/-
Copyright (c) 2024 Joseph Tooby-Smith. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Joseph Tooby-Smith
-/
import HepLean.FeynmanDiagrams.Wick.Species
/-!

# Operator algebra

Currently this file is only for an example of Wick strings, correpsonding to a
theory with two complex scalar fields. The concepts will however generalize.

This file is currently a stub.

We will formally define the operator ring, in terms of the fields present in the theory.

## Futher reading

- https://physics.stackexchange.com/questions/258718/ and links therein
- Ryan Thorngren (https://physics.stackexchange.com/users/10336/ryan-thorngren), Fermions,
  different species and (anti-)commutation rules, URL (version: 2019-02-20) :
  https://physics.stackexchange.com/q/461929
-/

namespace Wick
open CategoryTheory
open FeynmanDiagram
open PreFeynmanRule

informal_definition WickAlgebra where
  math :≈ "
    Modifications of this may be needed.
    A structure with the following data:
    - A ℤ₂-graded algebra A.
    - A map from `ψ : 𝓔 × SpaceTime → A` where 𝓔 are field colors.
    - A map `ψ₊ : 𝓔 × SpaceTime → A`.
    - A map `ψ₋ : 𝓔 × SpaceTime → A`.
    Subject to the conditions:
    - The sum of `ψ0` and `ψ1` is `ψ`.
    - Two fields super-commute if there colors are not dual to each other.
    - The super-commutator of two fields is always in the
      center of the algebra. "
  physics :≈ "This is defined to be an
    abstraction of the notion of an operator algebra."
  ref :≈ "https://physics.stackexchange.com/questions/24157/"

informal_definition WickMonomial where
  math :≈ "The type of elements of the Wick algebra which is a product of fields."
  deps :≈ [``WickAlgebra]

namespace WickMonomial

informal_definition toWickAlgebra where
  math :≈ "A function from WickMonomial to WickAlgebra which takes a monomial and
    returns the product of the fields in the monomial."
  deps :≈ [``WickAlgebra, ``WickMonomial]

informal_definition timeOrder where
  math :≈ "A function from WickMonomial to WickAlgebra which takes a monomial and
    returns the monomial with the fields time ordered, with the correct sign
    determined by the Koszul sign factor."
  deps :≈ [``WickAlgebra, ``WickMonomial]

informal_definition normalOrder where
  math :≈ "A function from WickMonomial to WickAlgebra which takes a monomial and
    returns the element in `WickAlgebra` defined as follows
    - The ψ₊ fields are move to the right.
    - The ψ₋ fields are moved to the left.
    - Othewise the order of the fields is preserved."
  ref :≈ "https://www.imperial.ac.uk/media/imperial-college/research-centres-and-groups/theoretical-physics/msc/current/qft/handouts/qftwickstheorem.pdf"
  deps :≈ [``WickAlgebra, ``WickMonomial]

end WickMonomial

informal_definition contraction where
  math :≈ "Given two `i j : 𝓔 × SpaceTime`, the element of WickAlgebra
    defined by subtracting the normal ordering of `ψ i ψ j` from the time-ordering of
    `ψ i ψ j`."
  deps :≈ [``WickAlgebra, ``WickMonomial]

informal_lemma contraction_in_center where
  math :≈ "The contraction of two fields is in the center of the algebra."
  deps :≈ [``WickAlgebra, ``contraction]

informal_lemma contraction_non_dual_is_zero where
  math :≈ "The contraction of two fields is zero if the fields are not dual to each other."
  deps :≈ [``WickAlgebra, ``contraction]

informal_lemma timeOrder_single where
  math :≈ "The time ordering of a single field is the normal ordering of that field."
  proof :≈ "Follows from the definitions."
  deps :≈ [``WickAlgebra, ``WickMonomial.timeOrder, ``WickMonomial.normalOrder]

informal_lemma timeOrder_pair where
  math :≈ "The time ordering of two fields is the normal ordering of the fields plus the
    contraction of the fields."
  proof :≈ "Follows from the definition of contraction."
  deps :≈ [``WickAlgebra, ``WickMonomial.timeOrder, ``WickMonomial.normalOrder,
    ``contraction]

/-! TODO: Need to set up data and structure for vaccum expectation values. -/

end Wick