Continuous Symmetries and Conservation Laws. Noether's

Home , Charge conservation, Conservation law, Conserved current, Conserved quantity, Constant of motion, Euclidean space

Nature always creates the best of all options. Aristoteles (384 BC–322 BC)

8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

In many physical systems, the action is invariant under some continuous set of transformations. In such systems, there exist local and global conservation laws analogous to current and charge conservation in electrodynamics. The analogs of the charges can be used to generate the symmetry transformation, from which they were derived, with the help of Poisson brackets, or after quantization, with the help of commutators.

8.1 Point Mechanics

Consider a simple mechanical system with a generic action

tb A = dt L(q(t), q˙(t), t). (8.1) Zta 8.1.1 Continuous Symmetries and Conservation Law Suppose A is invariant under a continuous set of transformations of the dynamical variables: q(t) → q′(t)= f(q(t), q˙(t)), (8.2) where f(q(t), q˙(t)) is some functional of q(t). Such transformations are called symmetry transformations. Thereby it is important that the equations of motion are not used when establishing the invariance of the action under (8.2). If the action is subjected successively to two symmetry transformations, the result is again a symmetry transformation. Thus, symmetry transformations form a group called the symmetry group of the system. For inﬁnitesimal symmetry transformations (8.2), the diﬀerence

′ δsq(t) ≡ q (t) − q(t) (8.3) will be called a symmetry variation. It has the general form

δsq(t)= ǫ∆(q(t), q˙(t), t). (8.4)

619 620 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

Symmetry variations must not be confused with ordinary variations δq(t) used in Section 1.1 to derive the Euler-Lagrange equations (1.8). While the ordinary variations δq(t) vanish at initial and ﬁnal times, δq(tb) = δq(ta) = 0 [recall (1.4)], the symmetry variations δsq(t) are usually nonzero at the ends. Let us calculate the change of the action under a symmetry variation (8.4). Using the chain rule of diﬀerentiation and an integration by parts, we obtain

tb tb ∂L ∂L ∂L δsA = dt − ∂t δsq(t)+ δsq(t) . (8.5) ta "∂q(t) ∂q˙(t)# ∂q˙(t) Z ta

For orbits q(t) that satisfy the Euler-Lagrange equations (1.8), only boundary terms survive, and we are left with

t ∂L a δsA = ǫ ∆(q, q,˙ t) . (8.6) ∂q˙ tb

Under the symmetry assumption, δsA vanishes for any orbit q(t), implying that the quantity ∂L Q(t) ≡ ∆(q, q,˙ t) (8.7) ∂q˙ is the same at times t = ta and t = tb. Since tb is arbitrary, Q(t) is independent of the time t, i.e., it satisﬁes

Q(t) ≡ Q. (8.8)

It is a conserved quantity, a constant of motion. The expression on the right-hand side of (8.7) is called Noether charge. The statement can be generalized to transformations δsq(t) for which the action is not directly invariant but its symmetry variation is equal to an arbitrary boundary term:

tb δsA = ǫ Λ(q, q,˙ t) . (8.9) ta

In this case, ∂L Q(t)= ∆(q, q,˙ t) − Λ(q, q,˙ t) (8.10) ∂q˙ is a conserved Noether charge. It is also possible to derive the constant of motion (8.10) without invoking the action, but starting from the Lagrangian. For it we evaluate the symmetry variation as follows: ∂L ∂L d ∂L δsL≡L (q+δsq, q˙+δsq˙) − L(q, q˙)= −∂t δsq(t)+ δsq(t) . (8.11) "∂q(t) ∂q˙(t)# dt "∂q˙(t) # 8.1 Point Mechanics 621

On account of the Euler-Lagrange equations (1.8), the ﬁrst term on the right-hand side vanishes as before, and only the last term survives. The assumption of invariance of the action up to a possible surface term in Eq. (8.9) is equivalent to assuming that the symmetry variation of the Lagrangian is a total time derivative of some function Λ(q, q,˙ t): d δ L(q, q,˙ t)= ǫ Λ(q, q,˙ t). (8.12) s dt Inserting this into the left-hand side of (8.11), we ﬁnd d ∂L ǫ ∆(q, q,˙ t) − Λ(q, q,˙ t) =0, (8.13) dt " ∂q˙ # thus recovering again the conserved Noether charge (8.8). The existence of a conserved quantity for every continuous symmetry is the content of Noether’s theorem [1].

8.1.2 Alternative Derivation Let us do the substantial variation in Eq. (8.5) explicitly, and change a classical orbit qc(t), that extremizes the action, by an arbitrary variation δaq(t). If this does not vanish at the boundaries, the action changes by a pure boundary term that follows directly from (8.5): t ∂L b δaA = δaq . (8.14) ∂q˙ ta

From this equation we can derive Noether’s theorem in yet another way. Suppose we subject a classical orbit to a new type of symmetry variation, to be called local symmetry transformations, which generalizes the previous symmetry variations (8.4) by making the parameter ǫ time-dependent:

t δsq(t)= ǫ(t)∆(q(t), q˙(t), t). (8.15) t The superscript t of δsq(t) indicates the new time dependence in the parameter ǫ(t). These variations may be considered as a special set of the general variations δaq(t) t introduced above. Thus also δsA must be a pure boundary term of the type (8.14). For the subsequent discussion it is useful to introduce the inﬁnitesimally transformed orbit ǫ t q (t) ≡ q(t)+ δsq(t)= q(t)+ ǫ(t)∆(q(t), q˙(t), t), (8.16) and the associated Lagrangian: Lǫ ≡ L(qǫ(t), q˙ǫ(t)). (8.17) Using the time-dependent parameter ǫ(t), the local symmetry variation of the action can be written as

tb tb ǫ ǫ ǫ t ∂L d ∂L d ∂L δsA = dt − ǫ(t)+ ǫ(t) . (8.18) ta "∂ǫ(t) dt ∂ǫ˙(t)# dt " ∂ǫ˙ # Z ta

622 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

Along the classical orbits, the action is extremal and satisﬁes the equation

δA =0, (8.19) δǫ(t) which translates for a local action to an Euler-Lagrange type of equation:

∂Lǫ d ∂Lǫ − =0. (8.20) ∂ǫ(t) dt ∂ǫ˙(t)

This can also be checked explicitly by diﬀerentiating (8.17) according to the chain rule of diﬀerentiation: ∂Lǫ ∂Lǫ ∂Lǫ = ∆(q, q,˙ t)+ ∆(˙ q, q,˙ t); (8.21) ∂ǫ(t) ∂q(t) ∂q˙(t) ∂Lǫ ∂Lǫ = ∆(q, q,˙ t), (8.22) ∂ǫ˙(t) ∂q˙(t) and inserting on the right-hand side the ordinary Euler-Lagrange equations (1.8). We now invoke the symmetry assumption that the action is a pure surface term under the time-independent transformations (8.15). This implies that

∂Lǫ d = Λ. (8.23) ∂ǫ dt Combining this with (8.20), we derive a conservation law for the charge:

∂Lǫ Q = − Λ. (8.24) ∂ǫ˙ Inserting here Eq. (8.22), we ﬁnd that this is the same charge as that derived by the previous method.

8.2 Displacement and Energy Conservation

As a simple but physically important example consider the case that the Lagrangian does not depend explicitly on time, i.e., that L(q, q,˙ t) ≡ L(q, q˙). Let us perform a time translation on the coordinate frame:

t′ = t − ǫ. (8.25)

In the new coordinate frame, the same orbit has the new description

q˙(t′)= q(t), (8.26) i.e., the orbitq ˙(t) at the translated time t′ is precisely the same as the orbit q(t) at the original time t. If we replace the argument ofq ˙(t) in (8.26) by t′, we describe a 8.2 Displacement and Energy Conservation 623 time-translated orbit in terms of the original coordinates. This implies the symmetry variation of the form (8.4):

′ ′ δsq(t) = q (t) − q(t)= q(t + ǫ) − q(t) = q(t′)+ ǫq˙(t′) − q(t)= ǫq˙(t). (8.27)

The symmetry variation of the Lagrangian is in general

∂L ∂L δ L = L(q′(t), q˙′(t)) − L(q(t), q˙(t)) = δ q(t)+ δ q˙(t). (8.28) s ∂q s ∂q˙ s

Inserting δsq(t) from (8.27) we ﬁnd, without using the Euler-Lagrange equation,

∂L ∂L d δsL = ǫ q˙ + q¨ = ǫ L. (8.29) ∂q˙ ∂q˙ ! dt

This has precisely the form of Eq. (8.12), with Λ = L as expected, since time translations are symmetry transformations. Here the function Λ in (8.12) happens to coincide with the Lagrangian. According to Eq. (8.10), we ﬁnd the Noether charge

∂L Q = q˙ − L(q, q˙) (8.30) ∂q˙ to be a constant of motion. This is recognized as the Legendre transform of the Lagrangian which is, of course, the Hamiltonian of the system. s Let us brieﬂy check how this Noether charge is obtained from the alternative formula (8.10). The time-dependent symmetry variation is here

t δsq(t) = ǫ(t)q ˙(t), (8.31) under which the Lagrangian is changed by

∂L ∂L ∂Lǫ ∂Lǫ δtL = ǫq˙ + (˙ǫq˙ + ǫq¨)= ǫ + ǫ,˙ (8.32) s ∂q ∂q˙ ∂ǫ˙ ∂ǫ˙ with ∂Lǫ ∂L = q˙ (8.33) ∂ǫ˙ ∂q˙ and ∂Lǫ ∂L ∂L d = q˙ + ǫq¨ = L. (8.34) ∂ǫ ∂q ∂q˙ dt This shows that time translations fulﬁll the symmetry condition (8.23), and that the Noether charge (8.24) coincides with the Hamiltonian found in Eq. (8.10). 624 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

8.3 Momentum and Angular Momentum

While the conservation law of energy follows from the symmetry of the action under time translations, conservation laws of momentum and angular momentum are found if the action is invariant under translations and rotations. Consider a Lagrangian of a point particle in a euclidean space L = L(xi(t), x˙ i(t), t). (8.35) In contrast to the previous discussion of time translation invariance, which was applicable to systems with arbitrary Lagrange coordinates q(t), we denote the coordinates here by xi to emphasize that we now consider cartesian coordinates. If the Lagrangian does depend only on the velocitiesx ˙ i and not on the coordinates xi themselves, the system is translationally invariant. If it depends, in addition, only on x˙ 2 =x ˙ ix˙ i, it is also rotationally invariant. The simplest example is the Lagrangian of a point particle of mass m in euclidean space: m L = x˙ 2. (8.36) 2 It exhibits both invariances, leading to conserved Noether charges of momentum and angular momentum, as we now demonstrate.

8.3.1 Translational Invariance in Space Under a spatial translation, the coordinates xi change to x′i = xi + ǫi, (8.37) where ǫi are small numbers. The inﬁnitesimal translations of a particle path are [compare (8.4)]

i i δsx (t)= ǫ . (8.38) Under these, the Lagrangian changes by

′i ′i i i δsL = L(x (t), x˙ (t), t) − L(x (t), x˙ (t), t) ∂L ∂L = δ xi = ǫi =0. (8.39) ∂xi s ∂xi By assumption, the Lagrangian is independent of xi, so that the right-hand side vanishes. This has to be compared with the symmetry variation of the Lagrangian around the classical orbit, calculated via the chain rule, and using the Euler- Lagrange equation:

∂L d ∂L i d ∂L i δsL = − δsx + δsx ∂xi dt ∂x˙ i ! dt "∂x˙ i # d ∂L = ǫi. (8.40) dt "∂x˙ i # 8.3 Momentum and Angular Momentum 625

This has the form (8.6), from which we extract a conserved Noether charge (8.7) for each coordinate xi: ∂L pi = . (8.41) ∂x˙ i These are simply the canonical momenta of the system.

8.3.2 Rotational Invariance Under rotations, the coordinates xi change to

′i i j x = R jx , (8.42)

i where R j is an orthogonal 3 × 3 -matrix. Inﬁnitesimally, this can be written as

i i R j = δ j − ωkǫkij, (8.43)

where ! is an inﬁnitesimal rotation vector. The corresponding rotation of a particle path is

i ′i i k j δsx (t)= x (t) − x (t)= −ω ǫkijx (τ). (8.44) It is useful to introduce the antisymmetric inﬁnitesimal rotation tensor

ωij ≡ ωkǫkij, (8.45) in terms of which

i j δsx = −ωijx . (8.46)

i Then we can write the change of the Lagrangian under δsx ,

′i ′i i i δsL = L(x (t), x˙ (t), t) − L(x (t), x˙ (t), t) ∂L ∂L = δ xi + δ x˙ i, (8.47) ∂xi s ∂x˙ i s as

∂L j ∂L j δsL = − x + x˙ ωij =0. (8.48) ∂xi ∂x˙ i ! If the Lagrangian depends only on the rotational invariants x2, x˙ 2, x · x˙ , and on powers thereof, the right-hand side vanishes on account of the antisymmetry of ωij. This ensures the rotational symmetry. We now calculate once more the symmetry variation of the Lagrangian via the chain rule and ﬁnd, using the Euler-Lagrange equations,

∂L d ∂L i d ∂L i δsL = − δsx + δsx ∂xi dt ∂x˙ i ! dt " ∂x˙ i #

d ∂L j 1 d ∂L = − x ωij = xi − (i ↔ j) ωij. (8.49) dt " ∂x˙ i # 2 dt " ∂x˙ j # 626 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

The right-hand side yields the conserved Noether charges of type (8.7), one for each antisymmetric pair i, j: ∂L ∂L Lij = xi − xj ≡ xipj − xjpi. (8.50) ∂x˙ j ∂x˙ i These are the antisymmetric components of angular momentum. Had we worked with the original vector form of the rotation angles ωk, we would have found the angular momentum in the more common form: 1 L = ǫ Lij =(x × p)k. (8.51) k 2 kij The quantum-mechanical operators associated with these, after replacing pi → −i∂/∂xi, have the well-known commutation rules

[Lˆi, Lˆj]= iǫijkLˆk. (8.52)

In the tensor notation (8.50), these become

[Lîj, Lˆkl]= −i δikLˆjl − δilLˆjk + δjlLîk − δjkLîl . (8.53) 8.3.3 Center-of-Mass Theorem Consider now the transformations corresponding to a uniform motion of the coordinate system. We shall study the behavior of a set of free massive point particles in euclidean space described by the Lagrangian

i mn 2 L(x ˙ )= x˙ n. (8.54) n 2 X Under Galilei transformations, the spatial coordinates and the time are changed to

x˙ i(t) = xi(t) − vit, (8.55) t′ = t, where vi is the relative velocity along the ith axis. The inﬁnitesimal symmetry variations are

i i i i δsx (t) =x ˙ (t) − x (t)= −v t, (8.56) which change the Lagrangian by

i i i i i i δsL = L(x − v t, x˙ − v ) − L(x , x˙ ). (8.57)

Inserting the explicit form (8.54), we ﬁnd

mn i i 2 i 2 δsL = (x ˙ n − v ) − (x ˙ n ) . (8.58) n 2 X h i 8.3 Momentum and Angular Momentum 627

This can be written as a total time derivative: 2 d d i i v δsL = Λ= mn −x˙ nv + t , (8.59) dt dt n " 2 # X proving that Galilei transformations are symmetry transformations in the Noether sense. By assumption, the velocities vi in (8.55) are inﬁnitesimal, so that the second term can be ignored. By calculating δsL once more via the chain rule with the help of the Euler- Lagrange equations, and by equating the result with (8.59), we ﬁnd the conserved Noether charge

∂L i Q = i δsx − Λ n ∂x˙ X i i i = − mnx˙ nt + mnxn v . (8.60) n n ! X X Since the direction of the velocity vi is arbitrary, each component is separately a constant of motion:

i i i N = − mnx˙ t + mnxn = const. (8.61) n n X X This is the well-known center-of-mass theorem [2]. Indeed, introducing the center- of-mass coordinates i i n mnxn xCM ≡ , (8.62) P n mn and the associated velocities P i i n mnx˙ n vCM = , (8.63) P n mn the conserved charge (8.61) can be writtenP as

i i i N = mn(−vCMt + xCM). (8.64) n X The time-independence of N i implies that the center-of-mass moves with uniform velocity according to the law

i i i xCM(t)= x0CM + vCMt, (8.65) where i i N x0CM = (8.66) n mn is the position of the center of mass at t =P 0. Note that in non-relativistic physics, the center-of-mass theorem is a consequence of momentum conservation since momentum ≡ mass × velocity. In relativistic physics, this is no longer true. 628 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

8.3.4 Conservation Laws Resulting from Lorentz Invariance In relativistic physics, particle orbits are described by functions in spacetime

xµ(τ), (8.67) where τ is an arbitrary Lorentz-invariant parameter. The action is an integral over some Lagrangian: A = dτL (xµ(τ), x˙ µ(τ), τ) , (8.68) Z wherex ˙ µ(τ) denotes the derivative with respect to the parameter τ. If the La- µ µ µ grangian depends only on invariant scalar products x xµ, x x˙ µ, x˙ x˙ µ, then it is invariant under Lorentz transformations

µ µ µ ν x → x˙ =Λ ν x , (8.69)

µ where Λ ν is a 4 × 4 matrix satisfying

ΛgΛT = g, (8.70) with the Minkowski metric 1 −1 g =   . (8.71) µν −1    −1      For a free massive point particle in spacetime, the Lagrangian is

µ ν L(x ˙(τ)) = −Mc gµν x˙ x˙ . (8.72) q It is reparametrization invariant under τ → f(τ), with an arbitrary function f(τ). Under translations µ µ µ δsx (τ)= x (τ) − ǫ (τ), (8.73) the Lagrangian is obviously invariant, satisfying δsL = 0. Calculating this variation once more via the chain rule with the help of the Euler-Lagrange equations, we ﬁnd

τν ∂L µ ∂L µ 0 = dτ µ δsx + µ δsx˙ Zτµ ∂x ∂x˙ ! τν µ d ∂L = −ǫ dτ µ . (8.74) Zτµ dτ ∂x˙ ! From this we obtain the Noether charges

∂L x˙ µ(τ) µ pµ ≡ − = Mc = Mcu , (8.75) µ µ ν ∂x˙ gµν x˙ x˙ q 8.3 Momentum and Angular Momentum 629 which satisfy the conservation law d p (t)=0. (8.76) dτ µ They are the conserved four-momenta of a free relativistic particle. The quantity x˙ µ uµ ≡ (8.77) µ ν gµνx˙ x˙ q is the dimensionless relativistic four-velocity of the particle. It has the property µ u uµ = 1, and it is reparametrization invariant. By choosing for τ the physical time t = x0/c, we can express uµ in terms of the physical velocities vi = dxi/dt as

uµ = γ(1, vi/c), with γ ≡ 1 − v2/c2. (8.78) q Note the minus sign in the deﬁnition (8.75) of the canonical momentum with respect to the nonrelativistic case. It is necessary to write Eq. (8.75) covariantly. The derivative with respect tox ˙ µ transforms like a covariant vector with a subscript µ, whereas the physical momenta are pµ. For small Lorentz transformations near the identity we write

µ µ µ Λ ν = δ ν + ω ν, (8.79) where µ µλ ω ν = g ωλν (8.80) is an arbitrary inﬁnitesimal antisymmetric matrix. An inﬁnitesimal Lorentz transformation of the particle path is

µ µ µ δsx (τ) =x ˙ (τ) − x (τ) µ ν = ω ν x (τ). (8.81)

Under it, the symmetry variation of a Lorentz-invariant Lagrangian vanishes:

∂L ν ∂L ν µ δsL = x + x˙ ω ν =0. (8.82) ∂xµ ∂x˙ µ ! This has to be compared with the symmetry variation of the Lagrangian calculated via the chain rule with the help of the Euler-Lagrange equation

∂L d ∂L µ d ∂L µ δsL = − δsx + δsx ∂xµ dτ ∂x˙ µ ! dτ " ∂x˙ µ #

d ∂L ν µ = x˙ ω ν dτ " ∂x˙ µ #

1 ν d µ ∂L ν ∂L = ωµ x − x . (8.83) 2 dτ ∂x˙ ν ∂x˙ µ ! 630 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

By equating this with (8.82), we obtain the conserved rotational Noether charges [containing again a minus sign as in (8.75)]: ∂L ∂L Lµν = −xµ + xν = xµpν − xνpµ. (8.84) ∂x˙ ν ∂x˙ µ

They are four-dimensional generalizations of the angular momenta (8.50). The quantum-mechanical operators

Lˆµν ≡ i(xµ∂ν − xν∂µ) (8.85)

µ obtained after the replacement p → i∂/∂xµ satisfy the four-dimensional spacetime generalization of the commutation relations (8.53):

[Lˆµν , Lˆκλ]= i gµκLˆνλ − gµλLˆνκ + gνλLˆµκ − gνκLˆµλ . (8.86) The quantities Lij coincide with the earlier-introduced angular momenta (8.50). The conserved components

0i 0 i i 0 L = x p − x p ≡ Mi (8.87) yield the relativistic generalization of the center-of-mass theorem (8.61):

Mi = const. (8.88)

8.4 Generating the Symmetry Transformations

As mentioned in the introduction to this chapter, the relation between invariances and conservation laws has a second aspect. With the help of Poisson brackets, the charges associated with continuous symmetry transformations can be used to generate the symmetry transformation from which they were derived. Explicitly,

δsxˆ = −iǫ[Q,ˆ xˆ(t)]. (8.89)

The charge derived in Section 7.2 from the invariance of the system under time displacement is the most famous example for this property. The charge (8.30) is by deﬁnition the Hamiltonian, Q ≡ H, whose operator version generates inﬁnitesimal time displacements by the Heisenberg equation of motion: xˆ˙ (t)= −i[H,ˆ xˆ(t)]. (8.90) This equation is obviously the same as (8.89). To quantize the system canonically, we may assume the Lagrangian to have the standard form M L(x, x˙)= x˙ 2 − V (x), (8.91) 2 8.4 Generating the Symmetry Transformations 631 so that the Hamiltonian operator becomes, with the canonical momentum p ≡ x˙:

pˆ2 Hˆ = + V (ˆx). (8.92) 2M Equation (8.90) is then a direct consequence of the canonical equal-time commutation rules [ˆp(t), xˆ(t)] = −i, [ˆp(t), pˆ(t)]=0, [ˆx(t), xˆ(t)]=0. (8.93) The charges (8.41), derived in Section 7.3 from translational symmetry, are another famous example. After quantization, the commutation rule (8.89) becomes, with (8.38), ǫj = iǫi[ˆpi(t), xˆj(t)]. (8.94) This coincides with one of the canonical commutation relations (here it appears only for time-independent momenta, since the system is translationally invariant). The relativistic charges (8.75) of spacetime generate translations via

µ µ ν µ δsxˆ = ǫ = −iǫ [ˆpν(t), xˆ (τ)], (8.95) in agreement with the relativistic canonical commutation rules (29.27). Similarly we ﬁnd that the quantized versions of the conserved charges Li in Eq. (8.51) generate inﬁnitesimal rotations:

j i k i j δsxˆ = −ω ǫijkxˆ (t)= iω [Lˆi, xˆ (t)], (8.96) whereas the quantized conserved charges N i of Eq. (8.61) generate inﬁnitesimal Galilei transformations, and that the charges Mi of Eq. (8.87) generate pure rotational Lorentz transformations:

j 0 j δsxˆ = ǫixˆ = iǫi[Mi, xˆ ], 0 i 0 δsxˆ = ǫixˆ = iǫi[Mi, xˆ ]. (8.97)

Since the quantized charges generate the rotational symmetry transformations, they form a representation of the generators of the symmetry group. When com- muted with each other, they obey the same commutation rules as the generators of the symmetry group. The charges (8.51) associated with rotations, for example, have the commutation rules [Lˆi, Lˆj]= iǫijkLˆj, (8.98) which are the same as those between the 3 × 3 generators of the three-dimensional rotations (Li)jk = −iǫijk. The quantized charges of the generators (8.84) of the Lorentz group satisfy the commutation rules (8.86) of the 4 × 4 generators (8.85)

[Lˆµν , Lˆµλ]= −igµµLˆνλ. (8.99)

This follows directly from the canonical commutation rules (8.95) [i.e., (29.27)]. 632 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

8.5 Field Theory

A similar relation between continuous symmetries and constants of motion holds in ﬁeld theory.

8.5.1 Continuous Symmetry and Conserved Currents Let A be the action of an arbitrary ﬁeld ϕ(x),

A = d4xL(ϕ,∂ϕ,x), (8.100) Z and suppose that a transformation of the ﬁeld

δsϕ(x)= ǫ∆(ϕ,∂ϕ,x) (8.101) changes the Lagrangian density L merely by a total derivative

µ δsL = ǫ∂µΛ , (8.102) or equivalently, that it changes the action A by a surface term

4 µ δsA = ǫ d x ∂µΛ . (8.103) Z

Then δsL is called a symmetry transformation. Given such a symmetry transformation, we can ﬁnd a current four-vector

∂L jµ = ∆ − Λµ (8.104) ∂∂µϕ that has no four-divergence

µ ∂µj (x)=0. (8.105)

The expression on the right-hand side of (8.104) is called a Noether current, and (8.105) is referred to as the associated current conservation law. It is a local conservation law. The proof of (8.105) is just as simple as that of the time-independence of the charge (8.10) associated with the corresponding symmetry of the mechanical action (8.1) in Section 8.1. We calculate the symmetry variation of L under the symmetry transformation in a similar way as in Eq. (8.11), and ﬁnd

∂L ∂L ∂L δsL = − ∂µ δsϕ + ∂µ δsϕ ∂ϕ ∂∂µϕ! ∂∂µϕ ! ∂L ∂L ∂L = ǫ − ∂µ ∆+ ∂µ ∆ . (8.106) ∂ϕ ∂∂µϕ! ∂∂µϕ ! 8.5 Field Theory 633

Then we invoke the Euler-Lagrange equation to remove the ﬁrst term. Equating the second term with (8.102), we obtain

µ ∂L µ ∂µj ≡ ∂µ ∆ − Λ =0. (8.107) ∂∂µϕ ! The relation between continuous symmetries and conservation is called Noether’s theorem [1]. Assuming all ﬁelds to vanish at spatial inﬁnity, we can derive from the local law (8.107) a global conservation law for the charge that is obtained from the spatial integral over the charge density j0:

Q(t)= d3x j0(x, t). (8.108) Z Indeed, we may write the time derivative of the charge as an integral

d 3 0 Q(t)= d x ∂0j (x, t) (8.109) dt Z and adding on the right-hand side a spatial integral over a total three-divergence, which vanishes due to the boundary conditions, we ﬁnd

d 3 0 3 0 i Q(t)= d x ∂0j (x, t)= d x [∂0j (x, t)+ ∂ij (x, t)]=0. (8.110) dt Z Z Thus the charge is conserved: d Q(t)=0. (8.111) dt

8.5.2 Alternative Derivation There is again an alternative derivation of the conserved current that is analogous to Eqs. (8.15)–(8.24). It is based on a variation of the ﬁelds under symmetry transformations whose parameter ǫ is made artiﬁcially spacetime-dependent ǫ → ǫ(x), thus extending (8.15) to

x δs ϕ(x)= ǫ(x)∆(ϕ(x), ∂µϕ(x), x). (8.112) As before in Eq. (8.17), we calculate the Lagrangian density for a slightly transformed ﬁeld ǫ x ϕ (x) ≡ ϕ(x)+ δs ϕ(x), (8.113) calling it Lǫ ≡ L(ϕǫ(t),∂ϕǫ(t)). (8.114) The corresponding action diﬀers from the original one by

ǫ ǫ ǫ x ∂L ∂L ∂L δs A = dx − ∂µ δǫ(x)+ ∂µ δǫ(x) . (8.115) Z (" ∂ǫ(x) ∂∂µǫ(x) # " ∂∂µǫ(x) #) 634 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

From this we obtain the Euler-Lagrange-like equation ∂Lǫ ∂Lǫ − ∂µ =0. (8.116) ∂ǫ(x) ∂∂µǫ(x) By assumption, the action is a pure surface term under x-independent transformations, implying that ∂Lǫ = ∂ Λµ. (8.117) ∂ǫ(x) µ Together with (8.116), we see that ∂δxL jµ = s − Λµ (8.118) ∂∂µǫ(x) has no four-divergence. By the chain rule of diﬀerentiation we calculate

t ∂L(x) ∂L δsL = ǫ∆+ ∂ν ǫ∆, (8.119) ∂ϕ ∂∂ν ϕ and see that ∂Lǫ ∂L = ∆(ϕ,∂ϕ,x), (8.120) ∂∂µǫ(x) ∂∂µϕ so that the current (8.118) coincides with (8.104).

8.5.3 Local Symmetries If we apply the alternative derivation of a conserved current to a local symmetry, such as a local gauge symmetry, the current density (8.118) vanishes identically. Let us illuminate the symmetry origin of this phenomenon. To be specific, we consider directly the field theory of electrodynamics. The theory does have a conserved charge resulting from the global U(1)-symmetry of the matter Lagrangian. There is a conserved current which is the source of a massless particle, the photon. This is described by a gauge field which is minimally coupled to the conserved current. A similar structure exists for many internal symmetries giving rise to nonabelian versions of the photon, such as gluons, whose exchange causes the strong interactions, and W - and Z-vector mesons, which mediate the weak interactions. It is useful to reconsider Noether’s derivation of conservation laws in such theories. The conserved matter current in a locally gauge-invariant theory cannot be found any more by the rule (8.118), which was so useful in the globally invariant theory. For the gauge transformation of quantum electrodynamics, the derivative with respect to the local field transformation ǫ(x) would simply be given by δL jµ = . (8.121) ∂∂µΛ 8.5 Field Theory 635

This would be identically equal to zero, due to local gauge invariance. We may, however, subject just the matter field to a local gauge transformation at fixed gauge fields. Then we obtain the correct current

∂L jµ ≡ . (8.122) ∂∂ Λ µ em

x Since the complete change under local gauge transformations δs L vanishes identically, we can alternatively vary only the gauge ﬁelds and keep the particle orbit ﬁxed: ∂L jµ = − . (8.123) ∂∂ Λ µ m

This is done most simply by forming the functional derivative with respect to the em gauge ﬁeld, thereby omitting the contribution of L:

m ∂L jµ = − . (8.124) ∂∂µΛ

An interesting consequence of local gauge invariance can be found for the gauge ﬁeld itself. If we form the variation of the pure gauge ﬁeld action

em em A δ = d4x tr δxA , (8.125) s A  s µ  Z δAµ   x and insert for δs A an infinitesimal pure gauge field configuration

x δs Aµ = −i∂µΛ(x), (8.126) the right-hand side must vanish for all Λ(x). After a partial integration this implies µ the local conservation law ∂µj (x) = 0 for the current:

em δ A jµ(x)= −i . (8.127) δAµ

In contrast to the earlier conservation laws derived for matter fields, which were valid only if the matter fields obey the Euler-Lagrange equations, the current conservation law for gauge fields is valid for all field configurations. It is an identity which we may call Bianchi identity due to its close analogy with the Bianchi identities in Riemannian geometry. To verify the conservation of (12.63), we insert the Lagrangian (12.3) into (12.63) ν µν and find j = ∂µF /2. This current is trivially conserved for any field configuration due to the antisymmetry of F µν . 636 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

8.6 Canonical Energy-Momentum Tensor

As an important example for the ﬁeld theoretic version of the theorem, consider the usual case that the Lagrangian density does not depend explicitly on the spacetime coordinates x:

L = L(ϕ,∂ϕ). (8.128)

We then perform a translation along an arbitrary direction ν =0, 1, 2, 3 of spacetime

x′µ = xµ − ǫµ, (8.129) under which ﬁeld ϕ(x) transforms as

ϕ′(x′)= ϕ(x). (8.130)

This equation expresses the fact that the field has the same value at the same absolute point in space and time, which in one coordinate system is labeled by the coordinates xµ and in the other by x′µ. Under an infinitesimal translation of the field configuration coordinate, the La- grangian density undergoes the following symmetry variation

′ ′ δsL ≡ L(ϕ (x),∂ϕ (x)) −L(ϕ(x),∂ϕ(x)) ∂L ∂L = δsϕ(x)+ ∂µδsϕ(x), (8.131) ∂ϕ(x) ∂∂µϕ where

′ δsϕ(x)= ϕ (x) − ϕ(x) (8.132) is the symmetry variation of the ﬁelds. For the particular transformation (8.130) the symmetry variation becomes simply

ν δsϕ(x)= ǫ ∂ν ϕ(x). (8.133)

The Lagrangian density (8.128) changes by

ν δsL(x)= ǫ ∂ν L(x). (8.134)

Hence the requirement (8.103) is satisﬁed and δsϕ(x) is a symmetry transformation. The function Λ happens to coincide with the Lagrangian density

Λ= L. (8.135)

µ ν We can now deﬁne a set of currents jν , one for each ǫ . In the particular case at µ µ hand, the currents jν are denoted by Θν , and read:

µ ∂L µ Θν = ∂ν ϕ − δν L. (8.136) ∂∂µϕ 8.6 Canonical Energy-Momentum Tensor 637

They have no four-divergence

µ ∂µΘν (x)=0. (8.137) As a consequence, the total four-momentum of the system, deﬁned by

P µ = d3xΘµ0(x), (8.138) Z is independent of time. The alternative derivation of the currents goes as follows. Introducing

x ν δs ϕ(x)= ǫ (x)∂ν ϕ(x), (8.139) we see that x ν δs ϕ(x)= ϕ (x)∂ν ϕ(x). (8.140) On the other hand, the chain rule of diﬀerentiation yields

x ∂L ν ∂L ν ν δs L = ǫ (x)∂ν ϕ(x)+ {[∂µǫ (x)]∂ν ϕ + ǫ ∂µ∂ν ϕ(x)} . (8.141) ∂ϕ(x) ∂∂µϕ(x) Hence ∂Lǫ ∂L ν = ∂ν ϕ, (8.142) ∂∂µǫ (x) ∂∂µϕ and we obtain once more the energy-momentum tensor (8.136). Note that (8.142) can also be written as

ǫ x ∂L ∂L ∂δs ϕ ν = ν . (8.143) ∂∂µǫ (x) ∂∂µϕ ∂ǫ (x) µ Since ν is a contravariant vector index, the set of currents Θν forms a Lorentz tensor called the canonical energy-momentum tensor. The component

0 ∂L Θ0 = ∂0ϕ −L (8.144) ∂∂0ϕ is recognized to be the Hamiltonian density in the canonical formalism.

8.6.1 Electromagnetism As an important physical application of the field theoretic Noether theorem, consider the free electromagnetic field with the action 1 L = − F F λκ, (8.145) 4c λκ where Fλκ are the components of the field strength Fλκ ≡ ∂λAκ − ∂κAλ. Under a µ µ µ translation in space and time from x to x − ǫδν , the vector potential undergoes a similar change as in (8.130): A′µ(x′)=Λµ(x). (8.146) 638 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

As before, this equation expresses the fact that at the same absolute spacetime point, which in the two coordinate frames is labeled once by x′ and once by x, the ﬁeld components have the same numerical values. The equation transformation law (8.146) can be rewritten in an inﬁnitesimal form as

λ µ ′λ µ λ µ δsA (x ) ≡ A (x ) − A (x ) ′λ ′µ µ λ µ = A (x + ǫδν ) − A (x ) (8.147) λ µ = ǫ∂ν A (x ). (8.148)

Under it, the ﬁeld tensor changes as follows

λκ λκ δsF = ǫ∂ν F , (8.149) so that the Lagrangian density is a total four-divergence: 1 δ L = −ǫ F ∂ F λκ = ǫ∂ L (8.150) s 2c λκ ν ν Thus, the spacetime translations (8.148) are symmetry transformations, and the currents

µ ∂L λ µ Θν = λ ∂νA − δν L (8.151) ∂∂µA are conserved:

µ ∂µΘν (x)=0. (8.152)

λ µ Using ∂L/∂∂µA = −F λ, the currents (8.151) become more explicitly

µ 1 µ λ 1 µ λκ Θν = − F λ∂νA − δν F Fλκ . (8.153) c 4 They form the canonical energy-momentum tensor of the electromagnetic ﬁeld.

8.6.2 Dirac Field We now turn to the Dirac ﬁeld which has the well-known action ↔ 4 4 µ A = d x L(x)= d xψ¯(x)(iγ ∂ µ − M)ψ(x), (8.154) Z Z where γµ are the Dirac matrices

µ µ 0 σ γ = µ . (8.155) σ˜ 0 ! Here σµ,σ ˜µ are four 2 × 2 matrices

σµ ≡ (σ0, σi).σ˜µ ≡ (σ0, −σi), (8.156) 8.6 Canonical Energy-Momentum Tensor 639 whose zeroth component is the unit matrix 1 0 σ0 = , (8.157) 0 1 ! and whose spatial components consist of the Pauli spin matrices 0 1 0 −i 1 0 σ1 = , σ2 = , σ3 = . (8.158) 1 0 ! i 0 ! 0 −1 ! On behalf of the algebraic properties of the Pauli matrices

σiσj = δij + iǫijkσk, (8.159) the Dirac matrices (8.155) satisfy the anticommutation rules

{γµ,γν} =2gµν. (8.160)

Under spacetime translations

x′µ = xµ − ǫµ, (8.161) the Dirac ﬁeld transforms in the same way as the previous scalar and vector ﬁelds:

ψ′(x′)= ψ(x), (8.162) or inﬁnitesimally: µ δsψ(x)= ǫ ∂µψ(x). (8.163) The same is true for the Lagrangian density, where

L′(x′)= L(x), (8.164) and µ δsL(x)= ǫ ∂µL(x). (8.165) Thus we obtain the Noether current

µ ∂L λ µ Θν = λ ∂ν ψ + c.c. − δν L, (8.166) ∂∂µψ with the local conservation law

µ ∂µΘν (x)=0. (8.167) From (8.154), we see that ∂L 1 ¯ µ λ = ψγ , (8.168) ∂∂µψ 2 so that we obtain the canonical energy-momentum tensor of the Dirac ﬁeld: 1 Θ µ = ψγ¯ µ∂ ψλ + c.c. − δ µL (8.169) ν 2 ν ν 640 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

8.7 Angular Momentum

Let us now turn to angular momentum in field theory. Consider first the case of a scalar field ϕ(x ˙). Under a rotation of the coordinates,

′i i j x = R jx , (8.170) the ﬁeld does not change, if considered at the same space point, i.e.,

ϕ′(x′i)= ϕ(xi). (8.171)

The inﬁnitesimal symmetry variation is:

′ δsϕ(x)= ϕ (x) − ϕ(x). (8.172)

Using the inﬁnitesimal form (8.46) of (8.170),

i j δx = −ωijx , (8.173) we see that

′ 0 ′i i δsϕ(x) = ϕ (x , x − δx ) − ϕ(x) j = ∂iϕ(x)x ωij. (8.174)

Suppose we are dealing with a Lorentz-invariant Lagrangian density that has no explicit x-dependence:

L = L(ϕ(x),∂ϕ(x)). (8.175)

Then the symmetry variation is

′ ′ δsL = L(ϕ (x),∂ϕ (x)) − ϕ(ϕ(x),∂ϕ(x)) ∂L ∂ϕ = δ ϕ(x)+ ∂ δ ϕ(x). (8.176) ∂ϕ(x) s ∂∂ϕ(x) µ s

For a Lorentz-invariant L, the derivative ∂L/∂∂µϕ is a vector proportional to ∂µϕ. For the Lagrangian density, the rotational symmetry variation Eq. (8.174) becomes

∂L j ∂L j δsL = ∂iϕ x + ∂µ(∂iLx ) ωij " ∂ϕ ∂µϕ #

j ∂L j = (∂iL)x + ∂iϕ ωij = ∂i Lx ωij . (8.177) " ∂∂j ϕ # The right-hand side is a total derivative. In arriving at this result, the antisymmetry of ϕij has been used twice: ﬁrst for dropping the second term in the brackets, which 8.8 Four-Dimensional Angular Momentum 641

is possible since ∂L/∂∂iϕ is proportional to ∂iϕ as a consequence of the assumed rotational invariance1 of L. Second it is used to pull xj inside the last parentheses. Calculating δsL once more with the help of the Euler-Lagrange equations gives ∂L ∂L δsL = δsϕ + ∂µδsϕ (8.178) ∂L ∂∂µϕ ∂L ∂L ∂L = − ∂µ δsϕ + ∂µ δsϕ ∂ϕ ∂∂µϕ! ∂∂µϕ !

∂L j = ∂µ ∂iϕ x ωij. ∂∂µϕ !

Thus the Noether charges

ij,µ ∂L j µ j L = ∂iϕx − δi L x − (i ↔ j) (8.179) ∂∂µϕ ! have no four-divergence

ij,µ ∂µL =0. (8.180)

The associated charges

Lij = d3xLij,µ (8.181) Z are called the total angular momenta of the ﬁeld system. In terms of the canonical energy-momentum tensor

µ ∂L µ Θν = ∂νϕ − δν L, (8.182) ∂∂µϕ the current density Lij,µ can also be rewritten as

Lij,µ = xiΘjµ − xjΘiµ. (8.183)

8.8 Four-Dimensional Angular Momentum

A similar procedure can be applied to pure Lorentz transformations. An inﬁnitesimal boost to rapidity ζi produces a coordinate change

′µ µ ν µ µ i ν µ i i x =Λ νx = x + δ iζ x + δ 0ζ x . (8.184)

This can be written as

µ µ ν δx = ω νx , (8.185)

1Recall the similar argument after Eq. (8.48) 642 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

where ωij =0, i ω0i = −ωi0 = ζ . (8.186)

µ With the tensor ω ν, the restricted Lorentz transformations and the inﬁnitesimal rotations can be treated on the same footing. The rotations have the form (8.185) for the particular choice

k ωij = ǫijkω ,

ω0i = ωi0 =0. (8.187)

We can now identify the symmetry variations of the ﬁeld as being

′ ′µ µ δsϕ(x) = ϕ (x − δx ) − ϕ(x) ν µ = −∂µϕ(x)x ω ν. (8.188)

Just as in (8.177), the Lagrangian density transforms as the total derivative

ν µ δsϕ = −∂µ(Lx )ω ν, (8.189) and we obtain the Noether currents ∂L Lµν,λ = − ∂λϕ xν − δµλL xν +(µ ↔ ν). (8.190) ∂∂λϕ ! The right-hand side can be expressed in terms of the canonical energy-momentum tensor (8.136), so that we ﬁnd

∂L Lµν,λ = − ∂λϕ xν − δµλLxν +(µ ↔ ν) ∂∂λϕ ! = xµΘνλ − xν Θµλ. (8.191)

These currents have no four-divergence

µν,λ ∂λL =0. (8.192)

The associated charges

Lµν ≡ d3x Lµν,0 (8.193) Z are independent of time. For the particular form of ωµν in (8.186), we ﬁnd time-independent components Li0. The components Lij coincide with the previously-derived angular momenta. The constancy of Li0 is the relativistic version of the center-of-mass theorem (8.65). Indeed, since

Li0 = d3x (xiΘ00 − x0Θi0), (8.194) Z 8.9 Spin Current 643 we can deﬁne the relativistic center of mass

3 00 i i d x Θ x xCM = 3 00 , (8.195) R d x Θ and the average velocity R

d3xΘi0 P i vi = c = c . (8.196) CM d3x Θ00 P 0

3 i0 i R i Since d xΘ = P is the constant momentum of the system, also vCM is a con- 0i stant.R Thus, the constancy of L implies the center-of-mass moves with the constant velocity i i i xCM(t)= x0CM + v0CMt, (8.197)

i 0i 0 µν with x0CM = L /P . The quantities L are referred to as four-dimensional orbital angular momenta. It is important to point out that the vanishing divergence of Lµν,λ makes Θνµ symmetric:

µν,λ µ νλ ν µλ ∂λL = ∂λ(x Θ − x Θ ) = Θνµ − Θνµ =0. (8.198)

Thus, translationally invariant ﬁeld theories whose orbital angular momentum is conserved have always a symmetric canonical energy-momentum tensor.

Θµν =Θνµ. (8.199)

8.9 Spin Current

If the ﬁeld ϕ(x) is no longer a scalar but carries spin degrees of freedom, the derivation of the four-dimensional angular momentum becomes slightly more involved.

8.9.1 Electromagnetic Fields Consider ﬁrst the case of electromagnetism where the relevant ﬁeld is the four-vector potential Aµ(x). When going to a new coordinate frame

′µ µ ν x =Λ ν x , (8.200) the vector field at the same point remains unchanged in absolute spacetime. How- ever, since the components Aµ refer to two different basic vectors in the different frames, they must be transformed accordingly. Indeed, since Aµ is a vector and transforms like xµ, it must satisfy the relation characterizing a vector field:

′µ ′ µ ν A (x )=Λ νA (x). (8.201) 644 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

For an inﬁnitesimal transformation

µ µ ν δsx = ω νx , (8.202) this implies a symmetry variation

µ ′µ µ ′µ µ δsA (x) = A (x) − A (x)= A (x − δx) − A (x) µ ν λ ν µ = ω νA (x) − ω νx ∂λA . (8.203)

The ﬁrst term is a spin transformation, the other an orbital transformation. The orbital transformation can also be written in terms of the generators Lˆµν of the Lorentz group deﬁned in (8.84) as

orb µ µν ˆ δs A (x) = −iω Lµν A(x). (8.204)

It is convenient to introduce 4 × 4 spin transformation matrices Lµν with the matrix elements:

(Lµν )λκ ≡ i (gµλgνκ − gµκgνλ) . (8.205)

They satisfy the same commutation relations (8.86) as the diﬀerential operators Lˆµν deﬁned in Eq. (8.85). By adding together the two generators Lˆµν and Lµν , we form the operator of total four-dimensional angular momentum

Jˆµν ≡ Lˆµν + Lµν , (8.206) and can write the symmetry variation (8.203) as

orb µ µν ˆ δs A (x) = −iω JµνA(x). (8.207)

If the Lagrangian density involves only scalar combinations of four-vectors Aµ, and if it has no explicit x-dependence, it changes under Lorentz transformations like a scalar ﬁeld:

L′(x′) ≡L(A′(x′), ∂′A′(x′)) = L(A(x), ∂A(x)) ≡L(x). (8.208)

Inﬁnitesimally, this makes the symmetry variation a pure gradient term:

ν µ δsL = −(∂µLx )ω ν. (8.209)

Thus Lorentz transformations are symmetry transformations in the Noether sense. Following Noether’s construction (8.179), we calculate the current of total four- dimensional angular momentum:

µν,λ ∂L ν ∂L µ κ ν µλ ν J = A − κ ∂ A x − δ Lx − (µ ↔ ν). (8.210) ∂∂λAµ ∂∂λA ! 8.9 Spin Current 645

The last two terms have the same form as the current Lµν,λ of the four-dimensional angular momentum of the scalar ﬁeld. Here they are the currents of the four- dimensional orbital angular momentum:.

µν,λ ∂L µ κ ν µλ ν L = − κ ∂ A x − δ Lx +(µ ↔ ν). (8.211) ∂∂λA !

Note that this current has the form

µν,λ ∂L ˆµν κ µλ ν L = −i κ L A + δ Lx − (µ ↔ ν) , (8.212) ∂∂λA h i where Lˆµν are the diﬀerential operators of four-dimensional angular momentum in the commutation rules (8.86). Just as the scalar case (8.191), the currents (8.211) can be expressed in terms of the canonical energy-momentum tensor as

Lµν,λ = xµΘνλ − xνΘµλ. (8.213)

The ﬁrst term in (8.210),

∂L Σµν,λ = Aν − (µ ↔ ν) , (8.214) " ∂∂λAν # is referred to as the spin current. It can be written in terms of the 4 × 4-generators (8.205) of the Lorentz group as

µν,λ ∂L µν σ Σ = −i κ (L )κσA . (8.215) ∂∂λA The two currents together,

J µν,λ(x) ≡ Lµν,λ(x) + Σµν,λ(x), (8.216)

µν,λ are conserved, satisfying ∂λJ (x) = 0. Individually, they are not conserved. The total angular momentum is given by the charge

J µν = d3xJ µν,0(x). (8.217) Z It is a constant of motion. Using the conservation law of the energy-momentum tensor we ﬁnd, just as in (8.198), that the orbital angular momentum satisﬁes

µν,λ µν νµ ∂λL (x)= − [Θ (x) − Θ (x)] . (8.218)

From this we ﬁnd the divergence of the spin current

µν,λ µν νµ ∂λΣ (x)= − [Θ (x) − Θ (x)] . (8.219) 646 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

For the charges associated with orbital and spin currents

Lµν (t) ≡ d3xLµν,0(x), Σµν (t) ≡ d3xΣµν,0(x), (8.220) Z Z this implies the following time dependence:

L˙ µν (t) = − d3x [Θµν (x) − Θνµ(x)] , Z Σ˙ µν (t) = d3x [Θµν(x) − Θνµ(x)] . (8.221) Z Thus ﬁelds with a nonzero spin density have always a non-symmetric energy momentum tensor. In general, the current density J µν,λ of total angular momentum reads ∂δxL J µν,λ = s − δµλLxν − (µ ↔ ν). (8.222) ∂∂λωµν(x) !

By the chain rule of differentiation, the derivative with respect to ∂λωµν(x) can come only from field derivatives, for a scalar field ∂δxL ∂L ∂δxϕ s = s , (8.223) ∂∂λωµν(x) ∂∂λϕ ∂ωµν (x) and for a vector field x x κ ∂δs L ∂L ∂δs A = κ . (8.224) ∂∂λωµν(x) ∂∂λA ∂ωµν The alternative rule of calculating angular momenta is to introduce spacetime- dependent transformations x µ ν δ x = ω ν(x)x , (8.225) under which the scalar fields transform as

λ ν δsϕ = −∂λϕω ν(x)x , (8.226) and the Lagrangian density as

x λ ν ν λ δs ϕ = −∂λL ω ν(x)x = −∂λ(x L)ω ν(x). (8.227)

x κ By separating spin and orbital transformations of δs A , we ﬁnd the two contributions σµν,λ and Lµν,λ to the current J µν,λ of the total angular momentum, the latter receiving a contribution from the second term in (8.222).

8.9.2 Dirac Field We now turn to the Dirac ﬁeld. Under a Lorentz transformation (8.200), this transforms according to the law

′ Λ ′ ψ(x ) −−−→ ψΛ(x)= D(Λ)ψ(x), (8.228) 8.9 Spin Current 647 where D(Λ) are the 4×4 spinor representation matrices of the Lorentz group. Their matrix elements can most easily be specified for infinitesimal transformations. For an infinitesimal Lorentz transformation

ν ν ν Λµ = δµ + ωµ , (8.229) under which the coordinates are changed by

µ µ ν δsx = ω νx , (8.230) the spin components transform under the representation matrix

ν ν 1 µν D(δµ + ωµ )= 1 − i ωµνσ , (8.231) 2 where σµν are the 4 × 4 matrices acting on the spinor space i σ = [γ ,γ ]. (8.232) µν 2 µ ν From the anticommutation rules (8.160), it is easy to verify that the spin matrices Sµν ≡ σµν /2 satisfy the same commutation rules (8.86) as the previous orbital and spin-1 generators Lˆµνµ and Lµν of Lorentz transformations. The ﬁeld has the symmetry variation [compare (8.203)]:

′ ν ν δsψ(x) = ψ (x) − ψ(x)= D(δµ + ωµ )ψ(x − δx) − ψ(x) 1 = −i ω σµνψ(x) − ωλ xν ∂ ψ(x) 2 µν ν λ 1 1 = −i ω Sµν + Lˆµν ψ(x) ≡ −i ω Jˆµνψ(x), (8.233) 2 µν 2 µν the last line showing the separation into spin and orbital transformation for a Dirac particle. Since the Dirac Lagrangian is Lorentz-invariant, it changes under Lorentz transformations like a scalar ﬁeld [compare (8.208)]:

L′(x′)= L(x). (8.234)

Inﬁnitesimally, this amounts to

ν µ δsL = −(∂µLx )ω ν. (8.235)

With the Lorentz transformations being symmetry transformations in the Noether sense, we calculate the current of total four-dimensional angular momentum extending the formulas (8.191) and (8.210) for scalar ﬁeld and vector potential. The result is ∂L ∂L J µν,λ = −i σµνψ − i Lˆµν ψ + c.c. + δµλLxν − (µ ↔ ν) . (8.236) ∂∂λψ ∂∂λψ ! h i 648 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

As before in (8.211) and (8.191), the orbital part of (8.236) can be expressed in terms of the canonical energy-momentum tensor as

Lµν,λ = xµΘνλ − xν Θµλ. (8.237)

The ﬁrst term in (8.236) is the spin current

1 ∂L Σµν,λ = −i σµν ψ + c.c. . (8.238) 2 ∂∂λψ !

Inserting (8.168), this becomes explicitly i 1 1 Σµν,λ = − ψγ¯ λσµν ψ = ψγ¯ [µγνγλ}]ψ = ǫµνλκψγ¯ κψ. (8.239) 2 2 2 The spin density is completely antisymmetric in the three indices [3]. The conservation properties of the three currents are the same as in Eqs. (8.217)– (8.221). Due to the presence of spin, the energy-momentum tensor is nonsymmetric.

8.10 Symmetric Energy-Momentum Tensor

Since the presence of spin is the cause of asymmetry of the canonical energy- momentum tensor, it is suggestive that an appropriate use of the spin current should help to construct a new modiﬁed momentum tensor

T µν =Θµν + ∆Θνµ, (8.240) that is symmetric, while still having the fundamental property of Θµν that its spatial integral P µ = d3x T µ0 yields the total energy-momentum vector of the system. µ0 This is ensuredR by the fact that ∆Θ being a three-divergence of a spatial vector. Such a construction was found in 1939 by Belinfante [4]. He introduced the tensor 1 T µν =Θµν − ∂ (Σµν,λ − Σνλ,µ + Σλµ,ν ), (8.241) 2 λ whose symmetry is manifest, due to (8.219) and the symmetry of the last two terms under the exchange µ ↔ ν. Moreover, the relation (8.241) for the µ0-components of (8.241), 1 T µ0 =Θµ0 − ∂ (Σµ0,λ − Σ0λ,µ + Σλµ,0), (8.242) 2 λ ensures that the spatial integral over J µν,0 ≡ xµT ν0 − xνT µ0 leads to the same total angular momentum

J µν = d3xJ µν,0 (8.243) Z 8.10 Symmetric Energy-Momentum Tensor 649 as the canonical expression (8.216). Indeed, the zeroth component of (8.242) is 1 xµΘν0 − xνΘµ0 − ∂ (Σµ0,k − Σ0k,µ + Σkµ,0)xν − (µ ↔ ν) . (8.244) 2 k h i Integrating the second term over d3x and performing a partial integration gives, for µ =0, ν = i:

1 3 0 i0,k 0k,i ki,0 i 00,k 0k,0 k0,0 3 0i,0 − d x x ∂k(Σ − Σ + Σ ) − x ∂k(Σ − Σ + Σ ) = d x Σ , 2 Z Z h i (8.245) and for µ = i, ν = j: 1 − d3x xi∂ (Σj0,k − Σ0k,j + Σkj,0) − (i ↔ j) = d3x Σij,0. (8.246) 2 k Z h i Z The right-hand sides are the contributions of spin to the total angular momentum. For the electromagnetic field, the spin current (8.214) reads explicitly 1 Σµν,λ = − F λµAν − (µ ↔ ν) . (8.247) c h i From this we calculate the Belinfante correction 1 ∆Θµν = [∂ (F λµAν − F λν Aµ) − ∂ (F µνAλ − F µλAν)+ ∂ (F νλAµ − F νµAλ)] 2c λ λ λ 1 = ∂ (F νλAµ). (8.248) c λ Adding this to the canonical energy-momentum tensor (8.153) 1 1 Θµν = − (F ν ∂µAλ − gµνF λκF ), (8.249) c λ 4 λκ we find the symmetric energy-momentum tensor 1 1 1 T µν = − (F ν F µλ − gµνF λκF )+ (∂ F νλ)Aµ. (8.250) c λ 4 λκ c λ µν The last term vanishes due to the free Maxwell field equations, ∂λF = 0. Therefore it can be dropped. Note that the proof of the symmetry of T µν involves the field equations via the divergence equation (8.219). It is useful to see what happens to Belinfante’s energy-momentum tensor in the presence of an external current, i.e., if T µν is calculated from the Lagrangian 1 1 L = − F F µν − jµA , (8.251) 4c µν c2 µ with an external current. The energy-momentum tensor is

µν 1 ν µ λ 1 µν λκ 1 µν λ Θ = F λ∂ A − g F Fλκ + 2 g j Aλ, (8.252) c 4 c 650 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem generalizing (8.25). The spin current is the same as before, and we ﬁnd Belinfante’s energy- momentum tensor [4]: 1 T µν = Θµν + ∂ (F νλAµ) (8.253) c λ 1 1 1 1 = − (F ν F µλ − gµνF λκF )+ gµνjλA + (∂ F νλ)Aµ. c λ 4 λκ c2 λ c λ νλ λ Using Maxwell’s equations ∂λF = −j , the last term can also be rewritten as 1 − jνAµ. (8.254) c This term prevents T µν from being symmetric, unless the current jλ vanishes.

8.10.1 Gravitational Field The derivation of the canonical energy-momentum tensor Θµν for the gravitational field is similar to that for the electromagnetic field in Subsection 8.9.1. We start from the quadratic action of the gravitational field (4.372),

f 1 4 νλ µ µ νλ µν µ A = − d x(∂µh ∂ hνλ − 2∂λhµν ∂ h +2∂µh ∂ν h − ∂µh∂ h), (8.255) 8κ Z and identify the canonically conjugate ﬁeld πλµν , f ∂ L π ≡ , (8.256) λµν ∂∂λhµν as being 1 π = [(∂ h −∂ h )(η ∂ h − η ∂ h)−η ∂σh +η ∂σh ]+(µ ↔ ν).(8.257) λµν 8κ λ µν µ λν λν µ µν λ λν σµ µν σλ It is antisymmetric under the exchange λ ↔ µ, and symmetric under µ ↔ ν. From the integrand in (8.255) we calculate, according to the general expression (8.136),

f µ ∂L µ λκ µν µ λκ µ Θ = ∂ h − η L = πνλκ∂ h − η L ν ∂∂ν hλκ ν 1 = (∂ h − ∂ h + η ∂ h − η ∂ h − η ∂σh + η ∂σh )∂µhλκ 2κ ν λκ κ δλ νκ λ νλ κ νκ σλ νκ σν ηµ − ν (∂ hσλ∂κh − 2∂ h ∂σhνλ +2∂ hσν ∂ h − ∂ h∂σh). (8.258) 8κ κ σλ λ σν σ ν σ

f In order to ﬁnd the symmetric energy-momentum tensor T µν , we follow Belinfante’s construction rule. The spin current density is calculated as in Subsection 8.9.1, starting from the substantial derivative of the tensor ﬁeld

µν µ κν ν µκ δsh = ω κh + ω κh . (8.259) 8.11 Internal Symmetries 651

Following the Noether rules, we ﬁnd, as in (8.215),

µν,λ ∂φ νκ Σ = 2 − (µ ↔ ν) = 2 [πλµκh − (µ ↔ ν)] . (8.260) ∂∂λhµκ

Combining the two results according to Belinfante’s formula (4.57), we obtain the symmetric energy-momentum tensor

f µν νλκ µ λµκ ν λνκ µ µνκ c µλκ ν νλκ µ νµκ µν T = π ∂ hλκ−∂c(π h κ−π h κ−π h d + π h κπ h κ−π hλκ)−η L. (8.261)

µνλ µν Using the ﬁeld equation ∂µπ = 0 and the Hilbert gauge (4.399) with ∂µφ = 0, this takes the simple form in φµν:

f µν 1 µ λκ ν µ ν µν κσ λ 1 λ T = 2∂ φ ∂ φλκ − ∂ φ∂ φ − η ∂λφ ∂ φκσ − ∂λφ∂ φ . (8.262) 8κ 2 8.11 Internal Symmetries

In quantum ﬁeld theory, an important role is played by internal symmetries. They do not involve any change in the spacetime coordinate of the ﬁelds, whose symmetry transformations have the simple form

φ′(x)= e−iαGφ(x), (8.263) where G are the generators of some Lie group and α the associated transformation parameters. The ﬁeld φ may have several indices on which the generators G act as a matrix. The symmetry variation associated with (8.263) is obviously

′ δsφ (x)= −iαGφ(x). (8.264)

The most important example is that of a complex field φ and a generator G = 1, where (8.263) is simply a multiplication by a constant phase factor. One also speaks of U(1)-symmetry. Other important examples are those of a triplet or an octet of fields φi with G being the generators of an SU(2) vector representation or an SU(3) octet representation (the adjoint representations of these groups). The first case is associated with charge conservation in electromagnetic interactions, the other two with isospin and SU(3) invariance in strong interactions. The latter symmetries are, however, not exact.

8.11.1 U(1)-Symmetry and Charge Conservation Suppose that a Lagrangian density L(x)= L(φ(x),∂φ(x), x) depends only on the absolute squares |φ|2, |∂φ|2, |φ∂φ|. Then L(x) is invariant under U(1)-transformations

δsφ(x)= −iφ(x). (8.265) 652 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

Indeed:

δsL =0. (8.266)

On the other hand, we ﬁnd by the chain rule of diﬀerentiation:

∂L ∂L ∂L δsL = − ∂µ δsφ + ∂µ δsφ =0. (8.267) ∂φ ∂µφ! " ∂∂µφ#

The Euler-Lagrange equation removes the ﬁrst part of this, and inserting (8.265) we ﬁnd by comparison with (8.266) that

∂L jµ = − φ (8.268) ∂∂µφ is a conserved current. For a free relativistic complex scalar ﬁeld with a Lagrangian density

∗ 2 ∗ L(x)= ∂µϕ ∂µϕ − m ϕ ϕ (8.269) we have to add the contributions of real and imaginary parts of the ﬁeld φ in formula (8.268). Then we obtain the conserved current

↔ ∗ jµ = −iϕ ∂ µϕ (8.270)

↔ ∗ where ϕ ∂ µϕ denotes the left-minus-right derivative:

↔ ∗ ∗ ∗ ϕ ∂ µϕ ≡ ϕ ∂µϕ − (∂µϕ )ϕ. (8.271)

For a free Dirac ﬁeld, we ﬁnd from (8.268) the conserved current

jµ(x)= ψ¯(x)γµψ(x). (8.272)

8.11.2 SU(N)-Symmetry

For more general internal symmetry groups, the symmetry variations have the form

δsϕ = −iαiGiϕ, (8.273) and the conserved currents are

µ ∂L ji = −i Giϕ. (8.274) ∂∂µϕ 8.12 Generating the Symmetry Transformations of Quantum Fields 653

8.11.3 Broken Internal Symmetries The physically important symmetries SU(2) of isospin and SU(3) are not exact. The Lagrange density is not strictly zero. In this case we remember the alternative derivation of the conservation law from (8.116). We introduce the spacetime-dependent parameters α(x) and conclude from the extremality property of the action that ∂Lǫ ∂Lǫ ∂µ = . (8.275) ∂∂µαi(x) ∂αi(x) This implies the divergence law for the above derived current

ǫ µ ∂L ∂µji (x)= . (8.276) ∂αi

8.12 Generating the Symmetry Transformations of Quantum Fields

As in quantum mechanical systems, the charges associated with the conserved currents of the previous section can be used to generate the transformations of the fields from which they were derived. One merely has to invoke the canonical field commutation rules. As an important example, consider the currents (8.274) of an internal U(N)- symmetry. Their charges i 3 ∂L Q = −i d x Giϕ (8.277) Z ∂∂µϕ can be written as i 3 Q = −i d xπGiϕ, (8.278) Z where π(x) ≡ ∂L(x)/∂∂µϕ(x) is the canonical momentum of the field ϕ(x). After quantization, these fields satisfy the canonical commutation rules:

[π(x, t),ϕ(x′, t)] = −iδ(3)(x − x′), [ϕ(x, t),ϕ(x′, t)] = 0, (8.279) [π(x, t), π(x′, t)] = 0.

From this we derive directly the commutation rule between the quantized charges (8.278) and the ﬁeld ϕ(x):

i i [Q , ϕˆ(x)] = −α Giϕ(x). (8.280)

We also ﬁnd that the commutation rules among the quantized charges are

[Qˆi, Qˆj] = [Gi,Gj]. (8.281)

i Since these coincide with those of the matrices Gi, the operators Q are seen to form a representation of the generators of the symmetry group in the Fock space. 654 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

It is important to realize that the commutation relations (8.280) and (8.281) remain also valid in the presence of symmetry breaking terms, as long as these do not contribute to the canonical momentum of the theory. Such terms are called soft symmetry breaking terms. The charges are no longer conserved, so that we must attach a time argument to the commutation relations (8.280) and (8.281). All times in these relations must be the same, in order to invoke the equal-time canonical commutation rules. The most important example is the canonical commutation relation (8.95) itself, which holds also in the presence of any potential V (q) in the Hamiltonian. This breaks translational symmetry, but does not contribute to the canonical momentum p = ∂L/∂q˙. In this case, the relation generalizes to

ǫj = iǫi[ˆpi(t), xˆj(t)], (8.282) which is correct thanks to the validity of the canonical commutation relations (8.93) at arbitrary equal times, also in the presence of a potential. Other important examples are the commutation rules of the conserved charges associated with the Lorentz generators (8.237):

J µν ≡ d3xJ µν,0(x), (8.283) Z which are the same as those of the 4 × 4-matrices (8.205), and those of the quantum mechanical generators (8.85):

[Jˆµν , Jˆµλ]= −igµµJˆνλ. (8.284)

The generators J µν ≡ d3xJ µν,0(x) are sums J µν = Lµν (t)+Σµν (t) of charges (8.220) associated with orbitalR and spin rotations. According to (8.221), the individual charges are time-dependent. Only their sum is conserved. Nevertheless, they both generate Lorentz transformations: Lµν (t) on the spacetime argument of the ﬁelds, and Σµν (t) on the spin indices. As a consequence, they both satisfy the commutation relations (8.284):

[Lˆµν , Lˆµλ] = −igµµLˆνλ, [Σˆ µν , Σˆ µλ] = −igµµΣˆ νλ. (8.285)

The commutators (8.281) have played an important role in developing a theory of strong interactions, where they ﬁrst appeared in the form of a charge algebra of the broken symmetry SU(3) × SU(3) of weak and electromagnetic charges. This symmetry will be discussed in more detail in Chapter 10. 8.13 Energy Momentum Tensor of a Relativistic Massive Point Particle 655

8.13 Energy Momentum Tensor of a Relativistic Massive Point Particle

If we want to study energy and momentum of charged relativistic point particles in an electromagnetic ﬁeld, it is useful to consider the action (8.68) with (8.72) as a spacetime integral over a Lagrangian density:

τb A = d4x L(x), with L(x)= L(x ˙ µ(τ))δ(4)(x − x(τ)). (8.286) Z Zτµ We can then derive for point particles local conservation laws that look very similar to those for ﬁelds. Instead of doing this from scratch, however, we shall simply take the already known global conservation laws and convert them into the local ones by inserting appropriate δ-functions with the help of the trivial identity

d4x δ(4)(x − x(τ))=1. (8.287) Z Consider for example the conservation law (8.74) for the momentum (8.75). With the help of (8.287) this becomes

∞ 4 d (4) 0= − d x dτ pλ(τ) δ (x − x(τ)). (8.288) Z Z−∞ " dτ # Note that the boundaries of the four volume in this expression contain the infor- mation on initial and ﬁnal times. We now perform a partial integration in τ, and rewrite (8.288) as

∞ ∞ 4 d (4) 4 (4) 0 = − d x dτ pλ(τ)δ (x − x(τ)) + d x dτ pλ(τ)∂τ δ (x − x(τ)). −∞ dτ −∞ Z Z h i Z Z (8.289)

The first term vanishes if the orbits come from and disappear into infinity. The second term can be rewritten as ∞ 4 ν (4) 0= − d x ∂ν dτ pλ(τ)x ˙ (τ)δ (x − x(τ)) . (8.290) Z Z−∞ This shows that the tensor ∞ Θλν (x) ≡ dτ pλ(τ)x ˙ ν (τ)δ(4)(x − x(τ)) (8.291) Z−∞ satisfies the local conservation law

λν ∂νΘ (x)=0. (8.292)

This is the conservation law of the energy-momentum tensor of a massive point particle. 656 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

The total momenta are obtained from the spatial integrals over Θλ0:

P µ(t) ≡ d3x Θλ0(x). (8.293) Z For point particles, they coincide with the canonical momenta pµ(t). If the La- grangian depends only on the velocityx ˙ µ(t) and not on the position xµ(t), the momenta pµ(t) are constants of motion: pµ(t) ≡ pµ. The Lorentz invariant quantity

2 2 µ ν M = P = gµν P P (8.294) is called the total mass of the system. For a single particle it coincides with the mass of the particle. Subjecting the orbits xµ(τ) to Lorentz transformations according to the rules of the last section we ﬁnd the currents of the total angular momentum

Lµν,λ ≡ xµΘνλ − xνΘµλ, (8.295) to satisfy the conservation law:

µν,λ ∂λL =0. (8.296)

A spatial integral over the zeroth component of the current Lµν,λ yields the conserved charges: Lµν (t) ≡ d3x Lµν,0(x)= xµpν(t) − xνpµ(t). (8.297) Z 8.14 Energy Momentum Tensor of a Massive Charged Particle in a Maxwell Field

Let us consider an important combination of a charged point particle and an electromagnetic ﬁeld Lagrangian

τν τν µ ν 1 4 µν e µ A = −mc dτ gµνx˙ (τ)x ˙ (τ) − d xFµν F − 2 dτx˙ (τ)Aµ(x(τ)). τµ 4c c τµ Z q Z Z (8.298)

By varying the action in the particle orbits, we obtain the Lorentz equation of motion dpµ e = F µ x˙ ν(τ). (8.299) dτ c ν We now vary the action in the vector potential, and find the Maxwell-Lorentz equation e −∂ F µν = x˙ ν(τ). (8.300) ν c The action (8.298) is invariant under translations of the particle orbits and the electromagnetic fields. The first term is obviously invariant, since it depends only 8.14 Energy Momentum Tensor of a Massive Charged Particle in a Maxwell Field 657 on the derivatives of the orbital variables xµ(τ). The second term changes under translations by a pure divergence [recall (8.134)]. The interaction term also changes by a pure divergence, which is seen as follows: Since the symmetry variation changes ν ν ν ν the coordinates as x (τ) → x (τ)−ǫ , and the field Aµ(x ) is transformed as follows:

ν ′ ν ν ν ν ν ν Aµ(x ) → Aµ(x )= Aµ(x + ǫ )= Aµ(x )+ ǫ ∂µAµ(x ), (8.301) we have altogether the symmetry variation

ν m δsL = ǫ ∂ν L . (8.302)

We now calculate the same variation once more using the equations of motion. This gives

em m d ∂L µ 4 ∂ L µ δsA = dτ ′µ δsx + d x µ δsA . (8.303) Z dτ ∂x Z ∂∂λA The ﬁrst term can be treated as in (8.289)–(8.290), after which it acquires the form

τν ∞ d e 4 d e (4) − dτ pµ + Aµ = − d x dτ pµ + Aµ δ (x − x(τ) Zτµ dτ c Z Z−∞ dτ c ∞ 4 e d (4) + d x dτ pµ + Aµ δ (x − x(τ)), (8.304) Z Z−∞ c dτ and thus, after dropping boundary terms,

τν ∞ λ d e 4 e dx (4) − dτ (pµ + Aµ)= ∂λ d x dτ pµ + Aµ δ (x − x(τ)). (8.305) Zτµ dτ c Z Z−∞ c dτ The electromagnetic part is the same as before, since the interaction contains no derivative of the gauge ﬁeld. In this way we ﬁnd the canonical energy-momentum tensor e Θµν(x) = dτ pµ + Aµ x˙ ν (τ)δ(4)(x − x(τ)) Z c 1 ν µ λ 1 µν λκ − F λ∂ A − g F Fλκ . (8.306) c 4 Let us check its conservation by calculating the divergence:

µν e ν (4) ∂ν Θ (x) = dτ p + Aµ x˙ (τ)∂ν δ (x − x(τ)) Z c 1 ν µ λ 1 ν µ λ 1 µ λκ − ∂ν F λ∂ A − F λ∂ν ∂ A − ∂ (F Fλκ) . (8.307) c c 4 The ﬁrst term is, up to a boundary term, equal to

e d d e − dτ pµ + Aµ δ(4)(x − x(τ))= dτ pµ + Aµ δ(4)(x − x(τ)).(8.308) Z τ dτ Z " dτ c # 658 8 Continuous Symmetries and Conservation Laws. Noether’s Theorem

Using the Lorentz equation of motion (8.299), this becomes

∞ e µ ν d µ (4) dτ F νx˙ (τ)+ A δ (x − x(τ)). (8.309) c Z−∞ dτ ! Inserting the Maxwell equation

µν µ (4) ∂ν F = −e dτ(dx /dτ)δ (x − x(τ)), (8.310) Z the second term in Eq. (8.307) can be rewritten as e ∞ dx − dτ λ ∂µAλδ(4)(x − x(τ)), (8.311) c Z−∞ dτ which is the same as e dx dx − dτ µ F µλ + λ ∂λAµ δ(4)(x − x(τ)), (8.312) c Z dτ dτ ! thus canceling (8.309). The third term in (8.307) is ﬁnally equal to

1 ν µ λ 1 µ λκ − F λ∂ Fν − ∂ (F Fλκ) , (8.313) c 4 due to the antisymmetry of F νλ. With the help of the homogeneous Maxwell equation we verify the Bianchi identity

∂λFµν + ∂µFνλ + ∂ν Fλµ =0. (8.314) is identically guaranteed. It is easy to construct from (8.306) Belinfante’s symmetric energy-momentum tensor. We merely observe that the spin density comes entirely from the vector potential, and is hence the same as before in (8.247). Hence the additional piece to be added to the canonical energy-momentum tensor is again [see (8.248)] 1 ∆Θµν = ∂ (F µνAµ) c λ 1 = (∂ F νλAµ + F νλ∂ Aµ). (8.315) 2 λ λ The second term in this expression serves to symmetrize the electromagnetic part of the canonical energy-momentum tensor and brings it to the Belinfante form:

em µν 1 ν µλ 1 µν λκ T = − F λF − g F Fλκ . (8.316) c 4 The first term in (8.315), which in the absence of charges vanishes, is now just what is needed to symmetrize the matter part of Θµν. Indeed, using once more Maxwell’s equation, it becomes e − dτx˙ ν (τ)Aµδ(4)(x − x(τ)), (8.317) c Z Notes and References 659 thus canceling the corresponding term in (8.306). In this way we find that the total energy-momentum tensor of charged particles plus electromagnetic fields is simply the sum of the two symmetric energy-momentum tensors: m em T µν = T µν + T µν ∞ 1 µ ν (4) 1 ν µλ 1 µν λκ = dτ u u δ (x − x(τ)) − F λF − g F Fλκ . (8.318) m Z−∞ c 4 For completeness, let us also cross-check its conservation:

µν ∂ν T =0. (8.319) Indeed, forming the divergence of the ﬁrst term gives [in contrast to (8.309)]

e ν µ dτ x˙ (τ)F ν(x(τ)), (8.320) c Z which is canceled by the divergence in the second term [in contrast to (8.312)]

1 ν µλ e µλ − ∂νF λF = − dτ x˙ λ(τ)F (x(τ)). (8.321) c c Z Notes and References

For more details see L.D. Landau and E.M. Lifshitz, The Classical Theory of Fields, Addison-Wesley, Reading, Mass., 1951; S. Schweber, Relativistic Quantum Fields, Harper and Row, New Yoerk, N.Y., 1961; A.O. Barut, Electrodynamics and Classical Theory of Fields and Particles, MacMillan, New York, N.Y. 1964; J.D. Jackson, Classical Electrodynamics, John Wiley & Sons, New York, N.Y., 1975; H. Ohanian, Classical Electrodynamics, Allyn and Bacon, Boston, Mass., 1988.

The individual citations refer to:

[1] E. Noether, Nachr. d. vgl. Ges. d. Wiss. Göttingen, Math-Phys. Klasse, 2, 235 (1918); See also E. Bessel-Hagen, Math. Ann. 84, 258 (1926); L. Rosenfeld, Me. Acad. Roy. Belg. 18, 2 (1938); F. Belinfante, Physica 6, 887 (1939). [2] S. Coleman and J.H. VanVleck, Phys. Rev. 171, 1370 (1968). [3] This property is important for being able to construct a consistent quantum mechanics in spacetime with torsion. See the textbook H. Kleinert, Path Integrals in Quantum Mechanics Statistics, Polymer Physics, and Finan- cial Markets, World Scientific, Singapore 2008 (http://klnrt.de/b5). [4] The Belinfante energy-momentum tensor is discussed further in H. Kleinert, Gauge Fields in Condensed Matter, Vol. II Stresses and Defects, World Sci- entific Publishing, Singapore 1989, pp. 744–1443 (http://klnrt.de/b2). H. Kleinert, Multivalued Fields in Condensed Matter, Electromagnetism, and Gravitation, World Scientific, Singapore 2009, pp. 1–497 (http://klnrt.de/b11).