In mathematics, the Whitehead manifold is an open 3-manifold that is contractible, but not homeomorphic to \R^3. discovered this puzzling object while he was trying to prove the Poincaré conjecture, correcting an error in an earlier paper where he incorrectly claimed that no such manifold exists. A contractible manifold is one that can continuously be shrunk to a point inside the manifold itself. For example, an open ball is a contractible manifold. All manifolds homeomorphic to the ball are contractible, too. One can ask whether all contractible manifolds are homeomorphic to a ball. For dimensions 1 and 2, the answer is classical and it is "yes". In dimension 2, it follows, for example, from the Riemann mapping theorem. Dimension 3 presents the first counterexample: the Whitehead manifold.

Construction

Take a copy of S^3, the three-dimensional sphere. Now find a compact unknotted solid torus T_1 inside the sphere. (A solid torus is an ordinary three-dimensional doughnut, that is, a filled-in torus, which is topologically a circle times a disk.) The closed complement of the solid torus inside S^3 is another solid torus. Now take a second solid torus T_2 inside T_1 so that T_2 and a tubular neighborhood of the meridian curve of T_1 is a thickened Whitehead link. Note that T_2 is null-homotopic in the complement of the meridian of T_1. This can be seen by considering S^3 as and the meridian curve as the z-axis together with \infty. The torus T_2 has zero winding number around the z-axis. Thus the necessary null-homotopy follows. Since the Whitehead link is symmetric, that is, a homeomorphism of the 3-sphere switches components, it is also true that the meridian of T_1 is also null-homotopic in the complement of T_2. Now embed T_3 inside T_2 in the same way as T_2 lies inside T_1, and so on; to infinity. Define W, the Whitehead continuum, to be or more precisely the intersection of all the T_k for The Whitehead manifold is defined as which is a non-compact manifold without boundary. It follows from our previous observation, the Hurewicz theorem, and Whitehead's theorem on homotopy equivalence, that X is contractible. In fact, a closer analysis involving a result of Morton Brown shows that However, X is not homeomorphic to \R^3. The reason is that it is not simply connected at infinity. The one point compactification of X is the space S^3/W (with W crunched to a point). It is not a manifold. However, is homeomorphic to \R^4. David Gabai showed that X is the union of two copies of \R^3 whose intersection is also homeomorphic to \R^3.

Related spaces

More examples of open, contractible 3-manifolds may be constructed by proceeding in similar fashion and picking different embeddings of T_{i+1} in T_i in the iterative process. Each embedding should be an unknotted solid torus in the 3-sphere. The essential properties are that the meridian of T_i should be null-homotopic in the complement of T_{i+1}, and in addition the longitude of T_{i+1} should not be null-homotopic in Another variation is to pick several subtori at each stage instead of just one. The cones over some of these continua appear as the complements of Casson handles in a 4-ball. The dogbone space is not a manifold but its product with \R^1 is homeomorphic to \R^4.