons.
- Pauli
exlusion principle: this vector in never zero,
**but**

since all the electrons are indistinguishable

**we can't tell what direction**the vector is pointing. - We disregard scaling.
- The state is given uniquely by a
**line**through the origin. - The
**space**of such lines is called the**projective plane**.

In general:

Bjørn Ian Dundas