# Spin structure

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{{Short description|Concept in differential geometry}}
In [differential geometry](/source/differential_geometry), a '''spin structure''' on an [orientable](/source/orientable) [Riemannian manifold](/source/Riemannian_manifold) {{nowrap|(''M'', ''g'')}} allows one to define associated [spinor bundle](/source/spinor_bundle)s, giving rise to the notion of a [spinor](/source/spinor) in differential geometry.

Spin structures have wide applications to [mathematical physics](/source/mathematical_physics), in particular to [quantum field theory](/source/quantum_field_theory) where they are an essential ingredient in the definition of any theory with uncharged [fermion](/source/fermion)s.  They are also of purely mathematical interest in [differential geometry](/source/differential_geometry), [algebraic topology](/source/algebraic_topology), and [K theory](/source/K_theory). They form the foundation for [spin geometry](/source/spin_geometry).

==Overview<!--'Spin frame' and 'Spin frames' redirect here-->==
In [geometry](/source/geometry) and in [field theory](/source/Field_theory_(physics)), mathematicians ask whether or not a given oriented Riemannian manifold (''M'',''g'') admits [spinor](/source/spinor)s. One method for dealing with this problem is to require that ''M'' have a spin structure.<ref name="autogenerated558">{{cite journal|title=Sur l'extension du groupe structural d'un espace fibré|first=A. |last=Haefliger|author-link=André Haefliger|journal=C. R. Acad. Sci. Paris|volume=243|year=1956|pages=558–560}}</ref><ref>{{cite journal|title=Spin structures on manifolds|author=J. Milnor|author-link=John Milnor|journal=L'Enseignement Mathématique|volume=9|year=1963|pages=198–203}}</ref><ref>{{cite journal|title=Champs spinoriels et propagateurs en rélativité générale  |first=A. |last=Lichnerowicz|author-link=André Lichnerowicz|journal=Bull. Soc. Math. Fr.|volume=92|year=1964|pages=11–100|doi=10.24033/bsmf.1604|doi-access=free}}</ref> This is not always possible since there is potentially a topological obstruction to the existence of spin structures. Spin structures will exist if and only if the second [Stiefel–Whitney class](/source/Stiefel%E2%80%93Whitney_class) ''w''<sub>2</sub>(''M'') ∈ H<sup>2</sup>(''M'', '''Z'''<sub>2</sub>) of ''M'' vanishes. Furthermore, if ''w''<sub>2</sub>(''M'') = 0, then the set of the isomorphism classes of spin structures on ''M'' is acted upon freely and transitively by H<sup>1</sup>(''M'', '''Z'''<sub>2</sub>) . As the manifold ''M'' is assumed to be oriented, the first Stiefel–Whitney class ''w''<sub>1</sub>(''M'') ∈ H<sup>1</sup>(''M'', '''Z'''<sub>2</sub>) of ''M'' vanishes too. (The Stiefel–Whitney classes ''w<sub>i</sub>''(''M'') ∈ H<sup>''i''</sup>(''M'', '''Z'''<sub>2</sub>) of a manifold ''M'' are defined to be the Stiefel–Whitney classes of its [tangent bundle](/source/tangent_bundle) ''TM''.)

The bundle of spinors π<sub>''S''</sub>: ''S'' → ''M'' over ''M'' is then the [complex vector bundle](/source/complex_vector_bundle) associated with the corresponding [principal bundle](/source/principal_bundle) π<sub>''P''</sub>: ''P'' → ''M'' of '''spin frames'''<!--boldface per WP:R#PLA--> over ''M'' and the spin representation of its structure group Spin(''n'') on the space of spinors Δ<sub>''n''</sub>. The bundle ''S'' is called the spinor bundle for a given spin structure on ''M''.

A precise definition of spin structure on manifold was possible only after the notion of [fiber bundle](/source/fiber_bundle) had been introduced; [André Haefliger](/source/Andr%C3%A9_Haefliger) (1956) found the topological obstruction to the existence of a spin structure on an orientable Riemannian manifold and [Max Karoubi](/source/Max_Karoubi) (1968) extended this result to the non-orientable pseudo-Riemannian case.<ref>{{cite journal|title=Algèbres de Clifford et K-théorie|first=M. |last=Karoubi|journal=Ann. Sci. Éc. Norm. Supér.|author-link=Max Karoubi|volume=1|year=1968|pages=161–270|issue=2|doi=10.24033/asens.1163|doi-access=free}}</ref><ref>
{{Citation
| last1 = Alagia
| first1 = H. R. 
| last2 = Sánchez
| first2 = C. U. 
| title = Spin structures on pseudo-Riemannian manifolds
| journal = Revista de la Unión Matemática Argentina
| volume = 32 
| pages = 64–78
| year = 1985
| url = http://inmabb.criba.edu.ar/revuma/pdf/v32n1/p064-078.pdf
}}
</ref>

==Spin structures on Riemannian manifolds==

===Definition===
A spin structure on an [orientable](/source/orientable) [Riemannian manifold](/source/Riemannian_manifold) <math>(M,g)</math> with an oriented vector bundle <math>E</math> is an [equivariant](/source/equivariant) ''lift'' of the orthonormal frame bundle <math>P_{\operatorname{SO}}(E) \rightarrow M</math>  with respect to the double covering <math>\rho : \operatorname{Spin}(n) \rightarrow \operatorname{SO}(n)</math>. In other words, a pair <math>(P_{\operatorname{Spin}}, \phi)</math> is a spin structure on the SO(''n'')-principal bundle <math>\pi: P_{\operatorname{SO}}(E) \rightarrow M</math> when
{{ordered list|type=lower-alpha|<math>\pi_{P} : P_{\operatorname{Spin}} \rightarrow M</math> is a principal Spin(''n'')-bundle over <math>M</math>, and
|<math>\phi: P_{\operatorname{Spin}} \rightarrow P_{\operatorname{SO}}(E)</math> is an [equivariant](/source/equivariant) 2-fold [covering map](/source/covering_map) such that
{{block indent|<math>\pi\circ \phi=\pi_P \quad </math>and<math>\quad \phi(pq) = \phi(p)\rho(q) \quad</math>for all <math>p \in P_{\operatorname{Spin}}</math> and <math>q \in \operatorname{Spin}(n) </math>.}}}}
Two spin structures <math>(P_1, \phi_1)</math> and <math>(P_2, \phi_2)</math> on the same oriented [Riemannian manifold](/source/Riemannian_manifold) are called "equivalent" if there exists a Spin(''n'')-equivariant map <math>f: P_1 \rightarrow P_2</math> such that
{{block indent|<math>\phi_2\circ f=\phi_1 \quad</math> and <math>\quad f(p q) = f(p)q \quad</math> for all <math>p\in P_1</math> and <math>q \in \operatorname{Spin}(n) </math>.}}
In this case <math>\phi_1</math> and <math>\phi_2</math> are two equivalent double coverings.

The definition of spin structure on <math>(M,g)</math> as a spin structure on the principal bundle <math>P_{\operatorname{SO}}(E) \rightarrow M</math> is due to [André Haefliger](/source/Andr%C3%A9_Haefliger) (1956).

===Obstruction===
Haefliger<ref name="autogenerated558"/> found necessary and sufficient conditions for the existence of a spin structure on an oriented Riemannian manifold (''M'',''g''). The obstruction to having a spin structure is a certain element [''k''] of H<sup>2</sup>(''M'', '''Z'''<sub>2</sub>) . For a spin structure the class [''k''] is the second [Stiefel–Whitney class](/source/Stiefel%E2%80%93Whitney_class) ''w''<sub>2</sub>(''M'') ∈ H<sup>2</sup>(''M'', '''Z'''<sub>2</sub>) of ''M''. Hence, a spin structure exists if and only if the second Stiefel–Whitney class ''w''<sub>2</sub>(''M'') ∈ H<sup>2</sup>(''M'', '''Z'''<sub>2</sub>) of ''M'' vanishes.

==Spin structures on vector bundles<!--'Spin manifold' redirects here-->==
Let ''M'' be a [paracompact](/source/paracompact) [topological manifold](/source/topological_manifold) and ''E'' an [oriented](/source/Oriented) vector bundle on ''M'' of dimension ''n'' equipped with a [fibre metric](/source/fibre_metric).  This means that at each point of ''M'', the fibre of ''E'' is an [inner product space](/source/inner_product_space).  A spinor bundle of ''E'' is a prescription for consistently associating a [spin representation](/source/spin_representation) to every point of ''M''.  There are topological obstructions to being able to do it, and consequently, a given bundle ''E'' may not admit any spinor bundle.  In case it does, one says that the bundle ''E'' is ''spin''.

This may be made rigorous through the language of [principal bundle](/source/principal_bundle)s.  The collection of oriented [orthonormal frame](/source/orthonormal_frame)s of a vector bundle form a [frame bundle](/source/frame_bundle) ''P''<sub>SO</sub>(''E''), which is a principal bundle under the action of the [special orthogonal group](/source/special_orthogonal_group) SO(''n'').  A spin structure for ''P''<sub>SO</sub>(''E'') is a ''lift'' of ''P''<sub>SO</sub>(''E'') to a principal bundle ''P''<sub>Spin</sub>(''E'') under the action of the [spin group](/source/spin_group) Spin(''n''), by which we mean that there exists a bundle map ''<math>\phi</math>'' : ''P''<sub>Spin</sub>(''E'') → ''P''<sub>SO</sub>(''E'') such that
:<math>\phi(pg) = \phi(p)\rho(g)</math>, for all {{nowrap|''p'' &isin; ''P''<sub>Spin</sub>(''E'')}} and {{nowrap|''g'' &isin; Spin(''n'')}},
where {{nowrap|''ρ'' : Spin(''n'') → SO(''n'')}} is the mapping of groups presenting the spin group as a double-cover of SO(''n'').

In the special case in which ''E'' is the [tangent bundle](/source/tangent_bundle) ''TM'' over the base manifold ''M'', if a spin structure exists then one says that ''M'' is a '''spin manifold'''<!--boldface per WP:R#PLA-->. Equivalently ''M'' is ''spin'' if the SO(''n'') principal bundle of [orthonormal bases](/source/orthonormal_basis) of the tangent fibers of ''M'' is a '''Z'''<sub>2</sub> quotient of a principal spin bundle.

If the manifold has a [cell decomposition](/source/CW_complex) or a [triangulation](/source/Triangulation_(topology)), a spin structure can equivalently be thought of as a [homotopy class](/source/homotopy_class) of a trivialization of the [tangent bundle](/source/tangent_bundle) over the 1-[skeleton](/source/N-skeleton) that extends over the 2-skeleton.  If the dimension is lower than 3, one first takes a Whitney sum with a trivial line bundle.

===Obstruction and classification===
For an [orientable](/source/orientable) [vector bundle](/source/vector_bundle) <math>\pi_E:E \to M</math> a spin structure exists on <math>E</math> if and only if the second [Stiefel–Whitney class](/source/Stiefel%E2%80%93Whitney_class) <math>w_2(E)</math> vanishes. This is a result of [Armand Borel](/source/Armand_Borel) and [Friedrich Hirzebruch](/source/Friedrich_Hirzebruch).<ref>{{cite journal|first1=A. |last1=Borel|first2=F. |last2=Hirzebruch |title=Characteristic classes and homogeneous spaces I |journal=[American Journal of Mathematics](/source/American_Journal_of_Mathematics)|volume=80|year=1958|pages=97–136|doi=10.2307/2372795|jstor=2372795|issue=2}}</ref> Furthermore, in the case <math>E \to M</math> is spin, the number of spin structures are in bijection with <math>H^1(M,\mathbf{Z}/2)</math>. These results can be easily proven<ref>{{Cite web|last=Pati|first=Vishwambhar|date=|title=Elliptic complexes and index theory|url=http://www.isibang.ac.in/~adean/infsys/database/notes/elliptic.pdf|url-status=live|archive-url=https://web.archive.org/web/20180820174233/http://www.isibang.ac.in/~adean/infsys/database/notes/elliptic.pdf|archive-date=20 Aug 2018|access-date=|website=}}</ref><sup>pg 110-111</sup> using a spectral sequence argument for the associated principal <math>\operatorname{SO}(n)</math>-bundle <math>P_E \to M</math>. Notice this gives a [fibration](/source/fibration)
<math display="block">\operatorname{SO}(n) \to P_E \to M,</math>
hence the [Serre spectral sequence](/source/Serre_spectral_sequence) can be applied. From general theory of spectral sequences, there is an exact sequence
<math display="block">0 \to E_3^{0,1} \to E_2^{0,1} \overset{w_2}{\rightarrow} E_2^{2,0} \to E_3^{2,0} \to 0</math>
where
<math display="block">\begin{align}
E_2^{0,1} &= H^0(M, H^1(\operatorname{SO}(n),\mathbf{Z}/2)) = H^1(\operatorname{SO}(n),\mathbf{Z}/2) = \mathbf{Z}/2 \\
E_2^{2,0} &= H^2(M, H^0(\operatorname{SO}(n),\mathbf{Z}/2)) = H^2(M,\mathbf{Z}/2).
\end{align}</math>
In addition, <math>E_\infty^{0,1} = E_3^{0,1}</math> and <math>E_\infty^{0,1} = H^1(P_E,\mathbf{Z}/2)/F^1(H^1(P_E,\mathbf{Z}/2))</math> for some filtration <math>F</math> on <math>H^1(P_E,\mathbf{Z}/2)</math>, hence we get a map
<math display="block">H^1(P_E,\mathbf{Z}/2) \to E_3^{0,1}</math>
giving an exact sequence
<math display="block">H^1(P_E,\mathbf{Z}/2) \to H^1(\operatorname{SO}(n),\mathbf{Z}/2) \overset{w_2}{\rightarrow}  H^2(M,\mathbf{Z}/2).</math>
Now, a spin structure is exactly a double covering of <math>P_E</math> fitting into a commutative diagram
<math display="block">\begin{matrix}
\operatorname{Spin}(n) & \to & \tilde{P}_E & \to & M \\
\downarrow & & \downarrow & & \downarrow \\
\operatorname{SO}(n) & \to & P_E & \to & M
\end{matrix}</math>
where the two left vertical maps are the double covering maps. Note double coverings of <math>P_E</math> are in bijection with index <math>2</math> subgroups of <math>\pi_1(P_E)</math>, equivalently, are in bijection with the set of group morphisms <math>\text{Hom}(\pi_1(P_E), \mathbf{Z}/2)</math>. But, from [Hurewicz theorem](/source/Hurewicz_theorem) and change of coefficients, this is exactly the cohomology group <math>H^1(P_E,\mathbf{Z}/2)</math>. Applying the same argument to <math>\operatorname{SO}(n)</math>, the non-trivial covering <math>\operatorname{Spin}(n) \to \operatorname{SO}(n)</math> corresponds to <math>1 \in H^1(\operatorname{SO}(n),\mathbf{Z}/2) = \mathbf{Z}/2</math>, and the map to <math>H^2(M,\mathbf{Z}/2)</math> is precisely the <math>w_2</math> of the second Stiefel–Whitney class, hence <math>w_2(1) = w_2(E)</math>. If it vanishes, then the inverse image of <math>1</math> under the map
<math display="block">H^1(P_E,\mathbf{Z}/2) \to H^1(\operatorname{SO}(n),\mathbf{Z}/2)</math>
is the set of double coverings giving spin structures. Now, this subset of <math>H^1(P_E,\mathbf{Z}/2)</math> can be identified with <math>H^1(M,\mathbf{Z}/2)</math>, showing this latter cohomology group classifies the various spin structures on the vector bundle <math>E \to M</math>. This can be done by looking at the long exact sequence of homotopy groups of the fibration
<math display="block">\pi_1(\operatorname{SO}(n)) \to \pi_1(P_E) \to \pi_1(M) \to 1</math>
and applying <math>\text{Hom}(-,\mathbf{Z}/2)</math>, giving the sequence of cohomology groups
<math display="block">0 \to H^1(M,\mathbf{Z}/2) \to H^1(P_E,\mathbf{Z}/2) \to H^1(\operatorname{SO}(n),\mathbf{Z}/2).</math>
Because <math>H^1(M,\mathbf{Z}/2)</math> is the kernel, and the inverse image of <math>1 \in H^1(\operatorname{SO}(n),\mathbf{Z}/2)</math> is in bijection with the kernel, we have the desired result.

====Remarks on classification====
When spin structures exist, the inequivalent spin structures on a manifold have a one-to-one correspondence (not canonical) with the elements of H<sup>1</sup>(''M'','''Z'''<sub>2</sub>), which by the [universal coefficient theorem](/source/universal_coefficient_theorem) is isomorphic to H<sub>1</sub>(''M'','''Z'''<sub>2</sub>). More precisely, the space of the isomorphism classes of spin structures is an [affine space](/source/affine_space) over H<sup>1</sup>(''M'','''Z'''<sub>2</sub>).

Intuitively, for each nontrivial cycle on ''M'' a spin structure corresponds to a binary choice of whether a section of the SO(''N'') bundle switches sheets when one encircles the loop.  If ''w''<sub>2</sub><ref>{{cite web|title=Spin manifold and the second Stiefel-Whitney class|url=https://math.stackexchange.com/a/808396/251222|website=Math.Stachexchange}}</ref> vanishes then these choices may be extended over the two-[skeleton](/source/N-skeleton), then (by [obstruction theory](/source/obstruction_theory)) they may automatically be extended over all of ''M''.  In [particle physics](/source/particle_physics) this corresponds to a choice of periodic or antiperiodic [boundary condition](/source/boundary_condition)s for [fermions](/source/fermions) going around each loop. Note that on a complex manifold <math>X</math> the second Stiefel-Whitney class can be computed as the first [Chern class](/source/Chern_class) <math>\text{mod } 2</math>.

===Examples===
# A [genus](/source/genus_(mathematics)) ''g'' [Riemann surface](/source/Riemann_surface) admits 2<sup>2''g''</sup> inequivalent spin structures; see [theta characteristic](/source/theta_characteristic).
# If ''H''<sup>2</sup>(''M'','''Z'''<sub>2</sub>) vanishes, ''M'' is spin. For example, ''S''<sup>''n''</sup> is spin for all <math> n\neq 2 </math>. (Note that ''S''<sup>2</sup> is also spin, but for different reasons; see below.)
# The [complex projective plane](/source/complex_projective_plane) '''CP'''<sup>2</sup> is not spin.
# More generally, all even-dimensional [complex projective space](/source/complex_projective_space)s '''CP'''<sup>2''n''</sup> are not spin.
# All odd-dimensional [complex projective space](/source/complex_projective_space)s '''CP'''<sup>2n+1</sup> are spin.
# All compact, [orientable manifold](/source/orientable_manifold)s of [dimension 3](/source/3-manifold) or less are spin.
# All [Calabi–Yau manifold](/source/Calabi%E2%80%93Yau_manifold)s are spin.

=== Properties ===
* The [Â genus](/source/%C3%82_genus) of a spin manifold is an integer, and is an even integer if in addition the dimension is 4 mod 8.
*:In general the [Â genus](/source/%C3%82_genus) is a rational invariant, defined for any manifold, but it is not in general an integer.
*:This was originally proven by [Hirzebruch](/source/Friedrich_Hirzebruch) and [Borel](/source/Armand_Borel), and can be proven by the [Atiyah–Singer index theorem](/source/Atiyah%E2%80%93Singer_index_theorem), by realizing the [Â genus](/source/%C3%82_genus) as the index of a [Dirac operator](/source/Dirac_operator) – a Dirac operator is a square root of a second order operator, and exists due to the spin structure being a "square root". This was a motivating example for the index theorem.

==Spin<sup>C</sup> structures==
A spin<sup>'''C'''</sup> structure is analogous to a spin structure on an oriented [Riemannian manifold](/source/Riemannian_manifold),<ref>{{Cite book | last1=Lawson | first1=H. Blaine | last2=Michelsohn | first2=Marie-Louise|author2-link=Marie-Louise Michelsohn | title=Spin Geometry | url=https://archive.org/details/spingeometry00hbla | url-access=limited | publisher=[Princeton University Press](/source/Princeton_University_Press) | isbn=978-0-691-08542-5 | year=1989  |page=[https://archive.org/details/spingeometry00hbla/page/n400 391]}}
</ref> but uses the Spin<sup>'''C'''</sup> group, which is defined instead by the [exact sequence](/source/exact_sequence) 
:<math>1 \to\mathbf Z_2\to \operatorname{Spin}^{\mathbf{C}}(n) \to \operatorname{SO}(n)\times\operatorname{U}(1) \to 1.</math>
To motivate this, suppose that {{nowrap|''κ'' : Spin(''n'') → U(''N'')}} is a complex spinor representation.  The center of U(''N'') consists of the diagonal elements coming from the inclusion {{nowrap|''i'' : U(1) → U(''N'')}}, i.e., the scalar multiples of the identity. Thus there is a [homomorphism](/source/homomorphism)
:<math>\kappa\times i\colon {\mathrm {Spin}}(n)\times {\mathrm U}(1)\to {\mathrm U}(N).</math>
This will always have the element (−1,−1) in the kernel. Taking the quotient modulo this element gives the group Spin<sup>'''C'''</sup>(''n''). This is the twisted product

:<math>{\mathrm {Spin}}^{\mathbf C}(n) = {\mathrm {Spin}}(n)\times_{\mathbf Z_2} {\mathrm U}(1)\, ,</math>

where U(1) = SO(2) = '''S'''<sup>1</sup>. In other words, the group Spin<sup>'''C'''</sup>(''n'') is a [central extension](/source/central_extension_(mathematics)) of SO(''n'') by '''S'''<sup>1</sup>.

Viewed another way, Spin<sup>'''C'''</sup>(''n'') is the quotient group obtained from {{nowrap|Spin(''n'') × Spin(2)}} with respect to the normal '''Z'''<sub>2</sub> which is generated by the pair of covering transformations for the bundles {{nowrap|Spin(''n'') → SO(''n'')}} and {{nowrap|Spin(2) → SO(2)}} respectively. This makes the Spin<sup>'''C'''</sup> group both a bundle over the circle with fibre Spin(''n''), and a bundle over SO(''n'') with fibre a circle.<ref>{{cite journal|title=''Spin<sup>c</sup>–structures and homotopy equivalences''|author=R. Gompf|doi=10.2140/gt.1997.1.41|journal=[Geometry & Topology](/source/Geometry_%26_Topology)|volume=1|year=1997|pages=41–50|arxiv=math/9705218|bibcode=1997math......5218G|s2cid=6906852}}</ref><ref>{{Cite book | last1=Friedrich|first1=Thomas| title = Dirac Operators in Riemannian Geometry| url=https://archive.org/details/diracoperatorsri00frie_506| url-access=limited| publisher=[American Mathematical Society](/source/American_Mathematical_Society) | year=2000|isbn=978-0-8218-2055-1 
|page=[https://archive.org/details/diracoperatorsri00frie_506/page/n40 26]}}</ref>

The fundamental group π<sub>1</sub>(Spin<sup>'''C'''</sup>(''n'')) is isomorphic to '''Z''' if ''n'' ≠ 2, and to '''Z''' ⊕ '''Z''' if ''n'' = 2.

If the manifold has a [cell decomposition](/source/CW_complex) or a [triangulation](/source/Triangulation_(topology)), a spin<sup>'''C'''</sup> structure can be equivalently thought of as a homotopy class of [complex structure](/source/Complex_manifold) over the 2-[skeleton](/source/N-skeleton) that extends over the 3-skeleton.  Similarly to the case of spin structures, one takes a Whitney sum with a trivial line bundle if the manifold is odd-dimensional.

Yet another definition is that a spin<sup>'''C'''</sup> structure on a manifold ''N'' is a complex line bundle ''L'' over ''N'' together with a spin structure on {{nowrap|T''N'' ⊕ ''L''}}.

===Obstruction===
A spin<sup>'''C'''</sup> structure exists when the bundle is orientable and the second [Stiefel–Whitney class](/source/Stiefel%E2%80%93Whitney_class) of the bundle ''E'' is in the image of the map {{nowrap|''H''<sup>2</sup>(''M'', '''Z''') → ''H''<sup>2</sup>(''M'', '''Z'''/2'''Z''')}} (in other words, the third integral Stiefel–Whitney class vanishes, see the discussion below).  In this case one says that ''E'' is spin<sup>'''C'''</sup>.  Intuitively, the lift gives the [Chern class](/source/Chern_class) of the square of the U(1) part of any obtained spin<sup>'''C'''</sup> bundle.

===Classification===
When a manifold carries a spin<sup>'''C'''</sup> structure at all, the set of spin<sup>'''C'''</sup> structures forms an affine space.  Moreover, the set of spin<sup>'''C'''</sup> structures has a free transitive action of {{nowrap|''H''<sup>2</sup>(''M'', '''Z''')}}.  Thus, spin<sup>'''C'''</sup>-structures correspond to elements of {{nowrap|''H''<sup>2</sup>(''M'', '''Z''')}} although not in a natural way.

===Geometric picture===
This has the following geometric interpretation, which is due to [Edward Witten](/source/Edward_Witten).  When the spin<sup>'''C'''</sup> structure is nonzero this square root bundle has a non-integral Chern class, which means that it fails the [triple overlap condition](/source/triple_overlap_condition).  In particular, the product of transition functions on a three-way intersection is not always equal to one, as is required for a [principal bundle](/source/principal_bundle).  Instead it is sometimes &minus;1.

This failure occurs at precisely the same intersections as an identical failure in the triple products of transition functions of the obstructed [spin bundle](/source/spinor_bundle).  Therefore, the triple products of transition functions of the full spin<sup>'''C'''</sup> bundle, which are the products of the triple product of the spin and U(1) component bundles, are either {{nowrap|1=1<sup>2</sup> = 1}} or {{nowrap|1=(−1)<sup>2</sup> = 1}} and so the spin<sup>'''C'''</sup> bundle satisfies the triple overlap condition and is therefore a legitimate bundle.

====The details====
The above intuitive geometric picture may be made concrete as follows.  Consider the [short exact sequence](/source/short_exact_sequence) {{nowrap|0 → '''Z''' → '''Z''' → '''Z'''<sub>2</sub> → 0}}, where the second [arrow](/source/arrow) is [multiplication](/source/multiplication) by 2 and the third is reduction modulo 2.  This  induces a [long exact sequence](/source/long_exact_sequence) on cohomology, which contains

::<math>\dots \longrightarrow \textrm H^2(M;\mathbf Z) \stackrel {2} {\longrightarrow} \textrm H^2(M;\mathbf Z) \longrightarrow \textrm H^2(M;\mathbf Z_2) \stackrel {\beta}\longrightarrow \textrm H^3(M;\mathbf Z) \longrightarrow \dots ,</math>

where the second [arrow](/source/arrow) is induced by multiplication by 2, the third is induced by restriction modulo 2 and the fourth is the associated [Bockstein homomorphism](/source/Bockstein_homomorphism) ''β''.

The obstruction to the existence of a spin bundle is an element ''w''<sub>2</sub> of {{nowrap|H<sup>2</sup>(''M'','''Z'''<sub>2</sub>)}}.  It reflects the fact that one may always locally lift an SO(n) bundle to a spin bundle, but one needs to choose a '''Z'''<sub>2</sub> lift of each transition function, which is a choice of sign.  The lift does not exist when the product of these three signs on a triple overlap is −1, which yields the [Čech cohomology](/source/%C4%8Cech_cohomology) picture of ''w''<sub>2</sub>.

To cancel this obstruction, one tensors this spin bundle with a U(1) bundle with the same obstruction ''w''<sub>2</sub>.  Notice that this is an abuse of the word ''bundle'', as neither the spin bundle nor the U(1) bundle satisfies the triple overlap condition and so neither is actually a bundle.

A legitimate U(1) bundle is classified by its [Chern class](/source/Chern_class), which is an element of H<sup>2</sup>(''M'','''Z'''). Identify this class with the first element in the above exact sequence.  The next arrow doubles this Chern class, and so legitimate bundles will correspond to even elements in the second {{nowrap|H<sup>2</sup>(''M'', '''Z''')}}, while odd elements will correspond to bundles that fail the triple overlap condition.  The obstruction then is classified by the failure of an element in the second H<sup>2</sup>(''M'','''Z''') to be in the image of the arrow, which, by exactness, is classified by its image in H<sup>2</sup>(''M'','''Z'''<sub>2</sub>) under the next arrow.

To cancel the corresponding obstruction in the spin bundle, this image needs to be ''w''<sub>2</sub>.  In particular, if ''w''<sub>2</sub> is not in the image of the arrow, then there does not exist any U(1) bundle with obstruction equal to ''w''<sub>2</sub> and so the obstruction cannot be cancelled.  By exactness, ''w''<sub>2</sub> is in the image of the preceding arrow only if it is in the kernel of the next arrow, which we recall is the [Bockstein homomorphism](/source/Bockstein_homomorphism) β.  That is, the condition for the cancellation of the obstruction is

:::<math>W_3=\beta w_2=0</math>

where we have used the fact that the third ''integral'' Stiefel–Whitney class ''W''<sub>3</sub> is the Bockstein of the second Stiefel–Whitney class ''w''<sub>2</sub> (this can be taken as a definition of ''W''<sub>3</sub>).

====Integral lifts of Stiefel–Whitney classes====
This argument also demonstrates that the second Stiefel–Whitney class defines elements not only of '''Z'''<sub>2</sub> cohomology but also of integral cohomology in one higher degree.  In fact this is the case for all even Stiefel–Whitney classes.  It is traditional to use an uppercase ''W'' for the resulting classes in odd degree, which are called the integral Stiefel–Whitney classes, and are labeled by their degree (which is always odd).

===Examples===
# All [oriented](/source/oriented) [smooth manifold](/source/smooth_manifold)s of dimension 4 or less are spin<sup>'''C'''</sup>.<ref>{{Cite book | last1=Gompf | first1=Robert E. | last2=Stipsicz | first2=Andras I. | title=4-Manifolds and Kirby Calculus | url=https://archive.org/details/manifoldskirbyca00gomp_530 | url-access=limited | publisher=[American Mathematical Society](/source/American_Mathematical_Society) | isbn=0-8218-0994-6 | year=1999 | pages=[https://archive.org/details/manifoldskirbyca00gomp_530/page/n66 55]–58, 186–187}}</ref>  (In particular, by a theorem of Hopf and Hirzebruch, closed orientable 4-manifolds always admit a spin<sup>'''C'''</sup> structure.)
# All [almost complex manifold](/source/almost_complex_manifold)s are spin<sup>'''C'''</sup>.
# All spin manifolds are spin<sup>'''C'''</sup>.

==Application to particle physics==

In [particle physics](/source/particle_physics) the [spin–statistics theorem](/source/spin%E2%80%93statistics_theorem) implies that the [wavefunction](/source/wavefunction) of an uncharged [fermion](/source/fermion) can be described as a section of the [associated vector bundle](/source/associated_vector_bundle) to the spin lift of an SO(''N'') bundle ''E''.  Therefore, the choice of spin structure is part of the data needed to define the wavefunction, and one often needs to sum over these choices in the [partition function](/source/partition_function_(quantum_field_theory)).  In many physical theories ''E'' is the [tangent bundle](/source/tangent_bundle), but for the fermions on the worldvolumes of [D-branes](/source/D-branes) in [string theory](/source/string_theory) it is a [normal bundle](/source/normal_bundle).

In [quantum field theory](/source/quantum_field_theory) charged spinors are sections of associated spin<sup>'''C'''</sup> bundles, and in particular no charged spinors can exist on a space that is not spin<sup>'''C'''</sup>.  An exception arises in some [supergravity](/source/supergravity) theories where additional interactions imply that other fields may cancel the third Stiefel–Whitney class. The mathematical description of spinors in supergravity and string theory is a particularly subtle open problem, which was recently addressed in references.<ref name=LazaroiuShahbaziI>{{cite journal |first1=C. |last1=Lazaroiu |first2=C.S. |last2=Shahbazi |arxiv=1606.07894 |title=Real pinor bundles and real Lipschitz structures|journal=Asian Journal of Mathematics |year=2019 |volume=23 |issue=5 |pages=749–836 |doi=10.4310/AJM.2019.v23.n5.a3 |s2cid=119598006 }}.</ref><ref>{{cite book |first1=C. |last1=Lazaroiu |first2=C.S. |last2=Shahbazi |title=Geometric Methods in Physics XXXVI |arxiv=1607.02103 |chapter=On the spin geometry of supergravity and string theory|series=Trends in Mathematics |year=2019 |pages=229–235 |doi=10.1007/978-3-030-01156-7_25 |isbn=978-3-030-01155-0 |s2cid=104292702 }}</ref> It turns out that the standard notion of spin structure is too restrictive for applications to supergravity and string theory, and that the correct notion of spinorial structure for the mathematical formulation of these theories is a "Lipschitz structure".<ref name=LazaroiuShahbaziI /><ref>{{cite journal |first1=Thomas |last1=Friedrich |first2=Andrzej |last2=Trautman |author-link2=Andrzej Trautman |arxiv=math/9901137 |title=Spin spaces, Lipschitz groups, and spinor bundles |journal=[Annals of Global Analysis and Geometry](/source/Annals_of_Global_Analysis_and_Geometry) |year=2000 |volume=18 |issue=3 |pages=221–240|doi=10.1023/A:1006713405277 |s2cid=118698159 }}</ref>

==See also==
* [Metaplectic structure](/source/Metaplectic_structure)
* [Orthonormal frame bundle](/source/Orthonormal_frame_bundle)
* [Spinor](/source/Spinor)

==References==
<references/>

==Further reading==
* {{Cite book | last1=Lawson | first1=H. Blaine | last2=Michelsohn | first2=Marie-Louise | title=Spin Geometry | publisher=[Princeton University Press](/source/Princeton_University_Press) | isbn=978-0-691-08542-5 | year=1989 }}
* {{Cite book | last1=Friedrich|first1=Thomas | title = Dirac Operators in Riemannian Geometry| publisher=[American Mathematical Society](/source/American_Mathematical_Society) | year=2000|isbn=978-0-8218-2055-1 }}
* {{Cite book | last1=Karoubi | first1=Max |title=K-Theory | publisher=Springer | isbn=978-3-540-79889-7 | year=2008 |pages=212–214}}
* {{Cite book | last1=Greub | first1=Werner | last2=Petry | first2=Herbert-Rainer |chapter=On the lifting of structure groups |series=Lecture Notes in Mathematics  |publisher=Springer-Verlag |volume=676| orig-year=1978 |pages=217–246 |title=Differential Geometrical Methods in Mathematical Physics II |chapter-url=https://link.springer.com/chapter/10.1007/BFb0063673 |year=2006 | doi=10.1007/BFb0063673 |isbn=9783540357216
}}
* {{Cite book | last1=Scorpan | first1=Alexandru |title=The wild world of 4-manifolds | publisher= American Mathematical Society | year=2005 |pages=174–189 |chapter=4.5 Notes Spin structures, the structure group definition; Equivalence of the definitions of |chapter-url=https://books.google.com/books?id=VgG9AwAAQBAJ&pg=PA173 |isbn=9780821837498}}

==External links==
*[http://earthlingsoft.net/ssp/studium/2001spring/Spin.pdf Something on Spin Structures] by Sven-S. Porst is a short introduction to [orientation](/source/orientability) and spin structures for mathematics students.

Structures on Riemannian manifolds
Category:Structures on manifolds
Category:Algebraic topology
Category:K-theory
Category:Mathematical physics

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Adapted from the Wikipedia article [Spin structure](https://en.wikipedia.org/wiki/Spin_structure) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Spin_structure?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.