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In mathematical physics, the gamma matrices, ${gamma ^{0},gamma ^{1},gamma ^{2},gamma ^{3}}$ , also known as the Dirac matrices, are a set of conventional matrices with specific anticommutation relations that ensure they generate a matrix representation of the Clifford algebra Cℓ_1,3(R). It is also possible to define higher-dimensional gamma matrices. When interpreted as the matrices of the action of a set of orthogonal basis vectors for contravariant vectors in Minkowski space, the column vectors on which the matrices act become a space of spinors, on which the Clifford algebra of spacetime acts. This in turn makes it possible to represent infinitesimal spatial rotations and Lorentz boosts. Spinors facilitate spacetime computations in general, and in particular are fundamental to the Dirac equation for relativistic spin-½ particles.

In Dirac representation, the four contravariant gamma matrices are

{displaystyle {begin{aligned}gamma ^{0}&={begin{pmatrix}1&0&0&0\0&1&0&0\0&0&-1&0\0&0&0&-1end{pmatrix}},&gamma ^{1}&={begin{pmatrix}0&0&0&1\0&0&1&0\0&-1&0&0\-1&0&0&0end{pmatrix}},\gamma ^{2}&={begin{pmatrix}0&0&0&-i\0&0&i&0\0&i&0&0\-i&0&0&0end{pmatrix}},&gamma ^{3}&={begin{pmatrix}0&0&1&0\0&0&0&-1\-1&0&0&0\0&1&0&0end{pmatrix}}.end{aligned}}}

$gamma ^{0}$ is the time-like, hermitian matrix. The other three are space-like, antihermitian matrices. More compactly, ${displaystyle gamma ^{0}=sigma ^{3}otimes I}$ , and ${displaystyle gamma ^{i}=isigma ^{2}otimes sigma ^{i}}$ .

Analogous sets of gamma matrices can be defined in any dimension and for any signature of the metric. For example, the Pauli matrices are a set of "gamma" matrices in dimension 3 with metric of Euclidean signature (3, 0). In 5 spacetime dimensions, the 4 gammas above together with the fifth gamma-matrix to be presented below generate the Clifford algebra.

Mathematical structure

The defining property for the gamma matrices to generate a Clifford algebra is the anticommutation relation

{displaystyle {gamma ^{mu },gamma ^{nu }}=gamma ^{mu }gamma ^{nu }+gamma ^{nu }gamma ^{mu }=2eta ^{mu nu }I_{4},}

where ${,}$ is the anticommutator, $eta ^{mu nu }$ is the Minkowski metric with signature (+ − − −), and $I_{4}$ is the 4 × 4 identity matrix.

This defining property is more fundamental than the numerical values used in the specific representation of the gamma matrices. Covariant gamma matrices are defined by

{displaystyle gamma _{mu }=eta _{mu nu }gamma ^{nu }=left{gamma ^{0},-gamma ^{1},-gamma ^{2},-gamma ^{3}right},}

and Einstein notation is assumed.

Note that the other sign convention for the metric, (− + + +) necessitates either a change in the defining equation:

{displaystyle {gamma ^{mu },gamma ^{nu }}=-2eta ^{mu nu }I_{4}}

or a multiplication of all gamma matrices by $i$ , which of course changes their hermiticity properties detailed below. Under the alternative sign convention for the metric the covariant gamma matrices are then defined by

{displaystyle gamma _{mu }=eta _{mu nu }gamma ^{nu }=left{-gamma ^{0},+gamma ^{1},+gamma ^{2},+gamma ^{3}right}.}

Physical structure

The Clifford algebra $Cl 1,3 (R)$ over spacetime $V$ can be regarded as the set of real linear operators from $V$ to itself, $End(V)$ , or more generally, when complexified to $Cl 1,3 (R) C$ , as the set of linear operators from any 4-dimensional complex vector space to itself. More simply, given a basis for $V$ , $Cl 1,3 (R) C$ is just the set of all $4 \times 4$ complex matrices, but endowed with a Clifford algebra structure. Spacetime is assumed to be endowed with the Minkowski metric $η μν$ . A space of bispinors, $U x$ , is also assumed at every point in spacetime, endowed with the bispinor representation of the Lorentz group. The bispinor fields $Ψ$ of the Dirac equations, evaluated at any point $x$ in spacetime, are elements of $U x$ , see below. The Clifford algebra is assumed to act on $U x$ as well (by matrix multiplication with column vectors $Ψ(x)$ in $U x$ for all $x$ ). This will be the primary view of elements of $Cl 1,3 (R) C$ in this section.

For each linear transformation $S$ of $U x$ , there is a transformation of $End(U x)$ given by $SES -1$ for $E$ in $Cl 1,3 (R) C \approx End(U x)$ . If $S$ belongs to a representation of the Lorentz group, then the induced action $E \mapsto SES -1$ will also belong to a representation of the Lorentz group, see Representation theory of the Lorentz group.

If $S(Λ)$ is the bispinor representation acting on $U x$ of an arbitrary Lorentz transformation $Λ$ in the standard (4-vector) representation acting on $V$ , then there is a corresponding operator on $End(U x) = Cl 1,3 (R) C$ given by

{displaystyle gamma ^{mu }mapsto S(Lambda )gamma ^{mu }S(Lambda )^{-1}={(Lambda ^{-1})^{mu }}_{nu }gamma ^{nu }:={Lambda _{nu }}^{mu }gamma ^{nu },}

showing that the $γ μ$ can be viewed as a basis of a representation space of the 4-vector representation of the Lorentz group sitting inside the Clifford algebra. This means that quantities of the form

{displaystyle a!!!/:=a_{mu }gamma ^{mu }}

should be treated as 4-vectors in manipulations. It also means that indices can be raised and lowered on the $γ$ using the metric $η μν$ as with any 4-vector. The notation is called the Feynman slash notation. The slash operation maps the basis $e μ$ of $V$ , or any 4-dimensional vector space, to basis vectors $γ μ$ . The transformation rule for slashed quantities is simply

{displaystyle {a!!!/}^{mu }mapsto {Lambda ^{mu }}_{nu }{a!!!/}^{nu }.}

One should note that this is different from the transformation rule for the $γ μ$ , which are now treated as (fixed) basis vectors. The designation of the 4-tuple $(γ μ) = (γ 0, γ 1, γ 2, γ 3)$ as a 4-vector sometimes found in the literature is thus a slight misnomer. The latter transformation corresponds to an active transformation of the components of a slashed quantity in terms of the basis $γ μ$ , and the former to a passive transformation of the basis $γ μ$ itself.

The elements $σ μν = γ μ γ ν - γ ν γ μ$ form a representation of the Lie algebra of the Lorentz group. This is a spin representation. When these matrices, and linear combinations of them, are exponentiated, they are bispinor representations of the Lorentz group, e.g., the $S(Λ)$ of above are of this form. The 6-dimensional space the $σ μν$ span is the representation space of a tensor representation of the Lorentz group. For the higher order elements of the Clifford algebra in general and their transformation rules, see the article Dirac algebra. But it is noted here that the Clifford algebra has no subspace being the representation space of a spin representation of the Lorentz group in the context used here.

Expressing the Dirac equation

In natural units, the Dirac equation may be written as

(igamma ^{mu }partial _{mu }-m)psi =0

where $psi$ is a Dirac spinor.

Switching to Feynman notation, the Dirac equation is

{displaystyle (i{partial !!!/}-m)psi =0.}

The fifth gamma matrix, `γ`⁵

It is useful to define the product of the four gamma matrices as follows:

{displaystyle gamma ^{5}:=igamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}={begin{pmatrix}0&0&1&0\0&0&0&1\1&0&0&0\0&1&0&0end{pmatrix}}}

(in the Dirac basis).

Although $gamma ^{5}$ uses the letter gamma, it is not one of the gamma matrices of Cℓ_1,3(R). The number 5 is a relic of old notation in which $gamma ^{0}$ was called " $gamma ^{4}$ ".

$gamma ^{5}$ has also an alternative form:

gamma ^{5}={frac {i}{4!}}varepsilon _{mu nu alpha beta }gamma ^{mu }gamma ^{nu }gamma ^{alpha }gamma ^{beta }

Proof

This can be seen by exploiting the fact that all the four gamma matrices anticommute, so

gamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}=gamma ^{[0}gamma ^{1}gamma ^{2}gamma ^{3]}={frac {1}{4!}}delta _{mu nu varrho sigma }^{0123}gamma ^{mu }gamma ^{nu }gamma ^{varrho }gamma ^{sigma }

,

where $delta _{mu nu varrho sigma }^{alpha beta gamma delta }$ is the type (4,4) generalized Kronecker delta in 4 dimensions, in full antisymmetrization. If $varepsilon _{alpha dots beta }$ denotes the Levi-Civita symbol in n dimensions, we can use the identity $delta _{mu nu varrho sigma }^{alpha beta gamma delta }=varepsilon ^{alpha beta gamma delta }varepsilon _{mu nu varrho sigma }$ .
Then we get

gamma ^{5}=igamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}={frac {i}{4!}}varepsilon ^{0123}varepsilon _{mu nu varrho sigma },gamma ^{mu }gamma ^{nu }gamma ^{varrho }gamma ^{sigma }={frac {i}{4!}}varepsilon _{mu nu varrho sigma },gamma ^{mu }gamma ^{nu }gamma ^{varrho }gamma ^{sigma }

This matrix is useful in discussions of quantum mechanical chirality. For example, a Dirac field can be projected onto its left-handed and right-handed components by:

{displaystyle psi _{L}={frac {1-gamma ^{5}}{2}}psi ,qquad psi _{R}={frac {1+gamma ^{5}}{2}}psi }

.

Some properties are:

It is hermitian:

${displaystyle (gamma ^{5})^{dagger }=gamma ^{5}.}$

Its eigenvalues are ±1, because:

${displaystyle (gamma ^{5})^{2}=I_{4}.}$

It anticommutes with the four gamma matrices:

${displaystyle left{gamma ^{5},gamma ^{mu }right}=gamma ^{5}gamma ^{mu }+gamma ^{mu }gamma ^{5}=0.}$

The set ${γ 0, γ 1, γ 2, γ 3, iγ 5}$ therefore, by the last two properties (keeping in mind that $i 2 = -1$ ) and those of the old gammas, forms the basis of the Clifford algebra in $5$ spacetime dimensions for the metric signature $(1,4)$ .^[1] In metric signature $(4,1)$ , the set ${γ 0, γ 1, γ 2, γ 3, γ 5}$ is used, where the $γ μ$ are the appropriate ones for the $(3,1)$ signature.^[2] This pattern is repeated for spacetime dimension $2 n$ even and the next odd dimension $2 n + 1$ for all $n \geq 1$ .^[3] For more detail, see Higher-dimensional gamma matrices.

Identities

The following identities follow from the fundamental anticommutation relation, so they hold in any basis (although the last one depends on the sign choice for $gamma ^{5}$ ).

Miscellaneous identities

{displaystyle gamma ^{mu }gamma _{mu }=4I_{4}}

Proof

Take the standard anticommutation relation:

{displaystyle left{gamma ^{mu },gamma ^{nu }right}=gamma ^{mu }gamma ^{nu }+gamma ^{nu }gamma ^{mu }=2eta ^{mu nu }I_{4}.}

One can make this situation look similar by using the metric $eta$ :

${displaystyle {begin{aligned}&gamma ^{mu }gamma _{mu }\={}&gamma ^{mu }eta _{mu nu }gamma ^{nu }=eta _{mu nu }gamma ^{mu }gamma ^{nu }end{aligned}}}$
${displaystyle ={frac {1}{2}}left(eta _{mu nu }+eta _{nu mu }right)gamma ^{mu }gamma ^{nu }}$	( $eta$ symmetric)
${displaystyle ={frac {1}{2}}left(eta _{mu nu }gamma ^{mu }gamma ^{nu }+eta _{nu mu }gamma ^{mu }gamma ^{nu }right)}$	(expanding)
${displaystyle ={frac {1}{2}}left(eta _{mu nu }gamma ^{mu }gamma ^{nu }+eta _{mu nu }gamma ^{nu }gamma ^{mu }right)}$	(relabeling term on right)
${displaystyle ={frac {1}{2}}eta _{mu nu }left{gamma ^{mu },gamma ^{nu }right}}$
${displaystyle ={frac {1}{2}}eta _{mu nu }left(2eta ^{mu nu }I_{4}right)=eta _{mu nu }eta ^{mu nu }I_{4}=4I_{4}.}$

${displaystyle gamma ^{mu }gamma ^{nu }gamma _{mu }=-2gamma ^{nu }}$

Proof

Similarly to the proof of 1, again beginning with the standard commutation relation:

${displaystyle {begin{aligned}gamma ^{mu }gamma ^{nu }gamma _{mu }&=gamma ^{mu }left(2eta _{mu }^{nu }I_{4}-gamma _{mu }gamma ^{nu }right)\&=2gamma ^{mu }eta _{mu }^{nu }-gamma ^{mu }gamma _{mu }gamma ^{nu }\&=2gamma ^{nu }-4gamma ^{nu }=-2gamma ^{nu }.end{aligned}}}$

{displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma _{mu }=4eta ^{nu rho }I_{4}}

Proof

To show

{displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma _{mu }=4eta ^{nu rho }I_{4}.}

Use the anticommutator to shift $gamma ^{mu }$ to the right

${displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma _{mu }}$	${displaystyle =left{gamma ^{mu },gamma ^{nu }right}gamma ^{rho }gamma _{mu }-gamma ^{nu }gamma ^{mu }gamma ^{rho }gamma _{mu }}$
	${displaystyle =2 eta ^{mu nu }gamma ^{rho }gamma _{mu }-gamma ^{nu }left{gamma ^{mu },gamma ^{rho }right}gamma _{mu }+gamma ^{nu }gamma ^{rho }gamma ^{mu }gamma _{mu }.}$

Using the relation ${displaystyle gamma ^{mu }gamma _{mu }=4I}$ we can contract the last two gammas, and get

${displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma _{mu }}$	${displaystyle =2gamma ^{rho }gamma ^{nu }-gamma ^{nu }2eta ^{mu rho }gamma _{mu }+4gamma ^{nu }gamma ^{rho }}$
	${displaystyle =2gamma ^{rho }gamma ^{nu }-2gamma ^{nu }gamma ^{rho }+4gamma ^{nu }gamma ^{rho }}$
	${displaystyle =2left(gamma ^{rho }gamma ^{nu }+gamma ^{nu }gamma ^{rho }right)}$
	${displaystyle =2left{gamma ^{nu },gamma ^{rho }right}.}$

Finally using the anticommutator identity, we get

{displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma _{mu }=4eta ^{nu rho }I_{4}.}

{displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma }gamma _{mu }=-2gamma ^{sigma }gamma ^{rho }gamma ^{nu }}

Proof

${displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma }gamma _{mu }}$	${displaystyle =(2eta ^{mu nu }-gamma ^{nu }gamma ^{mu })gamma ^{rho }gamma ^{sigma }gamma _{mu },}$ (anticommutator identity)
	${displaystyle =2eta ^{mu nu }gamma ^{rho }gamma ^{sigma }gamma _{mu }-4gamma ^{nu }eta ^{rho sigma },}$ (using identity 3)
	${displaystyle =2gamma ^{rho }gamma ^{sigma }gamma ^{nu }-4gamma ^{nu }eta ^{rho sigma }}$ (raising an index)
	${displaystyle =2left(2eta ^{rho sigma }-gamma ^{sigma }gamma ^{rho }right)gamma ^{nu }-4gamma ^{nu }eta ^{rho sigma }}$ (anticommutator identity)
	${displaystyle =-2gamma ^{sigma }gamma ^{rho }gamma ^{nu }}$ (2 terms cancel)

${displaystyle gamma ^{mu }gamma ^{nu }gamma ^{rho }=eta ^{mu nu }gamma ^{rho }+eta ^{nu rho }gamma ^{mu }-eta ^{mu rho }gamma ^{nu }-iepsilon ^{sigma mu nu rho }gamma _{sigma }gamma ^{5}}$

Proof

If $mu =nu =rho$ then ${displaystyle epsilon ^{sigma mu nu rho }=0}$ and it is easy to verify the identity. That is the case also when $mu =nu neq rho$ , $mu =rho neq nu$ or $nu =rho neq mu$ .

On the other hand, if all three indices are different, ${displaystyle eta ^{mu nu }=0}$ , ${displaystyle eta ^{mu rho }=0}$ and ${displaystyle eta ^{nu rho }=0}$ and both sides are completely antisymmetric; the left hand side because of the anticommutativity of the $gamma$ matrices, and on the right hand side because of the antisymmetry of $epsilon _{sigma mu nu rho }$ . It thus suffices to verify the identities for the cases of $gamma ^{0}gamma ^{1}gamma ^{2}$ , $gamma ^{0}gamma ^{1}gamma ^{3}$ , $gamma ^{0}gamma ^{2}gamma ^{3}$ and $gamma ^{1}gamma ^{2}gamma ^{3}$ .

${displaystyle {begin{aligned}-iepsilon ^{sigma 012}gamma _{sigma }gamma ^{5}&=-iepsilon ^{3012}left(-gamma ^{3}right)left(igamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}right)=-epsilon ^{3012}gamma ^{0}gamma ^{1}gamma ^{2}=epsilon ^{0123}gamma ^{0}gamma ^{1}gamma ^{2}\-iepsilon ^{sigma 013}gamma _{sigma }gamma ^{5}&=-iepsilon ^{2013}left(-gamma ^{2}right)left(igamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}right)=epsilon ^{2013}gamma ^{0}gamma ^{1}gamma ^{3}=epsilon ^{0123}gamma ^{0}gamma ^{1}gamma ^{3}\-iepsilon ^{sigma 023}gamma _{sigma }gamma ^{5}&=-iepsilon ^{1023}left(-gamma ^{1}right)left(igamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}right)=-epsilon ^{1023}gamma ^{0}gamma ^{2}gamma ^{3}=epsilon ^{0123}gamma ^{0}gamma ^{2}gamma ^{3}\-iepsilon ^{sigma 123}gamma _{sigma }gamma ^{5}&=-iepsilon ^{0123}left(gamma ^{0}right)left(igamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}right)=epsilon ^{0123}gamma ^{1}gamma ^{2}gamma ^{3}end{aligned}}}$

Trace identities

The gamma matrices obey the following trace identities:

${displaystyle operatorname {tr} left(gamma ^{mu }right)=0}$

Trace of any product of an odd number of $gamma ^{mu }$ is zero

Trace of $gamma ^{5}$ times a product of an odd number of $gamma ^{mu }$ is still zero

${displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }right)=4eta ^{mu nu }}$

${displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma }right)=4left(eta ^{mu nu }eta ^{rho sigma }-eta ^{mu rho }eta ^{nu sigma }+eta ^{mu sigma }eta ^{nu rho }right)}$

${displaystyle operatorname {tr} left(gamma ^{5}right)=operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{5}right)=0}$

${displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma }gamma ^{5}right)=-4iepsilon ^{mu nu rho sigma }}$

${displaystyle operatorname {tr} left(gamma ^{mu _{1}}dots gamma ^{mu _{n}}right)=operatorname {tr} left(gamma ^{mu _{n}}dots gamma ^{mu _{1}}right)}$

Proving the above involves the use of three main properties of the trace operator:

tr(A + B) = tr(A) + tr(B)

tr(rA) = r tr(A)

tr(ABC) = tr(CAB) = tr(BCA)

Proof of 0

From the definition of the gamma matrices,

{displaystyle gamma ^{mu }gamma ^{nu }+gamma ^{nu }gamma ^{mu }=2eta ^{mu nu }I}

We get

{displaystyle gamma ^{mu }gamma ^{mu }=eta ^{mu mu }I}

or equivalently,

{displaystyle {frac {gamma ^{mu }gamma ^{mu }}{eta ^{mu mu }}}=I}

where $eta ^{mu mu }$ is a number, and $gamma ^{mu }gamma ^{mu }$ is a matrix.

operatorname {tr} (gamma ^{nu })={frac {1}{eta ^{mu mu }}}operatorname {tr} (gamma ^{nu }gamma ^{mu }gamma ^{mu })

(inserting the identity and using tr(rA) = r tr(A))

=-{frac {1}{eta ^{mu mu }}}operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{mu })

(from anti-commutation relation, and given that we are free to select

mu neq nu

)

=-{frac {1}{eta ^{mu mu }}}operatorname {tr} (gamma ^{nu }gamma ^{mu }gamma ^{mu })

(using tr(ABC) = tr(BCA))

=-operatorname {tr} (gamma ^{nu })

(removing the identity)

This implies $operatorname {tr} (gamma ^{nu })=0$

Proof of 1

To show

{displaystyle operatorname {tr} (mathrm {odd num of } gamma )=0}

First note that

{displaystyle operatorname {tr} (gamma ^{mu })=0.}

We'll also use two facts about the fifth gamma matrix ${displaystyle gamma ^{5}}$ that says:

{displaystyle left(gamma ^{5}right)^{2}=I_{4},quad mathrm {and} quad gamma ^{mu }gamma ^{5}=-gamma ^{5}gamma ^{mu }}

So lets use these two facts to prove this identity for the first non-trivial case: the trace of three gamma matrices. Step one is to put in one pair of ${displaystyle gamma ^{5}}$ 's in front of the three original $gamma$ 's, and step two is to swap the ${displaystyle gamma ^{5}}$ matrix back to the original position, after making use of the cyclicity of the trace.

${displaystyle operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{rho })}$	${displaystyle =operatorname {tr} left(gamma ^{5}gamma ^{5}gamma ^{mu }gamma ^{nu }gamma ^{rho }right)}$
	${displaystyle =-operatorname {tr} left(gamma ^{5}gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{5}right)}$
	${displaystyle =-operatorname {tr} left(gamma ^{5}gamma ^{5}gamma ^{mu }gamma ^{nu }gamma ^{rho }right)}$ (using tr(ABC) = tr(BCA))
	${displaystyle =-operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{rho }right)}$

This can only be fulfilled if

{displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{rho }right)=0}

The extension to 2n+1 (n integer) gamma matrices, is found by placing two gamma-5s after (say) the 2n-th gamma-matrix in the trace, commuting one out to the right (giving a minus sign) and commuting the other gamma-5 2n steps out to the left [with sign change (-1)^2n =1 ]. Then we use cyclic identity to get the two gamma-5s together, and hence they square to identity, leaving us with the trace equalling minus itself, i.e. 0.

Proof of 2

If an odd number of gamma matrices appear in a trace followed by $gamma ^{5}$ , our goal is to move $gamma ^{5}$ from the right side to the left. This will leave the trace invariant by the cyclic property. In order to do this move, we must anticommute it with all of the other gamma matrices. This means that we anticommute it an odd number of times and pick up a minus sign. A trace equal to the negative of itself must be zero.

Proof of 3

To show

operatorname {tr} (gamma ^{mu }gamma ^{nu })=4eta ^{mu nu }

Begin with,

${displaystyle operatorname {tr} (gamma ^{mu }gamma ^{nu })}$	${displaystyle ={frac {1}{2}}left(operatorname {tr} (gamma ^{mu }gamma ^{nu })+operatorname {tr} (gamma ^{nu }gamma ^{mu })right)}$
	${displaystyle ={frac {1}{2}}operatorname {tr} (gamma ^{mu }gamma ^{nu }+gamma ^{nu }gamma ^{mu })={frac {1}{2}}operatorname {tr} left(left{gamma ^{mu },gamma ^{nu }right}right)}$
	${displaystyle ={frac {1}{2}}2eta ^{mu nu }operatorname {tr} (I_{4})=4eta ^{mu nu }}$

Proof of 4

${displaystyle operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma })}$	${displaystyle =operatorname {tr} left(gamma ^{mu }gamma ^{nu }(2eta ^{rho sigma }-gamma ^{sigma }gamma ^{rho })right)}$
	${displaystyle =2eta ^{rho sigma }operatorname {tr} left(gamma ^{mu }gamma ^{nu }right)-operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{sigma }gamma ^{rho }right)quad quad (1)}$

For the term on the right, we'll continue the pattern of swapping ${displaystyle gamma ^{sigma }}$ with its neighbor to the left,

${displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{sigma }gamma ^{rho }right)}$	${displaystyle =operatorname {tr} left(gamma ^{mu }(2eta ^{nu sigma }-gamma ^{sigma }gamma ^{nu })gamma ^{rho }right)}$
	${displaystyle =2eta ^{nu sigma }operatorname {tr} left(gamma ^{mu }gamma ^{rho }right)-operatorname {tr} left(gamma ^{mu }gamma ^{sigma }gamma ^{nu }gamma ^{rho }right)quad quad (2)}$

Again, for the term on the right swap ${displaystyle gamma ^{sigma }}$ with its neighbor to the left,

${displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{sigma }gamma ^{nu }gamma ^{rho }right)}$	${displaystyle =operatorname {tr} left((2eta ^{mu sigma }-gamma ^{sigma }gamma ^{mu })gamma ^{nu }gamma ^{rho }right)}$
	${displaystyle =2eta ^{mu sigma }operatorname {tr} left(gamma ^{nu }gamma ^{rho }right)-operatorname {tr} left(gamma ^{sigma }gamma ^{mu }gamma ^{nu }gamma ^{rho }right)quad quad (3)}$

Eq (3) is the term on the right of eq (2), and eq (2) is the term on the right of eq (1). We'll also use identity number 3 to simplify terms like so:

{displaystyle 2eta ^{rho sigma }operatorname {tr} left(gamma ^{mu }gamma ^{nu }right)=2eta ^{rho sigma }(4eta ^{mu nu })=8eta ^{rho sigma }eta ^{mu nu }.}

So finally Eq (1), when you plug all this information in gives

{displaystyle operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma })=8eta ^{rho sigma }eta ^{mu nu }-8eta ^{nu sigma }eta ^{mu rho }+8eta ^{mu sigma }eta ^{nu rho }}

{displaystyle - operatorname {tr} left(gamma ^{sigma }gamma ^{mu }gamma ^{nu }gamma ^{rho }right)quad quad quad quad quad quad (4)}

The terms inside the trace can be cycled, so

{displaystyle operatorname {tr} left(gamma ^{sigma }gamma ^{mu }gamma ^{nu }gamma ^{rho }right)=operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma }).}

So really (4) is

{displaystyle 2 operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma })=8eta ^{rho sigma }eta ^{mu nu }-8eta ^{nu sigma }eta ^{mu rho }+8eta ^{mu sigma }eta ^{nu rho }}

or

{displaystyle operatorname {tr} (gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma })=4left(eta ^{rho sigma }eta ^{mu nu }-eta ^{nu sigma }eta ^{mu rho }+eta ^{mu sigma }eta ^{nu rho }right)}

Proof of 5

To show

{displaystyle operatorname {tr} left(gamma ^{5}right)=0}

,

begin with

${displaystyle operatorname {tr} left(gamma ^{5}right)}$	${displaystyle =operatorname {tr} left(gamma ^{0}gamma ^{0}gamma ^{5}right)}$	(because ${displaystyle gamma ^{0}gamma ^{0}=I_{4}}$ )
	${displaystyle =-operatorname {tr} left(gamma ^{0}gamma ^{5}gamma ^{0}right)}$	(anti-commute the $gamma ^{5}$ with $gamma ^{0}$ )
	${displaystyle =-operatorname {tr} left(gamma ^{0}gamma ^{0}gamma ^{5}right)}$	(rotate terms within trace)
	${displaystyle =-operatorname {tr} left(gamma ^{5}right)}$	(remove $gamma ^{0}$ 's)

Add ${displaystyle operatorname {tr} left(gamma ^{5}right)}$ to both sides of the above to see

{displaystyle 2operatorname {tr} left(gamma ^{5}right)=0}

.

Now, this pattern can also be used to show

{displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{5}right)=0}

.

Simply add two factors of ${displaystyle gamma ^{alpha }}$ , with $alpha$ different from $mu$ and $nu$ . Anticommute three times instead of once, picking up three minus signs, and cycle using the cyclic property of the trace.

So,

{displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{5}right)=0}

.

Proof of 6

For a proof of identity 6, the same trick still works unless ${displaystyle left(mu nu rho sigma right)}$ is some permutation of (0123), so that all 4 gammas appear. The anticommutation rules imply that interchanging two of the indices changes the sign of the trace, so ${displaystyle operatorname {tr} left(gamma ^{mu }gamma ^{nu }gamma ^{rho }gamma ^{sigma }gamma ^{5}right)}$ must be proportional to $epsilon ^{{mu nu rho sigma }}$ ${displaystyle left(epsilon ^{0123}=eta ^{0mu }eta ^{1nu }eta ^{2rho }eta ^{3sigma }epsilon _{mu nu rho sigma }=eta ^{00}eta ^{11}eta ^{22}eta ^{33}epsilon _{0123}=-1right)}$ . The proportionality constant is ${displaystyle 4i}$ , as can be checked by plugging in ${displaystyle (mu nu rho sigma )=(0123)}$ , writing out $gamma ^{5}$ , and remembering that the trace of the identity is 4.

Proof of 7

Denote the product of $n$ gamma matrices by $Gamma =gamma ^{mu 1}gamma ^{mu 2}dots gamma ^{mu n}.$ Consider the Hermitian conjugate of $Gamma$ :

$Gamma ^{dagger }$	$=gamma ^{mu ndagger }dots gamma ^{mu 2dagger }gamma ^{mu 1dagger }$
	$=gamma ^{0}gamma ^{mu n}gamma ^{0}dots gamma ^{0}gamma ^{mu 2}gamma ^{0}gamma ^{0}gamma ^{mu 1}gamma ^{0}$	(since conjugating a gamma matrix with $gamma ^{0}$ produces its Hermitian conjugate as described below)
	$=gamma ^{0}gamma ^{mu n}dots gamma ^{mu 2}gamma ^{mu 1}gamma ^{0}$	(all $gamma ^{0}$ s except the first and the last drop out)

Conjugating with $gamma ^{0}$ one more time to get rid of the two $gamma ^{0}$ s that are there, we see that $gamma ^{0}Gamma ^{dagger }gamma ^{0}$ is the reverse of $Gamma$ . Now,

${displaystyle operatorname {tr} left(gamma ^{0}Gamma ^{dagger }gamma ^{0}right)}$	${displaystyle =operatorname {tr} left(Gamma ^{dagger }right)}$	(since trace is invariant under similarity transformations)
	${displaystyle =operatorname {tr} left(Gamma ^{*}right)}$	(since trace is invariant under transposition)
	${displaystyle =operatorname {tr} left(Gamma right)}$	(since the trace of a product of gamma matrices is real)

Normalization

The gamma matrices can be chosen with extra hermiticity conditions which are restricted by the above anticommutation relations however. We can impose

{displaystyle left(gamma ^{0}right)^{dagger }=gamma ^{0}}

, compatible with

{displaystyle left(gamma ^{0}right)^{2}=I_{4}}

and for the other gamma matrices (for k = 1, 2, 3)

{displaystyle left(gamma ^{k}right)^{dagger }=-gamma ^{k}}

, compatible with

{displaystyle left(gamma ^{k}right)^{2}=-I_{4}.}

One checks immediately that these hermiticity relations hold for the Dirac representation.

The above conditions can be combined in the relation

{displaystyle left(gamma ^{mu }right)^{dagger }=gamma ^{0}gamma ^{mu }gamma ^{0}.}

The hermiticity conditions are not invariant under the action $gamma ^{mu }to S(Lambda )gamma ^{mu }{S(Lambda )}^{-1}$ of a Lorentz transformation $Lambda$ because $S(Lambda )$ is not necessarily a unitary transformation due to the non-compactness of the Lorentz group.

Feynman slash notation used in Quantum field theory

The Feynman slash notation is defined by

{displaystyle {a!!!/}:=gamma ^{mu }a_{mu }}

for any 4-vector $a$ .

Here are some similar identities to the ones above, but involving slash notation:

${displaystyle {a!!!/}{b!!!/}=acdot b-ia_{mu }sigma ^{mu nu }b_{nu }}$

${displaystyle {a!!!/}{a!!!/}=a^{mu }a^{nu }gamma _{mu }gamma _{nu }={frac {1}{2}}a^{mu }a^{nu }left(gamma _{mu }gamma _{nu }+gamma _{nu }gamma _{mu }right)=eta _{mu nu }a^{mu }a^{nu }=a^{2}}$

${displaystyle operatorname {tr} left({a!!!/}{b!!!/}right)=4(acdot b)}$

${displaystyle operatorname {tr} left({a!!!/}{b!!!/}{c!!!/}{d!!!/}right)=4left[(acdot b)(ccdot d)-(acdot c)(bcdot d)+(acdot d)(bcdot c)right]}$

${displaystyle operatorname {tr} left(gamma _{5}{a!!!/}{b!!!/}right)=0}$

${displaystyle operatorname {tr} left(gamma _{5}{a!!!/}{b!!!/}{c!!!/}{d!!!/}right)=-4iepsilon _{mu nu rho sigma }a^{mu }b^{nu }c^{rho }d^{sigma }}$

${displaystyle gamma _{mu }{a!!!/}gamma ^{mu }=-2{a!!!/}}$

${displaystyle gamma _{mu }{a!!!/}{b!!!/}gamma ^{mu }=4acdot b}$

${displaystyle gamma _{mu }{a!!!/}{b!!!/}{c!!!/}gamma ^{mu }=-2{c!!!/}{b!!!/}{a!!!/}}$

where ${displaystyle epsilon _{mu nu rho sigma }}$ is the Levi-Civita symbol and ${displaystyle sigma ^{mu nu }={frac {i}{2}}left[gamma ^{mu },gamma ^{nu }right].}$ Actually traces of products of odd number of $gamma$ is zero and thus

${displaystyle operatorname {tr} left({a!!!/}right)=operatorname {tr} left({a!!!/}{b!!!/}{c!!!/}right)=operatorname {tr} left({a!!!/}{b!!!/}{c!!!/}{d!!!/}{e!!!/}right)=0}$ ^[4]

Other representations

The matrices are also sometimes written using the 2×2 identity matrix, $I_{2}$ , and

gamma ^{k}={begin{pmatrix}0&sigma ^{k}\-sigma ^{k}&0end{pmatrix}}

where k runs from 1 to 3 and the σ^k are Pauli matrices.

Dirac basis

The gamma matrices we have written so far are appropriate for acting on Dirac spinors written in the Dirac basis; in fact, the Dirac basis is defined by these matrices. To summarize, in the Dirac basis:

gamma ^{0}={begin{pmatrix}I_{2}&0\0&-I_{2}end{pmatrix}},quad gamma ^{k}={begin{pmatrix}0&sigma ^{k}\-sigma ^{k}&0end{pmatrix}},quad gamma ^{5}={begin{pmatrix}0&I_{2}\I_{2}&0end{pmatrix}}.

Weyl (chiral) basis

Another common choice is the Weyl or chiral basis,^[5] in which $gamma ^{k}$ remains the same but $gamma ^{0}$ is different, and so $gamma ^{5}$ is also different, and diagonal,

gamma ^{0}={begin{pmatrix}0&I_{2}\I_{2}&0end{pmatrix}},quad gamma ^{k}={begin{pmatrix}0&sigma ^{k}\-sigma ^{k}&0end{pmatrix}},quad gamma ^{5}={begin{pmatrix}-I_{2}&0\0&I_{2}end{pmatrix}},

or in more compact notation:

{displaystyle gamma ^{mu }={begin{pmatrix}0&sigma ^{mu }\{bar {sigma }}^{mu }&0end{pmatrix}},quad sigma ^{mu }equiv (1,sigma ^{i}),quad {bar {sigma }}^{mu }equiv (1,-sigma ^{i}).}

The Weyl basis has the advantage that its chiral projections take a simple form,

{displaystyle psi _{L}={frac {1}{2}}left(1-gamma ^{5}right)psi ={begin{pmatrix}I_{2}&0\0&0end{pmatrix}}psi ,quad psi _{R}={frac {1}{2}}left(1+gamma ^{5}right)psi ={begin{pmatrix}0&0\0&I_{2}end{pmatrix}}psi .}

The idempotence of the chiral projections is manifest.
By slightly abusing the notation and reusing the symbols $psi _{L/R}$ we can then identify

{displaystyle psi ={begin{pmatrix}psi _{L}\psi _{R}end{pmatrix}},}

where now $psi _{L}$ and $psi _{R}$ are left-handed and right-handed two-component Weyl spinors.

Another possible choice^[6] of the Weyl basis has

gamma ^{0}={begin{pmatrix}0&-I_{2}\-I_{2}&0end{pmatrix}},quad gamma ^{k}={begin{pmatrix}0&sigma ^{k}\-sigma ^{k}&0end{pmatrix}},quad gamma ^{5}={begin{pmatrix}I_{2}&0\0&-I_{2}end{pmatrix}}.

The chiral projections take a slightly different form from the other Weyl choice,

{displaystyle psi _{R}={begin{pmatrix}I_{2}&0\0&0end{pmatrix}}psi ,quad psi _{L}={begin{pmatrix}0&0\0&I_{2}end{pmatrix}}psi .}

In other words,

{displaystyle psi ={begin{pmatrix}psi _{R}\psi _{L}end{pmatrix}},}

where $psi _{L}$ and $psi _{R}$ are the left-handed and right-handed two-component Weyl spinors, as before.

Majorana basis

There is also the Majorana basis, in which all of the Dirac matrices are imaginary, and the spinors and Dirac equation are real. Regarding the Pauli matrices, the basis can be written as

{displaystyle {begin{aligned}gamma ^{0}&={begin{pmatrix}0&sigma ^{2}\sigma ^{2}&0end{pmatrix}},&gamma ^{1}&={begin{pmatrix}isigma ^{3}&0\0&isigma ^{3}end{pmatrix}},&gamma ^{2}&={begin{pmatrix}0&-sigma ^{2}\sigma ^{2}&0end{pmatrix}},\gamma ^{3}&={begin{pmatrix}-isigma ^{1}&0\0&-isigma ^{1}end{pmatrix}},&gamma ^{5}&={begin{pmatrix}sigma ^{2}&0\0&-sigma ^{2}end{pmatrix}},&C&={begin{pmatrix}0&-isigma ^{2}\-isigma ^{2}&0end{pmatrix}}.end{aligned}}}

The reason for making all gamma matrices imaginary is solely to obtain the particle physics metric (+, −, −, −) in which squared masses are positive. The Majorana representation, however, is real. One can factor out the $i$ to obtain a different representation with four component real spinors and real gamma matrices. The consequence of removing the $i$ is that the only possible metric with real gamma matrices is (−, +, +, +).

Cℓ_1,3(C) and Cℓ_1,3(R)

The Dirac algebra can be regarded as a complexification of the real algebra Cℓ_1,3(R), called the space time algebra:

Cl_{1,3}(mathbb {C} )=Cl_{1,3}(mathbb {R} )otimes mathbb {C}

Cℓ_1,3(R) differs from Cℓ_1,3(C): in Cℓ_1,3(R) only real linear combinations of the gamma matrices and their products are allowed.

Two things deserve to be pointed out. As Clifford algebras, Cℓ_1,3(C) and Cℓ₄(C) are isomorphic, see classification of Clifford algebras. The reason is that the underlying signature of the spacetime metric loses its signature (3,1) upon passing to the complexification. However, the transformation required to bring the bilinear form to the complex canonical form is not a Lorentz transformation and hence not "permissible" (at the very least impractical) since all physics is tightly knit to the Lorentz symmetry and it is preferable to keep it manifest.

Proponents of geometric algebra strive to work with real algebras wherever that is possible. They argue that it is generally possible (and usually enlightening) to identify the presence of an imaginary unit in a physical equation. Such units arise from one of the many quantities in a real Clifford algebra that square to −1, and these have geometric significance because of the properties of the algebra and the interaction of its various subspaces. Some of these proponents also question whether it is necessary or even useful to introduce an additional imaginary unit in the context of the Dirac equation.^[7]

However, in contemporary practice, the Dirac algebra rather than the space-time algebra continues to be the standard environment the spinors of the Dirac equation "live" in.

Euclidean Dirac matrices

In quantum field theory one can Wick rotate the time axis to transit from Minkowski space to Euclidean space. This is particularly useful in some renormalization procedures as well as lattice gauge theory. In Euclidean space, there are two commonly used representations of Dirac matrices:

Chiral representation

{displaystyle gamma ^{1,2,3}={begin{pmatrix}0&isigma ^{1,2,3}\-isigma ^{1,2,3}&0end{pmatrix}},quad gamma ^{4}={begin{pmatrix}0&I_{2}\I_{2}&0end{pmatrix}}}

Notice that the factors of $i$ have been inserted in the spatial gamma matrices so that the Euclidean Clifford algebra

{displaystyle left{gamma ^{mu },gamma ^{nu }right}=2delta ^{mu nu }I_{4}}

will emerge. It is also worth noting that there are variants of this which insert instead $-i$ on one of the matrices, such as in lattice QCD codes which use the chiral basis.

In Euclidean space,

{displaystyle gamma _{M}^{5}=ileft(gamma ^{0}gamma ^{1}gamma ^{2}gamma ^{3}right)_{M}={frac {1}{i^{2}}}left(gamma ^{4}gamma ^{1}gamma ^{2}gamma ^{3}right)_{E}=left(gamma ^{1}gamma ^{2}gamma ^{3}gamma ^{4}right)_{E}=gamma _{E}^{5}.}

Using the anti-commutator and noting that in Euclidean space ${displaystyle left(gamma ^{mu }right)^{dagger }=gamma ^{mu }}$ , one shows that

{displaystyle left(gamma ^{5}right)^{dagger }=gamma ^{5}}

In chiral basis in Euclidean space,

{displaystyle gamma ^{5}={begin{pmatrix}-I_{2}&0\0&I_{2}end{pmatrix}}}

which is unchanged from its Minkowski version.

Non-relativistic representation

{displaystyle gamma ^{1,2,3}={begin{pmatrix}0&-isigma ^{1,2,3}\isigma ^{1,2,3}&0end{pmatrix}},quad gamma ^{4}={begin{pmatrix}I_{2}&0\0&-I_{2}end{pmatrix}},quad gamma ^{5}={begin{pmatrix}0&-I_{2}\-I_{2}&0end{pmatrix}}}

References

^ The reason for the notation $γ 5$ is because that set of matrices $(Γ A) = (γ μ, iγ 5)$ with $A = (0, 1, 2, 3, 4)$ satisfy the five-dimensional Clifford algebra ${Γ A, Γ B} = 2 η AB$ . Tong 2007, p. 93.

^ Weinberg 2002 Section 5.5.

^ de Wit & Smith 1996, p. 679.

^ Lecture note from University of Texas at Austin

^ The matrices in this basis, provided below, are the similarity transforms of the Dirac basis matrices of the previous paragraph, ${displaystyle U^{dagger }gamma _{D}^{mu }U}$ , where ${displaystyle U={frac {1}{sqrt {2}}}left(1-gamma ^{5}gamma ^{0}right)={frac {1}{sqrt {2}}}{begin{pmatrix}I&I\-I&Iend{pmatrix}}}$ .

^ Michio Kaku, Quantum Field Theory, .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"""""""'""'"}.mw-parser-output .citation .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
ISBN 0-19-509158-2, appendix A

^ See e.g. Hestenes 1996.

Halzen, Francis; Martin, Alan (1984). Quarks & Leptons: An Introductory Course in Modern Particle Physics. John Wiley & Sons. ISBN 0-471-88741-2.

A. Zee, Quantum Field Theory in a Nutshell (2003), Princeton University Press: Princeton, New Jersey.
ISBN 0-691-01019-6. See chapter II.1.

M. Peskin, D. Schroeder, An Introduction to Quantum Field Theory (Westview Press, 1995)
ISBN 0-201-50397-2 See chapter 3.2.

W. Pauli (1936). "Contributions mathématiques à la théorie des matrices de Dirac". Annales de l'Institut Henri Poincaré. 6: 109.

Weinberg, S. (2002), The Quantum Theory of Fields, 1, Cambridge University Press, ISBN 0-521-55001-7

Tong, David (2007). "Quantum Field Theory". David Tong at University of Cambridge. p. 93. Retrieved 2015-03-07.

de Wit, B.; Smith, J. (1986). Field Theory in Particle Physics. North-Holland Personal Library. 1. North-Holland. ISBN 978-0444869999.
Appendix E

David Hestenes, Real Dirac Theory, in J. Keller and Z. Oziewicz (Eds.), The Theory of the Electron, UNAM, Facultad de Estudios Superiores, Cuautitlan, Mexico (1996), pp. 1–50.

External links

Dirac matrices on mathworld including their group properties

Dirac matrices as an abstract group on GroupNames

Hazewinkel, Michiel, ed. (2001) [1994], "Dirac matrices", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4

[1] The reason for the notation $γ 5$ is because that set of matrices $(Γ A) = (γ μ, iγ 5)$ with $A = (0, 1, 2, 3, 4)$ satisfy the five-dimensional Clifford algebra ${Γ A, Γ B} = 2 η AB$ . Tong 2007, p. 93.

[2] Weinberg 2002 Section 5.5.

[3] Wit & Smith 1996, p. 679.

[4] Lecture note from University of Texas at Austin

[5] The matrices in this basis, provided below, are the similarity transforms of the Dirac basis matrices of the previous paragraph, ${displaystyle U^{dagger }gamma _{D}^{mu }U}$ , where ${displaystyle U={frac {1}{sqrt {2}}}left(1-gamma ^{5}gamma ^{0}right)={frac {1}{sqrt {2}}}{begin{pmatrix}I&I\-I&Iend{pmatrix}}}$ .

[6] Michio Kaku, Quantum Field Theory, .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"""""""'""'"}.mw-parser-output .citation .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
ISBN 0-19-509158-2, appendix A

[Hestenes1-7] See e.g. Hestenes 1996.

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Vfrdtyky

Gamma matrices

Contents

Mathematical structure

Physical structure

Expressing the Dirac equation

The fifth gamma matrix, `γ`⁵

Identities

Miscellaneous identities

Trace identities

Normalization

Feynman slash notation used in Quantum field theory

Other representations

Dirac basis

Weyl (chiral) basis

Majorana basis

Cℓ_1,3(C) and Cℓ_1,3(R)

Euclidean Dirac matrices

Chiral representation

Non-relativistic representation

See also

References

External links

Popular posts from this blog

Bressuire

Vorschmack

Quarantine

Gamma matrices

Contents

Mathematical structure

Physical structure

Expressing the Dirac equation

The fifth gamma matrix, γ5

Identities

Miscellaneous identities

Trace identities

Normalization

Feynman slash notation used in Quantum field theory

Other representations

Dirac basis

Weyl (chiral) basis

Majorana basis

Cℓ1,3(C) and Cℓ1,3(R)

Euclidean Dirac matrices

Chiral representation

Non-relativistic representation

See also

References

External links

Popular posts from this blog

Bressuire

Vorschmack

Quarantine

The fifth gamma matrix, `γ`⁵

Cℓ_1,3(C) and Cℓ_1,3(R)