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The theory of **causal fermion systems** is an approach to describe fundamental physics. It provides a unification of the weak, the strong and the electromagnetic forces with gravity at the level of classical field theory.^{[1]}^{[2]} Moreover, it gives quantum mechanics as a limiting case and has revealed close connections to quantum field theory.^{[3]}^{[4]} Therefore, it is a candidate for a unified physical theory.
Instead of introducing physical objects on a preexisting spacetime manifold, the general concept is to derive spacetime as well as all the objects therein as secondary objects from the structures of an underlying causal fermion system. This concept also makes it possible to generalize notions of differential geometry to the non-smooth setting.^{[5]}^{[6]} In particular, one can describe situations when spacetime no longer has a manifold structure on the microscopic scale (like a spacetime lattice or other discrete or continuous structures on the Planck scale). As a result, the theory of causal fermion systems is a proposal for quantum geometry and an approach to quantum gravity.

Causal fermion systems were introduced by Felix Finster and collaborators.

The physical starting point is the fact that the Dirac equation in Minkowski space has solutions of negative energy which are usually associated to the Dirac sea. Taking the concept seriously that the states of the Dirac sea form an integral part of the physical system, one finds that many structures (like the causal and metric structures as well as the bosonic fields) can be recovered from the wave functions of the sea states. This leads to the idea that the wave functions of all occupied states (including the sea states) should be regarded as the basic physical objects, and that all structures in spacetime arise as a result of the collective interaction of the sea states with each other and with the additional particles and "holes" in the sea. Implementing this picture mathematically leads to the framework of causal fermion systems.

More precisely, the correspondence between the above physical situation and the mathematical framework is obtained as follows. All occupied states span a Hilbert space of wave functions in Minkowski space . The observable information on the distribution of the wave functions in spacetime is encoded in the *local correlation operators* which in an orthonormal basis have the matrix representation

(where is the adjoint spinor).
In order to make the wave functions into the basic physical objects, one considers the set as a set of linear operators on an *abstract* Hilbert space. The structures of Minkowski space are all disregarded, except for the volume measure , which is transformed to a corresponding measure on the linear operators (the *"universal measure"*). The resulting structures, namely a Hilbert space together with a measure on the linear operators thereon, are the basic ingredients of a causal fermion system.

The above construction can also be carried out in more general spacetimes. Moreover, taking the abstract definition as the starting point, causal fermion systems allow for the description of generalized "quantum spacetimes." The physical picture is that one causal fermion system describes a spacetime together with all structures and objects therein (like the causal and the metric structures, wave functions and quantum fields). In order to single out the physically admissible causal fermion systems, one must formulate physical equations. In analogy to the Lagrangian formulation of classical field theory, the physical equations for causal fermion systems are formulated via a variational principle, the so-called *causal action principle*. Since one works with different basic objects, the causal action principle has a novel mathematical structure where one minimizes a positive action under variations of the universal measure. The connection to conventional physical equations is obtained in a certain limiting case (the *continuum limit*) in which the interaction can be described effectively by gauge fields coupled to particles and antiparticles, whereas the Dirac sea is no longer apparent.

In this section the mathematical framework of causal fermion systems is introduced.

A **causal fermion system** of spin dimension is a triple where

- is a complex Hilbert space.
- is the set of all self-adjoint linear operators of finite rank on which (counting multiplicities) have at most positive and at most negative eigenvalues.
- is a measure on .

The measure is referred to as the **universal measure**.

As will be outlined below, this definition is rich enough to encode analogs of the mathematical structures needed to formulate physical theories. In particular, a causal fermion system gives rise to a spacetime together with additional structures that generalize objects like spinors, the metric and curvature. Moreover, it comprises quantum objects like wave functions and a fermionic Fock state.^{[7]}

Inspired by the Langrangian formulation of classical field theory, the dynamics on a causal fermion system is described by a variational principle defined as follows.

Given a Hilbert space and the spin dimension , the set is defined as above. Then for any , the product is an operator of rank at most . It is not necessarily self-adjoint because in general . We denote the non-trivial eigenvalues of the operator (counting algebraic multiplicities) by

Moreover, the **spectral weight** is defined by

The **Lagrangian** is introduced by

The **causal action** is defined by

The **causal action principle** is to minimize under variations of within the class of (positive) Borel measures under the following constraints:

- Boundedness constraint: for some positive constant .
- Trace constraint: is kept fixed.
- The total volume is preserved.

Here on one considers the topology induced by the -norm on the bounded linear operators on .

The constraints prevent trivial minimizers and ensure existence, provided that is finite-dimensional.^{[8]}
This variational principle also makes sense in the case that the total volume is infinite if one considers variations of bounded variation with .

In contemporary physical theories, the word spacetime refers to a Lorentzian manifold . This means that spacetime is a set of points enriched by topological and geometric structures. In the context of causal fermion systems, spacetime does not need to have a manifold structure. Instead, spacetime is a set of operators on a Hilbert space (a subset of ). This implies additional inherent structures that correspond to and generalize usual objects on a spacetime manifold.

For a causal fermion system ,
we define **spacetime** as the support of the universal measure,

With the topology induced by , spacetime is a topological space.

For , we denote the non-trivial eigenvalues of the operator (counting algebraic multiplicities) by .
The points and are defined to be **spacelike** separated if all the have the same absolute value. They are **timelike** separated if the do not all have the same absolute value and are all real. In all other cases, the points and are **lightlike** separated.

This notion of causality fits together with the "causality" of the above causal action in the sense that if two spacetime points are space-like separated, then the Lagrangian vanishes. This corresponds to the physical notion of causality that spatially separated spacetime points do not interact. This causal structure is the reason for the notion "causal" in causal fermion system and causal action.

Let denote the orthogonal projection on the subspace . Then the sign of the functional

distinguishes the **future** from the **past**. In contrast to the structure of a partially ordered set, the relation "lies in the future of" is in general not transitive. But it is transitive on the macroscopic scale in typical examples.^{[5]}^{[6]}

For every the **spin space** is defined by ; it is a subspace of of dimension at most . The **spin scalar product** defined by

is an indefinite inner product on of signature with .

A **wave function** is a mapping

On wave functions for which the norm defined by

is finite (where is the absolute value of the symmetric operator ), one can define the inner product

Together with the topology induced by the norm , one obtains a Krein space .

To any vector we can associate the wave function

(where is again the orthogonal projection to the spin space).
This gives rise to a distinguished family of wave functions, referred to as the
wave functions of the **occupied states**.

The **kernel of the fermionic projector** is defined by

(where is again the orthogonal projection on the spin space,
and denotes the restriction to ). The **fermionic projector** is the operator

which has the dense domain of definition given by all vectors satisfying the conditions

As a consequence of the causal action principle, the kernel of the fermionic projector has additional normalization properties^{[9]} which justify the name projector.

Being an operator from one spin space to another, the kernel of the fermionic projector gives relations between different spacetime points. This fact can be used to introduce a **spin connection**

The basic idea is to take a polar decomposition of . The construction becomes more involved by the fact that the spin connection should induce a corresponding **metric connection**

where the tangent space is a specific subspace of the linear operators on endowed with a Lorentzian metric.
The **spin curvature** is defined as the holonomy of the spin connection,

Similarly, the metric connection gives rise to **metric curvature**. These geometric structures give rise to a proposal for a quantum geometry.^{[5]}

A minimizer of the causal action satisfies corresponding **Euler-Lagrange equations**.^{[10]} They state that the function
defined by

(with two Lagrange parameters and ) vanishes and is minimal on the support of ,

For the analysis, it is convenient to introduce **jets** consisting of a real-valued function on and a vector field on along , and to denote the combination of multiplication and directional derivative by . Then the Euler-Lagrange equations imply that the **weak Euler-Lagrange equations**

hold for any test jet .

Families of solutions of the Euler-Lagrange equations are generated infinitesimally by a jet which satisfies the **linearized field equations**

to be satisfied for all test jets , where the Laplacian $\Delta$ is defined by

The Euler-Lagrange equations describe the dynamics of the causal fermion system, whereas small perturbations of the system are described by the linearized field equations.

In the setting of causal fermion systems, spatial integrals are expressed by so-called **surface layer integrals**.^{[9]}^{[10]}^{[11]} In general terms, a surface layer integral is a double integral of the form

where one variable is integrated over a subset , and the other variable is integrated over the complement of . It is possible to express the usual conservation laws for charge, energy, ... in terms of surface layer integrals. The corresponding conservation laws are a consequence of the Euler-Lagrange equations of the causal action principle and the linearized field equations. For the applications, the most important surface layer integrals are the *current integral* , the *symplectic form* , the *surface layer inner product* and the *nonlinear surface layer integral* .

Based on the conservation laws for the above surface layer integrals, the dynamics of a causal fermion system as described by the Euler-Lagrange equations corresponding to the causal action principle can be rewritten as a linear, norm-preserving dynamics on the bosonic Fock space built up of solutions of the linearized field equations.^{[4]} In the so-called *holomorphic approximation*, the time evolution respects the complex structure, giving rise to a unitary time evolution on the bosonic Fock space.

If has finite dimension , choosing an orthonormal basis of and taking the wedge product of the corresponding wave functions

gives a state of an -particle fermionic Fock space. Due to the total anti-symmetrization, this state depends on the choice of the basis of only by a phase factor.^{[12]} This correspondence explains why the vectors in the particle space are to be interpreted as fermions. It also motivates the name causal **fermion** system.

Causal fermion systems incorporate several physical principles in a specific way:

- A
**local gauge principle**: In order to represent the wave functions in components, one chooses bases of the spin spaces. Denoting the signature of the spin scalar product at by , a pseudo-orthonormal basis of is given by

- Then a wave function can be represented with component functions,
- The freedom of choosing the bases independently at every spacetime point corresponds to local unitary transformations of the wave functions,
- These transformations have the interpretation as local gauge transformations. The gauge group is determined to be the isometry group of the spin scalar product. The causal action is gauge invariant in the sense that it does not depend on the choice of spinor bases.

- The
**equivalence principle**: For an explicit description of spacetime one must work with local coordinates. The freedom in choosing such coordinates generalizes the freedom in choosing general reference frames in a spacetime manifold. Therefore, the equivalence principle of general relativity is respected. The causal action is generally covariant in the sense that it does not depend on the choice of coordinates. - The
**Pauli exclusion principle**: The fermionic Fock state associated to the causal fermion system makes it possible to describe the many-particle state by a totally antisymmetric wave function. This gives agreement with the Pauli exclusion principle. - The principle of
**causality**is incorporated by the form of the causal action in the sense that spacetime points with spacelike separation do not interact.

Causal fermion systems have mathematically sound limiting cases that give a connection to conventional physical structures.

Starting on any globally hyperbolic Lorentzian spin manifold with spinor bundle , one gets into the framework of causal fermion systems by choosing as a subspace of the solution space of the Dirac equation. Defining the so-called **local correlation operator** for by

(where is the inner product on the fibre ) and introducing the universal measure as the push-forward of the volume measure on ,

one obtains a causal fermion system. For the local correlation operators to be well-defined, must consist of continuous sections, typically making it necessary to introduce a regularization on the microscopic scale . In the limit , all the intrinsic structures on the causal fermion system (like the causal structure, connection and curvature) go over to the corresponding structures on the Lorentzian spin manifold.^{[5]} Thus the geometry of spacetime is encoded completely in the corresponding causal fermion systems.

The Euler-Lagrange equations corresponding to the causal action principle have a well-defined limit if the spacetimes of the causal fermion systems go over to Minkowski space. More specifically, one considers a sequence of causal fermion systems (for example with finite-dimensional in order to ensure the existence of the fermionick Fock state as well as of minimizers of the causal action), such that the corresponding wave functions go over to a configuration of interacting Dirac seas involving additional particle states or "holes" in the seas. This procedure, referred to as the *continuum limit*, gives effective equations having the structure of the Dirac equation coupled to classical field equations. For example, for a simplified model involving three elementary fermionic particles
in spin dimension two, one obtains an interaction via a classical axial gauge field ^{[2]} described by the coupled Dirac- and Yang-Mills equations

Taking the non-relativistic limit of the Dirac equation, one obtains the Pauli equation or the Schrödinger equation, giving the correspondence to quantum mechanics. Here and depend on the regularization and determine the coupling constant as well as the rest mass.

Likewise, for a system involving neutrinos in spin dimension 4, one gets effectively a massive gauge field coupled to the left-handed component of the Dirac spinors.^{[2]} The fermion configuration of the standard model can be described in spin dimension 16.^{[1]}

For the just-mentioned system involving neutrinos,^{[2]} the continuum limit also yields the Einstein field equations coupled to the Dirac spinors,

up to corrections of higher order in the curvature tensor. Here the cosmological constant is undetermined, and denotes the energy-momentum tensor of the spinors and the gauge field. The gravitation constant depends on the regularization length.

Starting from the coupled system of equations obtained in the continuum limit and expanding in powers of the coupling constant, one obtains integrals which correspond to Feynman diagrams on the tree level. Fermionic loop diagrams arise due to the interaction with the sea states, whereas bosonic loop diagrams appear when taking averages over the microscopic (in generally non-smooth) spacetime structure of a causal fermion system (so-called *microscopic mixing*).^{[3]} The detailed analysis and comparison with standard quantum field theory is work in progress.^{[4]}

- ^
^{a}^{b}Finster, Felix (2006).*The Principle of the Fermionic Projector*. Providence, R.I: American Mathematical Society. ISBN 978-0-8218-3974-4. OCLC 61211466.Chapters 1-4Chapters 5-8Appendices - ^
^{a}^{b}^{c}^{d}Finster, Felix (2016).*The Continuum Limit of Causal Fermion Systems*. Fundamental Theories of Physics. Vol. 186. Cham: Springer International Publishing. arXiv:1605.04742. doi:10.1007/978-3-319-42067-7. ISBN 978-3-319-42066-0. ISSN 0168-1222. - ^
^{a}^{b}Finster, Felix (2014). "Perturbative quantum field theory in the framework of the fermionic projector".*Journal of Mathematical Physics*.**55**(4): 042301. arXiv:1310.4121. doi:10.1063/1.4871549. ISSN 0022-2488. - ^
^{a}^{b}^{c}Finster, Felix; Kamran, Niky (2018). "Complex structures on jet spaces and bosonic fock space dynamics for causal variational principles". arXiv:1808.03177.`{{cite journal}}`

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(help) - ^
^{a}^{b}^{c}^{d}Finster, Felix; Grotz, Andreas (2012). "A Lorentzian quantum geometry".*Advances in Theoretical and Mathematical Physics*.**16**(4): 1197–1290. arXiv:1107.2026. doi:10.4310/atmp.2012.v16.n4.a3. ISSN 1095-0761. - ^
^{a}^{b}Finster, Felix; Kamran, Niky (2019). "Spinors on singular spaces and the topology of causal fermion systems".*Memoirs of the American Mathematical Society*.**259**(1251): v+83. arXiv:1403.7885. doi:10.1090/memo/1251. ISSN 0065-9266. **^**Finster, Felix; Grotz, Andreas; Schiefeneder, Daniela (2012). "Causal Fermion Systems: A Quantum Space-Time Emerging From an Action Principle".*Quantum Field Theory and Gravity*. Basel: Springer Basel. pp. 157–182. arXiv:1102.2585. doi:10.1007/978-3-0348-0043-3_9. ISBN 978-3-0348-0042-6.**^**Finster, Felix (2010). "Causal variational principles on measure spaces".*Journal für die reine und angewandte Mathematik*.**2010**(646): 141–194. arXiv:0811.2666. doi:10.1515/crelle.2010.069. ISSN 0075-4102.- ^
^{a}^{b}Finster, Felix; Kleiner, Johannes (2016). "Noether-like theorems for causal variational principles".*Calculus of Variations and Partial Differential Equations*.**55**: 35. arXiv:1506.09076. doi:10.1007/s00526-016-0966-y. ISSN 0944-2669. - ^
^{a}^{b}Finster, Felix; Kleiner, Johannes (2017). "A Hamiltonian formulation of causal variational principles".*Calculus of Variations and Partial Differential Equations*.**56**: 73. arXiv:1612.07192. doi:10.1007/s00526-017-1153-5. ISSN 0944-2669. **^**Finster, Felix; Kleiner, Johannes (2019). "A class of conserved surface layer integrals for causal variational principles".*Calculus of Variations and Partial Differential Equations*.**58**: 38. arXiv:1801.08715. doi:10.1007/s00526-018-1469-9. ISSN 0944-2669.**^**Finster, Felix (2010). "Entanglement and second quantization in the framework of the fermionic projector".*Journal of Physics A: Mathematical and Theoretical*.**43**(39): 395302. arXiv:0911.0076. doi:10.1088/1751-8113/43/39/395302. ISSN 1751-8113.

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