Physics[DiracConjugate] - compute the Dirac conjugate a given mathematical expression
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Calling Sequence
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DiracConjugate(psi)
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Parameters
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psi
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any mathematical expression, possibly involving spinors or matrices
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Description
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The DiracConjugate command represents and computes the Dirac conjugate of its argument; the returned result is built as follows:
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- If is a scalar, return its conjugate, .
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- If is a spinor, so defined with one spinor index using the Define command, or is an anticommutative quantum operator (see Setup) with a spinor index, then return unevaluated, as DiracConjugate(psi), displayed as , representing , where is the Hermitian conjugate Dagger(psi) and is the contravariant Dirac matrix Dgamma[~0].
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- If is the Dirac conjugate of - say - , then return .
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- If is a Dirac matrix (represented by the Dgamma command), then return the matrix itself, also when .
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- If is a Matrix - say - return the matrix product , where is the Dagger(M), the Hermitian conjugate of .
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- If is a sum of terms, return the sum of the DiracConjugate of each term.
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- If is a product, return , that is the product of the DiracConjugate of each of the factors with the ordering reversed.
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- If is one of the d_ or dAlembertian operators, return the operator applied to the DiracConjugate of the first operand of .
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- Otherwise, return the operation unevaluated, DiracConjugate(A).
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The %DiracConjugate command is the inert form of DiracConjugate; that is, it represents the same mathematical operation while displaying the operation unevaluated. To evaluate the operation, use the value command.
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About Spinors
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After loading Physics, you indicate the letter to represent a spinor index using the Setup command, for example: Setup(spinorindices = lowercaselatin). A spinor is then any symbol indexed with one spinor index, and defined as a tensor using Define. If the symbol is anticommutative, and has one spinor index, and was defined as a tensor using Define or alternatively was set as a quantum operator using Setup, then it represents a DiracSpinor,
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You can check whether say is a spinor using the Library:-PhysicsType:-Spinor, as in type(psi[j], Library:-PhysicsType:-Spinor). Likewise, you can check if the object is a DiracSpinor using the Library:-PhysicsType:-DiracSpinor. Note that the object can have more than one index, but to be a spinor (Dirac or not), only one of them must be a spinor index. The related types Library:-PhysicsType:-SpinorWithoutIndices and Library:-PhysicsType:-DiracSpinorWithoutIndices return true when the argument passed to them, say , has no indices but would be a Spinor if it had a spinor index.
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Any tensor with more than one spinor index, is not "a spinor" but it is spinorial, for the example the Dirac matrices . You can check whether an object is spinorial using the Library:-IsSpinorial command, say as in Library:-IsSpinorial(Dgamma[mu][j,k]), which will returns true in this case, and false when the object is not spinorial. Likewise, a sum of spinorial objects, or a product of them with at least one free spinor index, is considered spinorial, for which Library:-IsSpinorial returns true.
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Examples
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The Dirac conjugate of a scalar is the standard conjugate
In general, the Dirac conjugate of the Dirac conjugate of an object is the object itself
The Dirac conjugate of a Dirac matrix is the Dirac matrix itself
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Note the display is different from the display of conjugate: the above has the bar in black and bold, instead of blue and thin as in (1)
The Dirac conjugate of is also equal to itself
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Set coordinates, a quantum operator, an anticommutative prefix and a kind of letter to represent spinor indices
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Define then one spinor using the anticommutative prefix and also a generic noncommutative spinor
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Take their product
Sum over the repeated indices, then take the Dirac conjugate of the sum
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This result is expressed in terms of the conjugate of the spinor components of and . Reversing the order of operations results in the same: take first the Dirac conjugate of the product , then sum over the repeated indices
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Unlike conjugate, DiracConjugate allows for constructing true scalars using contracted products of spinors
The Dirac conjugate of a Matrix
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The output involves the conjugates of the components of the transpose of multiplied at both sides by the Dirac matrix
If the matrix components are real,
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The Lagrangian of QED: to load the StandardModel package, clear first the letters used to represent spinor indices
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This Lagrangian is a scalar, constructed as a sum of products, where each term and each product involves noncommutative objects; the first term includes the contracted spinor product of the DiracConjugate of the electron field . Computing the Dirac conjugate of this Lagrangian is thus expected to result in several intermediate computations such that, at the end, the result is the same Lagrangian
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For Annihilation and Creation operators, DiracConjugate returns the same as the Dagger command, that is the dual, respectively.
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DiracConjugate understands Commutator and AntiCommutator
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Thus, the DiracConjugate of an AntiCommutator of Hermitian operators is equal to itself (however, the product of two Hermitian operators is Hermitian only if they commute).
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In the generic, non-Hermitian case:
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For linear operators, differential and others, DiracConjugate is applied to the first operand.
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See Also
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Annihilation, AntiCommutator, Bra, Bracket, Commutator, Creation, d_, Dagger, dAlembertian, g_, Ket, Physics, Physics conventions, Physics examples, Physics Updates, Tensors - a complete guide, Mini-Course Computer Algebra for Physicists, Setup, value
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Compatibility
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The Physics[DiracConjugate] command was introduced in Maple 2024.
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