I know the solution for R2. That is a for an infinite plane you can have one of 2 things (from the classification of 2D surfaces): 1) cross cap (cut a circle out of the plane and identify opposite points). 2) a oriented handle (cut two circles out and identify points on one with reflected points on the other - like a wormhole) A non-oriented handle (cut two circles out and identify points on one with equivalent points on the other) is equivalent to two cross-caps. Each of these "particles" adds negative curvature to the surface. So that got me thinking, in R3 what kind of topological "particles" could you get? I think there will be more since you can cut out spherical holes or toroidal holes (which could be knotted). You could get 3 dimensional equivalents of (1) and (2) but can you get anything else interesting? And will they all add negative curvature? The ones I can think of are: 1) Cut out a spherical hole and identify opposite points (a 3D cross-cap - whatever that is called!!) 2) Cut out two spherical holes and identify reflected points (like a wormhole) 3) Cut out a torus (perhaps knotted) and identify opposite points at each cross-section. 4) Cut out a torus (perhaps knotted) and identify opposite points but reflected 5) Cut out two tori and identify points - (Like a toroidal wormhole - not sure if this can be composed of others) I know the wormholes (2) are solutions of General Relativity. Are any of the others? Does that mean that these things exist or not? Are non-orientable topological defects allowed in General Relativity? If so, would they act like fermions? Also, can there be any chiral topological particles? Maybe made out of a trefoil knotted torus or something simpler? Would something like (3) act like a string from string theory or something else? What is their curvature? I imagine it is zero. Hence they might be solutions to empty space in GR. In 2D space there is no-such thing as an anti-topological particle, since two cross-caps don't cancel each other out, they produce a non-oriented handle. (Being negatively curved they just add together). Are there any such things as anti-toplogical particles in 3D? (i.e. if both particle and anti-particle exist on the same plane it is equivalent to R3). Sorry, lots of questions! This was just on my mind today!