# Spin group

In mathematics the **spin group** Spin(*n*)^{[1]}^{[2]} is the double cover of the special orthogonal group SO(*n*) = SO(*n*, **R**), such that there exists a short exact sequence of Lie groups (when *n* ≠ 2)

As a Lie group, Spin(*n*) therefore shares its dimension, *n*(*n* − 1)/2, and its Lie algebra with the special orthogonal group.

For *n* > 2, Spin(*n*) is simply connected and so coincides with the universal cover of SO(*n*).

The non-trivial element of the kernel is denoted −1, which should not be confused with the orthogonal transform of reflection through the origin, generally denoted −*I*.

Spin(*n*) can be constructed as a subgroup of the invertible elements in the Clifford algebra Cl(*n*). A distinct article discusses the spin representations.

The spin group is used in physics to describe the symmetries of (electrically neutral, uncharged) fermions. Its complexification, Spinc, is used to describe electrically charged fermions, most notably the electron. Strictly speaking, the spin group describes a fermion in a zero-dimensional space; but of course, space is not zero-dimensional, and so the spin group is used to define spin structures on (pseudo-)Riemannian manifolds: the spin group is the structure group of a spinor bundle. The affine connection on a spinor bundle is the spin connection; the spin connection is useful as it can simplify and bring elegance to many intricate calculations in general relativity. The spin connection in turn enables the Dirac equation to be written in curved spacetime (effectively in the tetrad coordinates), which in turn provides a footing for quantum gravity, as well as a formalization of Hawking radiation (where one of a pair of entangled, virtual fermions fall past the event horizon, and the other does not). In short, the spin group is a vital cornerstone, centrally important for understanding advanced concepts in modern theoretical physics. In mathematics, the spin group is interesting in its own right: not only for these reasons, but for many more.

Construction of the Spin group often starts with the construction of a Clifford algebra over a real vector space *V* with a definite quadratic form *q*.^{[3]} The Clifford algebra is the quotient of the tensor algebra T*V* of *V* by a two-sided ideal. The tensor algebra (over the reals) may be written as

This has important applications in 4-manifold theory and Seiberg–Witten theory. In physics, the Spin group is appropriate for describing uncharged fermions, while the Spin^{C} group is used to describe electrically charged fermions. In this case, the U(1) symmetry is specifically the gauge group of electromagnetism.

In low dimensions, there are isomorphisms among the classical Lie groups called *exceptional isomorphisms*. For instance, there are isomorphisms between low-dimensional spin groups and certain classical Lie groups, owing to low-dimensional isomorphisms between the root systems (and corresponding isomorphisms of Dynkin diagrams) of the different families of simple Lie algebras. Writing **R** for the reals, **C** for the complex numbers, **H** for the quaternions and the general understanding that Cl(*n*) is a short-hand for Cl(**R**^{n}) and that Spin(*n*) is a short-hand for Spin(**R**^{n}) and so on, one then has that^{[3]}

There are certain vestiges of these isomorphisms left over for *n* = 7, 8 (see Spin(8) for more details). For higher *n*, these isomorphisms disappear entirely.

In indefinite signature, the spin group Spin(*p*, *q*) is constructed through Clifford algebras in a similar way to standard spin groups. It is a double cover of SO_{0}(*p*, *q*), the connected component of the identity of the indefinite orthogonal group SO(*p*, *q*). For *p* + *q* > 2, Spin(*p*, *q*) is connected; for (*p*, *q*) = (1, 1) there are two connected components.^{[4]}^{: 193 } As in definite signature, there are some accidental isomorphisms in low dimensions:

Connected and simply connected Lie groups are classified by their Lie algebra. So if *G* is a connected Lie group with a simple Lie algebra, with *G*′ the universal cover of *G*, there is an inclusion

The definite signature Spin(*n*) are all simply connected for *n* > 2, so they are the universal coverings of SO(*n*).

In indefinite signature, Spin(*p*, *q*) is not necessarily connected, and in general the identity component, Spin_{0}(*p*, *q*), is not simply connected, thus it is not a universal cover. The fundamental group is most easily understood by considering the maximal compact subgroup of SO(*p*, *q*), which is SO(*p*) × SO(*q*), and noting that rather than being the product of the 2-fold covers (hence a 4-fold cover), Spin(*p*, *q*) is the "diagonal" 2-fold cover – it is a 2-fold quotient of the 4-fold cover. Explicitly, the maximal compact connected subgroup of Spin(*p*, *q*) is

This allows us to calculate the fundamental groups of Spin(*p*, *q*), taking *p* ≥ *q*:

Thus once *p*, *q* > 2 the fundamental group is Z_{2}, as it is a 2-fold quotient of a product of two universal covers.

The maps on fundamental groups are given as follows. For *p*, *q* > 2, this implies that the map π_{1}(Spin(*p*, *q*)) → π_{1}(SO(*p*, *q*)) is given by 1 ∈ Z_{2} going to (1, 1) ∈ Z_{2} × Z_{2}. For *p* = 2, *q* > 2, this map is given by 1 ∈ **Z** → (1,1) ∈ **Z** × Z_{2}. And finally, for *p* = *q* = 2, (1, 0) ∈ **Z** × **Z** is sent to (1,1) ∈ **Z** × **Z** and (0, 1) is sent to (1, −1).

The center of the spin groups, for *n* ≥ 3, (complex and real) are given as follows:^{[4]}^{: 208 }

Quotient groups can be obtained from a spin group by quotienting out by a subgroup of the center, with the spin group then being a covering group of the resulting quotient, and both groups having the same Lie algebra.

The homotopy groups of the cover and the quotient are related by the long exact sequence of a fibration, with discrete fiber (the fiber being the kernel) – thus all homotopy groups for *k* > 1 are equal, but π_{0} and π_{1} may differ.

For *n* > 2, Spin(*n*) is simply connected (π_{0} = π_{1} = Z_{1} is trivial), so SO(*n*) is connected and has fundamental group Z_{2} while PSO(*n*) is connected and has fundamental group equal to the center of Spin(*n*).

In indefinite signature the covers and homotopy groups are more complicated – Spin(*p*, *q*) is not simply connected, and quotienting also affects connected components. The analysis is simpler if one considers the maximal (connected) compact SO(*p*) × SO(*q*) ⊂ SO(*p*, *q*) and the component group of Spin(*p*, *q*).

The spin group appears in a Whitehead tower anchored by the orthogonal group:

The tower is obtained by successively removing (killing) homotopy groups of increasing order. This is done by constructing short exact sequences starting with an Eilenberg–MacLane space for the homotopy group to be removed. Killing the π_{3} homotopy group in Spin(*n*), one obtains the infinite-dimensional string group String(*n*).

Discrete subgroups of the spin group can be understood by relating them to discrete subgroups of the special orthogonal group (rotational point groups).

For point groups that reverse orientation, the situation is more complicated, as there are two pin groups, so there are two possible binary groups corresponding to a given point group.