Types of quantum field thories
A Bohr topos (a notion going back to ideas of (Butterfield-Hamilton-Isham) and further expored in (Isham-Döring 07) and (Heunen-Landsman-Spitters 09)) is a topos associated with any quantum mechanical system which is such that the observables and states of the physical system are more or less naturally encoded in the internal logic of the topos. A detailed motivation/derivation of this from classical theorems on the foundations of standard quantum mechanics is at order-theoretic structure in quantum mechanics.
One might think of a Bohr topos as (part of) a formalization of the “coordination” of the physical theory of quantum mechanics, providing a formalized prescription of how to map the theory to propositions about (experimental) observables of the system. The internal logic of Bohr toposes has been argued (e.g. Heunen-Landsman-Spitters 09) to be a better formal context for such considerations than the old quantum logic going back to von Neumann.
The notion of Bohr topos is named after Niels Bohr, whose informal ideas about the nature of quantum mechanics (e.g. Scheibe 73) it is supposed to formalize, see at interpretation of quantum mechanics – Bohr’s standpoint.
Sometimes in the literature the discussion of Bohr toposes is referred to as “the topos-theoretic formulation of physics”, but actually Bohr toposes currently formalize but one aspect of quantum mechanics, namely “the quantum mechanical phase space” in the form of the quantum observables and the quantum states. The plain Bohr topos does not even encode any dynamics, though in the spirit of AQFT a certain presheaf of Bohr toposes on spacetime does encode dynamics (Nuiten 11). For other and more comprehensive usages of topos theory in the formalization of physics see at geometry of physics and at higher category theory and physics.
To every quantum mechanical system is associated its Bohr topos: a ringed topos which plays the role of the quantum phase space. The idea of this construction – Bohrification – is that it naturally captures the geometric and logical aspects of quantum physics in terms of higher geometry/topos theory.
One way to understand Bohrification is as a generalization of the construction of the Gelfand spectrum of commutative C-star algebras to a context of noncommutative -algebras. It assigns to a noncommutative -algebra essentially a system of Gelfand dual spaces to each of its commutative subalgebras. Together, this system yields a generalized Gelfand spectrum in the form of a locale internal to the sheaf topos over the poset of commutative subalgebras of , or equivalently its externalization regarded as a locale over the locale of open subalgebras.
As a topos, the Bohr topos is just a presheaf topos, the topos of presheaves on this poset of commutative subalgebras of , but the point is that it is naturally a ringed topos with the original non-commutative algebra appearing as a commutative -algebra internal to the Bohr topos. This allows to talk about quantum states on much like classical states, but internal to the Bohr topos.
In fact, under mild assumptions on , its poset of commutative subalgebras, and hence the Bohr topos over it, encodes precisely the Jordan algebra underlying . As discussed at Jordan algebra, this is precisely that part of which knows about the quantum observables themselves. In order to have the Bohr topos remember the full non-commutative algebra structure of , it needs to be equipped with the information about Hamiltonian flows induced on by automorphisms of the form , where is the commutator (that gets discarded as one passes to the Jordan algebra). According to Andreas Döring (private communication at MPI Bonn, April 2013), this can be formulated nicely in topos theory.
The system as such is encoded by a C-star algebra ;
(Often in the literature quantum mechanical systems are instead dually conceived of in terms of Hilbert spaces of pure states. The relation between these two descriptions is established by the GNS-construction which allows to pass from one to the other.)
Notice that these axioms are naturally thought of as exhibiting the -algebra as a formal dual of the would-be quantum phase space of the physical system – not quite an ordinary topological space (which by Gelfand duality it would be if were commutative) but still a kind of generalized space. Traditionally it is common to regard this as a space in noncommutative geometry. Notice that if we do so – hence if we regard the object that is the incarnation of in the opposite category – then the definition of observable above reduces to the evident notion of real-valued functions on quantum phase space: such a function is a morphism in , which dually is a -algebra homomorphism (where on the left we have the unitization of functions with compact support). That such correspond to self-adjoint operators of is the statement of functional calculus for -algebras.
More subtle is the interpretation of the axiom for states. Historically this had been been subject to some discussion: by the spectral theorem two different observables have a compatible set of observable values if and only if these elements commute with each other in . Generally, a set of observables has a jointly consistent set of observable values if and only if the sub--algebra generated by them is commutative. Therefore for the phenomenological interpretation of the axioms it seems to make no sense to demand that a state be linear on non-commuting observables: if and do not commute, it is not a-priori clear that it makes sense to require that . This might experimentally fail, and hold only for commuting .
Therefore the notion of quasi-state was introduced: a quasi-state on is defined to be a (positive and normalized) function which is required to be -linear only on all commutative subalgebras of . Operationally, quasi-states should be the genuine states!
One would therefore tend to think that the terminology has been chosen in an unfortunate way. While maybe true, it turns out – non-trivially – that in a major class of cases of interest the distinction does not matter: namely Gleason's theorem states that for a separable complex Hilbert space with and the -algebra of bounded operators on , all quasi-states on are automatically states: a function that is linear on all commutative subalgebras is automatically also linear on all of .
While this means that the distinction between states and quasi-states disappears in a major case of interest, it does not disappear in all cases of interest. In particular, other foundational theorems about quantum mechanics concern the collection of commutative subalgebras, too.
Notably, one may wonder about the evident strengthening of the notion of quasi-states to that of a map which is not just linear but also an algebra homomorphism on each commuting subalgebra. Notice that by Gelfand duality every commutative -algebra is the algebra of continuous functions on some topological space . Under this duality a state on is a probability distribution on , while an algebra homomorphism is a point of . Therefore a quasi-state which is commutative-subalgebra-wise an algebra homomorphism may be thought of as encoding a collection of precise numerical values (as opposed to just expectation values) of all possible observables. Such a hypothetical quasi-state is sometimes called a collection of hidden variables of the quantum mechanical system: its existence would mean that despite the apparent probabilistic nature of quantum mechanics, there are “hidden” non-probabilistic states. But there are not. This is the statement of the Kochen-Specker theorem: under precisely the assumptions that make Gleason's theorem work, there is no quasi-state which is commutative-subalgebra-wise an algebra homomorphism.
In summary, this means:
Since the notion of commutative-subalgebra-wise homomorphism is at the heart of quantum physics, it seems worthwhile to consider natural formalizations of this notion. There is indeed a very natural and general abstract one: whenever any notion of function is defined only locally it is natural to consider the sheaf of such functions over all possible local patches.
The historically motivating example, and possibly still the most widely familiar one, is that of holomorphic functions on a complex manifold: there are in general very few holomorphic functions defined over all of a complex manifold, but plenty of them defined over any small enough subset. And it is of fundamental interest to consider the collections of holomorphic functions over each such subset, and how these restrict to each other under restriction of subsets. This collection of local data is a sheaf of functions on the complex manifold.
There is an evident analog setup of this situation that applies in the present case of interest, that of functions defined on commutative subalgebras:
For any C-star algebra, write for the set of all its commutative -subalgebras. This is naturally a poset under inclusion of subalgebras. A (co)presheaf of this set is a functor . Any such functor we may think of as a collection of commutative-subalgebra-wise data on , consistent with restriction of subalgebras. The collection of all such functors – which we write – is a category called a presheaf topos.
Below we shall discuss (here) that in a precise sense this commutative internal captures precisely all the kinematical information encoded in the quantum mechanical system of – everything related to states and observables but not information about (time) evolution. So everything we have discussed so far.
The pair of these two ingredients
constitutes what is called a ringed topos – a special case of the notion of a locally ringed topos. This notion is a fundamental notion for generalized spaces in higher geometry. The most advanced general theory of higher geometry (Lurie09) is based on modelling spaces as ringed toposes.
We shall call this ringed topos the Bohr topos of .
This terminology is meant to indicate that one may think of this construction as formalizing faithfully and usefully a heuristic that has been emphasized by Nils Bohr – one of the founding fathers of quantum mechanics – and is known as the doctrine of classical concepts (Scheibe) in quantum mechanics. This states that nonclassical/noncommutative as the logic/geometry of quantum mechanics may be, it is to be probed and detected by classical/commutative logic/geometry.
Namely in terms of the Bohr topos we have the following equivalent reformulations of the foundational facts about quantum physics discussed above, now internally in .
These two statements might be taken as suggesting that a quantum mechanical system is naturally regarded in terms of its Bohr topos – somewhat more naturally than as a -algebra . (The second, in a slightly different setup, was emphasized in IshamHamiltonButterfield, which inspired all of the following discussion, the first in Spitters). In fact, thinking of ringed toposes as generalized spaces in higher geometry, it suggests that the Bohr topos itself is the quantum phase space of the quantum mechanical system in question.
To which extent this perspective is genuinely useful is maybe still to be established. For pointers to the literature see the references below. Discussion along the above lines may suggest that this perspective is indeed useful, but what is probably still missing is a statement about quantum physics that can be formulated and proven in terms of Bohr toposes, while being hardly conceivable or at least more unnatural without. It is probably currently not clear if such statements have been found.
In the formalization of quantum field theory by the Haag-Kastler axioms – called AQFT – every quantum field theory is entirely encoded in terms of its local net of observables over spacetime . This is a copresheaf of C-star algebras
which assigns to every open subset of spacetime the quantum subsystem of quantum fields supported in that region. By the above, we may consider for each of these quantum systems their associated quantum phase spaces given by the correspondong Bohr toposes . This yields a presheaf
of ringed toposes whose internal ring object has the structure of a commutative -algebra. With the copresheaf thus turned into a presheaf it is natural to ask under which conditions this is a sheaf: under which conditions this presheaf satisfies descent.
In (Nuiten) the following is observed: if satisfies what is called the split property (a strong form of the time slice axiom) then the Bohr-presheaf of quantum phase spaces satisfies spatial descent by local geometric morphisms precisely if the original copresheaf of observables is indeed local – spatially and causally. So this means that a natural property of quantum physics – spatial and causal locality – corresponds from the perspective of Bohr toposes to a natural property of presheaves of quantum phase spaces: descent.
One can probably view this as further suggestive evidence that indeed quantum physics is naturally regarded from the point of view of the Bohr topos. But for seeing where this perspective is headed, it seems that more insights along these lines would be useful.
The discussion below proceeds in the following steps (following (Nuiten11))
We discuss the definitions and some basic properties of Bohr toposes: certain ringed toposes – in fact ringed spaces – associated with any (possibly non-commutative) algebra. We formulate the construction for C-star algebras, since this is the standard model for quantum mechanical systems, but actually much of it does not depend on either the topology or the star-algebra structure until we come to the discussion of the kinematics in a Bohr topos below.
A C-star algebra is (…)
Every element of a -algebra is the sum of two normal elements, because
This means whenever a linear morphism between the vector spaces underlying two -algebras is defined on normal elements, it is already defined on all elements. This will be used in several of the arguments below.
A partial -algebra is a set equipped with
a symmetric and reflexive binary relation ;
an involution ;
a function ;
a function ;
(partial) binary operations
such that every set of elements that are pairwise in is contained in a set whose elements are also pairwise in and on which the above operations yield the structure of a commutative C-star algebra.
This appears as (vdBergHeunen, def. 11,12).
For a C-star algebra, write
This construction extends to a functor
Notice the following fact about Alexandroff spaces:
from posets to topological spaces that sends a poset to the topological space whose underlying set is the underlying set of and whose open subsets are the upward closed subsets exhibits an equivalence of categories
For we call the Bohr site of .
A morphism in is called commutativity reflecting if for all we have that if commutes with in then already commutes with in .
Every monomorphism in is commutativity reflecting.
A morphism in is commutativity reflecting precisely if the morphism has a right adjoint
This appears as (Nuiten, lemma 2.6).
for the subcategories of on the monomorphisms and on the commutativity-reflecting morphisms, respectively.
for the categories of ringed spaces and ringed toposes, where the internal ring object is equipped with the structure of an internal commutative C-star algebra and the morphisms respect the -algebra structure.
Hence a morphism in is
For the Bohr topos of is the -space/topos
in equipped with the evident objectwise commutative -algebra structure.
where and are left and right Kan extension along , respectively.
The next proposition asserts that all essential geometric morphisms between Bohr toposes arise this way:
Analogously, essential geometric morphisms of the underlying toposes are precisely those in image under the functor of functors .
By the discussion in the section In terms of essential geometric morphisms at Cauchy complete category we have a full and faithful embedding of Cauchy-complete catgeories . But posets are trivially Cauchy, complete, hence this restricts to an embedding .
In terms of Alexandroff topologies: by the discussion of Alexandroff locales (in the entry Alexandroff space) we have that the functor takes values precisely on those morphisms of locales whose inverse image has a left adjoint. The statement then follows using the properties of localic reflection, which says that the 2-functor is a full and faithful 2-functor.
For an essential geometric morphism, the inverse image is naturally a -algebra in .
Using this we now discuss morphisms of Bohr toposes in .
with the special property that any is sent to
This is (Nuiten, lemma 2.7). (Essentially this argument also appears as (vdBergHeunen, prop. 33), where however the extra right adjoint is not made use of and instead the variances of the morphisms involved in the definition of are redefined in order to make the statement come out.)
we assign the essential geometric morphism
(wher and denote left and right Kan extension, respectively) equipped with the morphism of internal -algebras
which over is the restriction of to and corestriction to
(to the -completion of the algebraic image of ).
For , morphisms in for which
the underlying geometric morphism has an extra right adjoint
the morphism of internal algebras is an epimorphism
are in bijection with functions
that are homomorphisms on all commutative subalgebras and reflect commutativity.
In particular when is already commutative, morphisms with an extra right adjoint and epi ring homomorphism are in bijection with algebra homomorphisms .
By prop 4 every essential geometric morphism comes from a morphism of locales , which by the discussion at Alexandroff space is equivalently a morphism of posets . By the assumption of the extra right adjoint we also have a geometric morphism the other way round, and hence, again by prop. 4, an adjoint pair
that induces functors between toposes as in prop. 6. Then the fact that is a morphisms of -toposes implies algebra homomorphisms
natural in .
A key aspect about the Bohr topos is that that classical kinematics of the commutative internal -algebra is the quantum kinematics of . In fact, the very definition of provides a formal context in which Gleason's theorem has a natural formulation:
which is a -linear map when restricted to each commutative subalgebra .
A function that preserves certain structure locally – here: over each commutative subalgebra – is precisely an internal fully structure preserving homomorphism in the presheaf topos over these local objects – here: over commutative subalgebras. Hence we have the following direct topos-theoretic equivalent reformulation of Gleason’s theorem.
The following discussion makes this precise by exhibiting this formal dual as an internal locale. Since is a spatial topos, this internal locale is in fact an ordinary topological space bundle over the Alexandroff space .
for the corresponding external locale, given under the equivalence of categories
discussed at internal locale.
For a (noncommutative) C-star algebra, the assignment
is called the Bohrification of .
Regarded as an object
of external locales over , this is the topological space whose underlying set is given by the disjoint union
for all commutative subalgebras ;
For all , if and such that , then .
Regarded equivalently as an internal locale in this
As a presheaf on the poset this is given by
where for we set
with the relative topology inherited from .
This appears as (HLSW, theorem 1).
The Bohrification of only depends on its partial C-star algebra of normal elements
This is highlighted in (vdBergHeunen).
This is (Spitters06, theorem 9, corollary 10).
extends to a functor
where on the right the morphisms of internal rings are even morphisms of internal C* algebras.
With the components of the morphism of internal rings the evident objectwise inclusions, this is directly checked.
For a C*-algebra, then in quantum physics a self-adjoint operator is a quantum observable. The following statement asserts that quantum observables on are in a precise sense the -valued “functions” on the Bohr topos of .
Write for the -algebra of continuous complex functions on the real line. We think of as the incarnation of in the context of Bohr toposes.
Morphisms with an extra right adjoint and epi are in bijection to the observables on .
By prop. 7 such morphisms are in bijection to algebra homomorphisms
By functional calculus: every self-adjoint operator provides such a homomorphism by . Conversely, given such an algebra homomorphism, its image of is a self-adjoint operator in , and these two constructions are clearly inverses of each other.
In the different but related context of the spectral presheaf (Isham-Döring 07) the identification of quantum observables with a topos-theoretic construction, as far as possible, has been called “daseinisation”. This is a bit more involved than the above direct characterization in terms of maps of ringed toposes.
where on the right we have the copresheaves over the under-category . This is precisely the sub-poset of commutative subalgebras on those commutative subalgebras that contain . This means that a classically consistent observation in the slice topos is one that is consistent with the observation of .
For write for the ringed topos as indicated, where denotes the copresheaf constant on .
The internal -algebra is an internal -module. Forgetting the algebra structure and only remembering the -module structure, we get a category of ”-module toposes”.
There is a canonical morphism of ringed toposes
whose underlying geometric morphism is the identity (and whose morphism of internal ring objects is the unique one).
Let be a Lorentzian manifold and
This appears as (Nuiten, def. 17).
Assume that a net of observables satisfies the split property?. Then it is strongly local precisely if the corresponding presheaf of Bohr toposes satisfies spatial descent by local geometric morphisms (meaning that for every spatial hyperslice the induced presheaf satisfies descent by local geometric morphisms.)
This appears as (Nuiten, theorem 4.2).
Above is discussed the notion of Bohr topos given by covariant functors on the poset of commutative subalgebras of a C*-algebra. The fact that the functors here are covariant is related to the fact that the algebra itself naturally exists inside the presheaf topos.
Alternatively one can explore the situation for contravariant functors on the poset of commutative subalgebras (Isham-Döring 07). The resulting presheaf topos then does not directly contain the given -algebra, but by Gelfand duality, does directly contain an internal locale which is its Gelfand spectrum. This is called the “spectral presheaf”.
The assertion by Bohr that all experiments in quantum mechanics must be possible to describe in “classical terms” is in
however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms .
Niels Bohr’s views on quantum mechanics that give the construction of Bohrification its name are reviewed further in
The term Bohrification and the investigations associated with it are initiated in
The computation of the internal Gelfand spectrum was initiated in
with some results in section 5 and 6 of
and completed in
An complete outline of the full proof is given in
Applications and examples for a matrix algebra are discussed in
The functoriality of Bohrification is observed in
Jeremy Butterfield, John Hamilton, Chris Isham, A topos perspective on the Kochen-Specker theorem, I. quantum states as generalized valuations, Internat. J. Theoret. Phys. 37(11):2669–2733, 1998, MR2000c:81027, doi; II. conceptual aspects and classical analogues Int. J. of Theor. Phys. 38(3):827–859, 1999, MR2000f:81012, doi; III. Von Neumann algebras as the base category, Int. J. of Theor. Phys. 39(6):1413–1436, 2000, arXiv:quant-ph/9911020, MR2001k:81016,doi; IV. Interval valuations, Internat. J. Theoret. Phys. 41 (2002), no. 4, 613–639, MR2003g:81009, doi
Disucssion of aspects of the process of quantization in terms of Bohr toposes is in