Static forces and virtual-particle exchange: Difference between revisions

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{{Short description|Physical interaction in post-classical physics}}

Static force fields are fields, such as a simple electric, magnetic or gravitational fields, that exist without excitations that result in the carrying of information from place to place. The modern view of static forces is that the interactions between two bodies are mediated by [[virtual particles]], particles that exist for only a short time determined by the [[uncertainty principle]]. The virtual particles, also known as [[force carriers]], are [[bosons]] with a particular type of boson associated with each type of field.<ref> {{cite book | author=A. Zee | title=Quantum Field Theory in a Nutshell| publisher= Princeton University| year=2003 | id=ISBN 0-691-01019-6}} pp. 16-37 </ref>

{{Use American English|date = February 2019}}

'''Static force fields''' are fields, such as a simple [[Electric field|electric]], [[Magnetic field|magnetic]] or [[gravitational field]]s, that exist without excitations. The [[Perturbation theory (quantum mechanics)|most common approximation method]] that physicists use for [[Scattering theory|scattering calculations]] can be interpreted as static forces arising from the interactions between two bodies mediated by '''[[virtual particle]]s''', particles that exist for only a short time determined by the [[uncertainty principle]].<ref>{{cite journal|last1=Jaeger|first1=Gregg|title=Are virtual particles less real?| journal=Entropy |volume=21 |issue=2|page=141|date=2019|doi=10.3390/e21020141|pmid=33266857 |pmc=7514619 |bibcode=2019Entrp..21..141J|doi-access=free}}</ref> The virtual particles, also known as [[force carrier]]s, are [[boson]]s, with different bosons associated with each force.<ref name="Zee">{{cite book | first = A. | last = Zee | title=Quantum Field Theory in a Nutshell| publisher= Princeton University| year=2003 | isbn=0-691-01019-6}}</ref>{{rp|pp=16–37}}

The virtual-particle description of static forces is capable of identifying the spacialspatial form of the forces, such as the inverse-square behavior in [[Newton's Universal Lawlaw of Gravitationuniversal gravitation]] and in [[Coulomb's Lawlaw]]. It is also able to predict whether the forces are attractive or repulsive for like bodies. Also, the description has been able to relate the mass and range of the force carriers for such forces as the strong force.

The [[path integral formulation]] is the natural language for describing force carriers. This article uses the path integral formulation to describe the force carriers for [[Spin (physics) | spin]] 0, 1, and 2 fields. [[Pion]]s, [[photon]]s, and [[graviton]]s fall into these respective categories.

There are limits to the validity of the virtual particle picture. The virtual-particle formulation is derived from a method known as [[perturbation theory]] which is an approximation assuming interactions are not too strong, and was intended for scattering problems, not bound states such as atoms. For the strong force binding [[quark]]s into [[nucleon]]s at low energies, perturbation theory has never been shown to yield results in accord with experiments,<ref>{{cite web |url=http://www.hep.phy.cam.ac.uk/theory/research/hadronic.html |title=High Energy Physics Group - Hadronic Physics |accessdate=2010-08-31 |url-status=dead |archiveurl=https://web.archive.org/web/20110717002648/http://www.hep.phy.cam.ac.uk/theory/research/hadronic.html |archivedate=2011-07-17 }}</ref> thus, the validity of the "force-mediating particle" picture is questionable. Similarly, for [[bound state]]s the method fails.<ref>{{cite web| url=http://galileo.phys.virginia.edu/classes/752.mf1i.spring03/Time_Ind_PT.htm|title=Time-Independent Perturbation Theory| work=virginia.edu}}</ref> In these cases, the physical interpretation must be re-examined. As an example, the calculations of atomic structure in atomic physics or of molecular structure in quantum chemistry could not easily be repeated, if at all, using the "force-mediating particle" picture.{{citation needed|date=October 2014}}

<!-- Text below hidden for the time being (vs. being deleted) because it seems valuable, but needs rewriting in an encyclopedic form. Admonishment to look critically and avoid fallacy is unwikipedic. The CERN experiments are cited, but then criticized in a way appearing to be original research. Please reinstate with improvements. -->

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Use of the "force-mediating particle" picture (FMPP) is unnecessary in [[Quantum mechanics|nonrelativistic quantum mechanics]], and Coulomb's law is used as given in atomic physics and quantum chemistry to calculate both bound and scattering states. A non-perturbative [[relativistic quantum theory]], in which Lorentz invariance is preserved, is achievable by evaluating Coulomb's law as a 4-space interaction using the 3-space position vector of a reference electron obeying Dirac's equation and the quantum trajectory of a second electron which depends only on the scaled time. The quantum trajectory of each electron in an ensemble is inferred from the Dirac current for each electron by setting it equal to a velocity field times a quantum density, calculating a position field from the time integral of the velocity field, and finally calculating a quantum trajectory from the expectation value of the position field. The quantum trajectories are of course spin dependent, and the theory can be validated by checking that [[Pauli's exclusion principle]] is obeyed for a collection of [[fermion]]s.

==Classical forces==

The force exerted by one mass on another and the force exerted by one charge on another are strikingly similar. Both fall off as the square of the distance between the bodies. Both are proportional to the product of properties of the bodies, mass in the case of gravitation and charge in the case of electrostatics.

They also have a striking difference. Two masses attract each other, while two like charges repel each other.

In both cases, the bodies appear to act on each other over a distance. The concept of [[Field (physics) | field]] was invented to mediate the interaction among bodies thus eliminating the need for [[Action at a distance (physics)|action at a distance]]. The gravitational force is mediated by the [[gravitational field]] and the Coulomb force is mediated by the [[electromagnetic field]].

=== Gravitational force ===

The [[Newton's law of universal gravitation | gravitational force]] on a mass <math> m </math> exerted by another mass <math> M </math> is

<math display="block">\mathbf{F} = - G \frac{m M}{r^2} \, \hat\mathbf{r} = m \mathbf{g} \left ( \mathbf{r} \right ),</math>

where {{math|''G''}} is the [[Newtonian constant of gravitation]], {{mvar|r}} is the distance between the masses, and <math> \hat\mathbf{r} </math> is the [[unit vector]] from mass <math> M </math> to mass <math> m </math>.

: <math>

\mathbf{F} =

- G {m M \over {r}^2}

\, \mathbf{\hat{r}} =

m \mathbf{g} \left ( \mathbf{r} \right )

</math>

where ''G'' is the gravitational constant, r is the distance between the masses, and <math> \mathbf{\hat{r}} </math> is the [[unit vector]] from mass <math> m </math> to mass <math> M </math>.

The force can also be written

<math display="block">\mathbf{F} = m \mathbf{g} \left ( \mathbf{r} \right ),</math>

: <math>

\mathbf{F} =

m \mathbf{g} \left ( \mathbf{r} \right )

</math>

where <math> \mathbf{g} \left ( \mathbf{r} \right ) </math> is the [[gravitational field]] described by the field equation

<math display="block">\nabla\cdot \mathbf{g} = -4\pi G\rho_m, </math>

:<math>\nabla\cdot \mathbf{g} = -4\pi G\rho_m </math>

where <math>\rho_m</math> is the [[density|mass density]] at each point in space.

=== Coulomb force ===

The electrostatic [[Coulomb force]] on a charge <math> q </math> exerted by a charge <math> Q </math> is ([[SI units]])

<math display="block">\mathbf{F} = \frac{1}{4\pi\varepsilon_0}\frac{q Q}{r^2}\mathbf{\hat{r}},</math>

where <math> \varepsilon_0 </math> is the [[vacuum permittivity]], <math>r</math> is the separation of the two charges, and <math>\mathbf{\hat{r}}</math> is a [[unit vector]] in the direction from charge <math> Q </math> to charge <math> q </math>.

:<math>\mathbf{F} = {1 \over 4\pi\varepsilon_0}{q Q \over r^2}\mathbf{\hat{r}},</math>

where <math> \varepsilon_0 </math> is the [[vacuum permittivity]], <math>r</math> is the separation of the two charges, and <math>\mathbf{\hat{r}}</math> is a [[unit vector]] in the direction from charge <math> q </math> to charge <math> Q </math>.

The Coulomb force can also be written in terms of an [[electrostatic field]]

:<math>\mathbf{F} = q \mathbf{E} \left ( \mathbf{r} \right )</math>

The Coulomb force can also be written in terms of an [[electrostatic field]]:

<math display="block">\mathbf{F} = q \mathbf{E} \left ( \mathbf{r} \right ),</math>

where

<math display="block"> \nabla \cdot \mathbf{E} = \frac {\rho_q}{\varepsilon _0};</math>

<math>\rho_q</math> being the [[density|charge density]] at each point in space.

== Virtual-particle exchange ==

:<math> \nabla \cdot \mathbf{E} = \frac { \rho_q } { \varepsilon _0 }</math>

where <math>\rho_q</math> is the [[density|charge density]] at each point in space.

==Virtual-particle exchange==

In the modern picture, forces are generated by the exchange of [[virtual particles]]. A virtual particle acts as a carrier of force information between two bodies. The mechanics of virtual-particle exchange is best described with the [[path integral formulation]] of quantum mechanics. There are insights that can be obtained, however, without going into the machinery of path integrals, such as why classical gravitational and electrostatic forces fall off as the inverse square of the distance between bodies.

===Inverse square law===

The virtual-particle picture can be used to provide a [[heuristic]] explanation of the inverse square law for gravitational and electrostatic forces. If we imagine that a body emits a virtual particle and that virtual particle is absorbed by another body a distance r away, then the [[uncertainty principle]] states that

:<math> \Delta E \Delta t \approx \hbar </math>

where <math> \Delta t </math> is the time it takes the virtual particle to travel between bodies, and <math> \Delta E </math> is the energy of the virtual particle. If we imagine that <math> \Delta E </math> is a kind of [[potential energy]] and we assume the virtual particle travels at the speed of light, then

:<math> \Delta t \approx { r \over c } </math>

and

:<math> \Delta E \approx { \hbar c \over r } </math>.

The force is the gradient of the potential energy, therefore

:<math> F \propto { 1 \over r^2 } </math>,

which yields the inverse square law seen in both electrostatic and gravitational forces.

===Path integral formulation of virtual-particle exchange===

In perturbation theory, forces are generated by the exchange of [[virtual particle]]s. The mechanics of virtual-particle exchange is best described with the [[path integral formulation]] of quantum mechanics. There are insights that can be obtained, however, without going into the machinery of path integrals, such as why classical gravitational and electrostatic forces fall off as the inverse square of the distance between bodies.

A virtual particle is created by a disturbance to the [[vacuum state]], and the virtual particle is destroyed when it is absorbed back into the vacuum state by another disturbance. The disturbances are imagined to be due to bodies that interact with the virtual particle field.

=== Path-integral formulation of virtual-particle exchange ===

====The probability amplitude====

A virtual particle is created by a disturbance to the [[vacuum state]], and the virtual particle is destroyed when it is absorbed back into the vacuum state by another disturbance. The disturbances are imagined to be due to bodies that interact with the virtual particle’s field.

The probability amplitude for the creation, propagation, and destruction of a virtual particle is give, in the [[path integral formulation]] by

==== Probability amplitude ====

:<math> Z \equiv

Using [[natural units]], <math> \hbar = c = 1 </math>, the probability amplitude for the creation, propagation, and destruction of a virtual particle is given, in the [[path integral formulation]] by

<math display="block"> Z \equiv

\langle 0 | \exp\left ( -i \hat H T \right ) |0 \rangle

= \exp\left ( -i E T \right )

= \int D\varphi \; \exp\left ( i \mathcal{S} [\varphi] \right )\;

= \exp\left ( i W \right )

</math>

where <math> \hat H </math> is the [[Hamiltonian operator]], <math> T </math> is elapsed time, <math> E </math> is the energy change due to the disturbance, <math> W = - E T </math> is the change in action due to the disturbance, <math> \varphi </math> is the field of the virtual particle, the integral is over all paths, and the classical [[Action (physics)|action]] is given by

<math display="block">\mathcal{S} [\varphi] = \int \mathrm{d}^4x\; {\mathcal{L} [\varphi (x)]\,} </math>

where <math> \hat H </math> is the [[Hamiltonian operator]], <math> T </math> is elapsed time, <math> E </math> is the energy change due to the disturbance, <math> W = - E T </math> is the change in action due to the disturbance, <math> \varphi </math> is the field of the virtual particle, the integral is over all paths, and the classical [[Action (physics) | action]] is given by

where <math> \mathcal{L} [\varphi (x)] </math> is the [[Lagrangian (field theory)|Lagrangian]] density.

:<math>\mathcal{S} [\varphi] = \int \mathrm{d}^4x\; {\mathcal{L} [\varphi (x)]\,} </math>

where <math> \mathcal{L} [\varphi (x)] </math> is the [[Lagrangian]] density. We are using [[natural units]], <math> \hbar = c = 1 </math>.

Here, the [[spacetime]] metric is given by

<math display="block">\eta_{\mu\nu} = \begin{pmatrix}

1 & 0 & 0 & 0 \\

:<math>\eta_{\mu\nu} = \begin{pmatrix}

10 & 0-1 & 0 & 0 \\

0 & -10 & 0-1 & 0 \\

0 & 0 & -1 & 0\\

0 & 0 & 0 & -1

\end{pmatrix}.</math>

.</math>

The path integral often can be converted to the form

<math display="block"> Z = \int \exp\left[ i \int d^4x \left ( \frac 1 2 \varphi \hat O \varphi + J \varphi \right) \right ] D\varphi</math>

:<math> Z=

\int \exp\left[ i \int d^4x \left ( \frac 1 2 \varphi \hat O \varphi + J \varphi \right) \right ] D\varphi

</math>

where <math> \hat O </math> is a differential operator with <math> \varphi </math> and <math> J </math> functions of [[spacetime]]. The first term in the argument represents the free particle and the second term represents the disturbance to the field from an external source such as a charge or a mass.

The integral can be written (see [[{{slink|Common integrals in quantum field theory]]#Integrals with differential operators in the argument}})

<math display="block"> Z \propto \exp\left( i W\left ( J \right )\right)</math>

where

<math display="block"> W\left ( J \right ) = -\frac{1}{2} \iint d^4x \; d^4y \; J\left ( x \right ) D\left ( x-y \right ) J\left ( y \right )</math>

is the change in the action due to the disturbances and the [[propagator]] <math> D\left ( x-y \right ) </math> is the solution of

<math display="block">\hat O D\left ( x - y \right ) = \delta^4 \left ( x - y \right ).</math>

==== Energy of interaction ====

:<math> Z \propto

\exp\left( i W\left ( J \right )\right)

</math>

We assume that there are two point disturbances representing two bodies and that the disturbances are motionless and constant in time. The disturbances can be written

where

<math display="block"> J(x) = \left( J_1 +J_2,0,0,0 \right)</math>

<math display="block">\begin{align}

J_1 &= a_1 \delta^3\left ( \mathbf x - \mathbf x_1 \right ) \\

J_2 &= a_2 \delta^3\left ( \mathbf x - \mathbf x_2 \right )

\end{align}</math>

where the delta functions are in space, the disturbances are located at <math> \mathbf x_1 </math> and <math> \mathbf x_2 </math>, and the coefficients <math> a_1 </math> and <math> a_2 </math> are the strengths of the disturbances.

If we neglect self-interactions of the disturbances then W becomes

:<math> W\left ( J \right ) =

-{1<math display="block"> W\overleft 2}( J \right ) = - \iint d^4x \; d^4y \; JJ_1\left ( x \right ) \frac{1}{2} \left [ D\left ( x-y \right ) J+ D\left ( y-x \right )\right ] J_2\left ( y \right ),</math>

which can be written

<math display="block"> W\left ( J \right ) =

- T a_1 a_2\int \frac{d^3k}{(2 \pi )^3} \; \; D\left ( k \right )\mid_{k_0=0} \; \exp\left ( i \mathbf k \cdot \left ( \mathbf x_1 - \mathbf x_2 \right ) \right ).

</math>

is the change in the action due to the disturbances and the [[propagator]]Here <math> D\left ( x-yk \right ) </math> is the solutionFourier transform of

<math display="block"> \frac{1}{2} \left [ D\left ( x-y \right ) + D\left ( y-x \right )\right ].</math>

Finally, the change in energy due to the static disturbances of the vacuum is

:<math>

<math display="block"> E = - \frac{W}{T} = a_1 a_2\int \frac{d^3k}{(2 \pi )^3} \; \; D\left ( k \right )\mid_{k_0=0} \; \exp\left ( i \mathbf k \cdot \left ( \mathbf x_1 - \mathbf x_2 \right ) \right ).</math>

\hat O D\left ( x - y \right ) = \delta^4 \left ( x - y \right )

</math>.

If this quantity is negative, the force is attractive. If it is positive, the force is repulsive.

====Static fields====

Examples of static, motionless, interacting currents are the [[#The Yukawa potential: The force between two nucleons in an atomic nucleus|Yukawa potential]], the [[#The Coulomb potential in a vacuum|Coulomb potential in a vacuum]], and the [[#Coulomb potential in a simple plasma or electron gas|Coulomb potential in a simple plasma or electron gas]].

We assume that there are two disturbances representing two bodies and that the disturbances are constant in time. The disturbances can be written

The expression for the interaction energy can be generalized to the situation in which the point particles are moving, but the motion is slow compared with the speed of light. Examples are the Darwin interaction [[#Darwin interaction in a vacuum|in a vacuum]] and [[#Darwin interaction in a plasma|in a plasma]].

:<math> J\left ( x \right ) = J_1 +J_2

Finally, the expression for the interaction energy can be generalized to situations in which the disturbances are not point particles, but are possibly line charges, tubes of charges, or current vortices. Examples include: [[#Two line charges embedded in a plasma or electron gas|two line charges embedded in a plasma or electron gas]], [[#Coulomb potential between two current loops embedded in a magnetic field|Coulomb potential between two current loops embedded in a magnetic field]], and the [[#Magnetic interaction between current loops in a simple plasma or electron gas|magnetic interaction between current loops in a simple plasma or electron gas]]. As seen from the Coulomb interaction between tubes of charge example, shown below, these more complicated geometries can lead to such exotic phenomena as [[Fractional quantum Hall effect|fractional quantum numbers]].

</math>

== Selected examples ==

:<math> J_1 =

a_1 \delta^3\left ( \vec x - \vec x_1 \right )

=== Yukawa potential: the force between two nucleons in an atomic nucleus ===

</math>

Consider the [[Spin (physics)|spin]]-0 Lagrangian density<ref name="Zee"/>{{rp|pp=21–29}}

:<math> J_2 =

<math display="block">

a_2 \delta^3\left ( \vec x - \vec x_2 \right )

\mathcal{L} [\varphi (x)]

= \frac{1}{2} \left [ \left ( \partial \varphi \right )^2 -m^2 \varphi^2 \right ].</math>

The equation of motion for this Lagrangian is the [[Klein–Gordon equation]]

</math>

<math display="block"> \partial^2 \varphi + m^2 \varphi =0.</math>

If we add a disturbance the probability amplitude becomes

where the delta functions are in space, the disturbances are located at <math> \vec x_1 </math> and <math> \vec x_2 </math>, and the coefficients <math> \vec a_1 </math> and <math> \vec a_2 </math> are the strengths of the disturbances.

<math display="block"> Z =

\int D\varphi \; \exp \left \{ i \int d^4\mathbf{x}\; \left [ \frac{1}{2} \left ( \left ( \partial \varphi \right )^2 - m^2\varphi^2 \right ) + J\varphi \right ] \right \}.</math>

If we integrate by parts and neglect self-interactionsboundary ofterms theat disturbancesinfinity the thenprobability Wamplitude becomes

<math display="block"> Z =

\int D\varphi \; \exp \left \{ i \int d^4x\; \left [ -\frac{1}{2}\varphi \left ( \partial^2 + m^2\right )\varphi + J\varphi \right ] \right \}.</math>

With the amplitude in this form it can be seen that the propagator is the solution of

:<math> W\left ( J \right ) =

-<math \iintdisplay="block"> d^4x \; d^4y \; J_1-\left ( x \rightpartial^2 ) {1\over+ m^2} \leftright [) D\left ( x-y \right ) += D\delta^4\left ( yx-x \right )\right ] J_2\left ( y \right ).</math>

</math>,

whichFrom this it can be writtenseen that

<math display="block">D\left ( k \right )\mid_{k_0=0} \; = \; -\frac{1}{ k^2 + m^2}.</math>

The energy due to the static disturbances becomes (see {{slink|Common integrals in quantum field theory#Yukawa Potential: The Coulomb potential with mass}})

:<math> W\left ( J \right ) =

-<math Tdisplay="block">E =-\frac{a_1 a_2\int }{d^3k \over (24 \pi )^3 r} \; \; D\left ( k \right )\mid_{k_0=0} \; \exp\left ( i \vec k \cdot \left ( \vec x_1 -m \vec x_2 \right )r \right )</math>

with

</math>.

<math display="block">r^2 = \left (\mathbf x_1 - \mathbf x_2 \right )^2</math>

which is attractive and has a range of <math display="block">\frac{1}{m}.</math>

[[Hideki Yukawa|Yukawa]] proposed that this field describes the force between two [[nucleon]]s in an atomic nucleus. It allowed him to predict both the range and the mass of the particle, now known as the [[pion]], associated with this field.

Here <math> D\left ( k \right ) </math> is the Fourier transform of

=== Electrostatics ===

:<math> {1\over 2} \left [ D\left ( x-y \right ) + D\left ( y-x \right )\right ]

</math>.

==== Coulomb potential in vacuum ====

Finally, the change in energy due to the static disturbances of the vacuum is

Consider the [[Spin (physics)|spin]]-1 [[Proca action|Proca Lagrangian]] with a disturbance<ref name="Zee"/>{{rp|pp=30–31}}

<math display="block">\mathcal{L} [\varphi (x)] = -\frac{1}{4} F_{\mu \nu} F^{\mu \nu} + \frac{1}{2} m^2 A_{\mu} A^{\mu} + A_{\mu} J^{\mu}</math>

:<math> E =

where

-{W\over T}=

<math display="block"> F_{\mu \nu} = \partial_{\mu} A_{\nu} - \partial_{\nu} A_{\mu},</math>

a_1 a_2\int {d^3k \over (2 \pi )^3 } \; \; D\left ( k \right )\mid_{k_0=0} \; \exp\left ( i \vec k \cdot \left ( \vec x_1 - \vec x_2 \right ) \right )

charge is conserved

</math>.

<math display="block"> \partial_{\mu} J^{\mu} = 0,</math>

and we choose the [[Lorenz gauge]]

<math display="block"> \partial_{\mu} A^{\mu} = 0.</math>

Moreover, we assume that there is only a time-like component <math>J^0 </math> to the disturbance. In ordinary language, this means that there is a charge at the points of disturbance, but there are no electric currents.

If this quantity is negative, the force is attractive. If it is positive, the force is repulsive.

If we follow the same procedure as we did with the Yukawa potential we find that

==The Yukawa potential: The force between two nucleons in an atomic nucleus==

<math display="block">\begin{align}

-\frac{1}{4} \int d^4x F_{\mu \nu}F^{\mu \nu}

&= -\frac{1}{4}\int d^4x \left( \partial_{\mu} A_{\nu} - \partial_{\nu} A_{\mu} \right)\left( \partial^{\mu} A^{\nu} - \partial^{\nu} A^{\mu} \right) \\

&= \frac{1}{2}\int d^4x \; A_{\nu} \left( \partial^{2} A^{\nu} - \partial^{\nu} \partial_{\mu} A^{\mu} \right) \\

&= \frac{1}{2}\int d^4x \; A^{\mu} \left( \eta_{\mu \nu} \partial^{2} \right) A^{\nu},

\end{align}</math>

which implies

<math display="block"> \eta_{\mu \alpha} \left ( \partial^2 + m^2\right ) D^{\alpha \nu}\left ( x-y \right ) = \delta_{\mu}^{\nu} \delta^4\left ( x-y \right )</math>

and

<math display="block">D_{\mu \nu}\left ( k \right )\mid_{k_0=0} \; = \; \eta_{\mu \nu}\frac{1}{- k^2 + m^2}.</math>

This yields

Consider the [[Spin (physics) | spin]]-0 Lagrangian density<ref> Zee, pp. 21-29 </ref>

<math display="block">D\left( k \right)\mid_{k_0=0}\; = \; \frac{1}{\mathbf k^2 + m^2}</math>

for the [[timelike]] propagator and

<math display="block">E = + \frac{a_1 a_2}{4 \pi r} \exp \left( -m r \right)</math>

which has the opposite sign to the Yukawa case.

In the limit of zero [[photon]] mass, the Lagrangian reduces to the Lagrangian for [[electromagnetism]]

:<math>

<math display="block">E = \frac{a_1 a_2}{4 \pi r}.</math>

\mathcal{L} [\varphi (x)]

= {1\over 2} \left [ \left ( \partial \varphi \right )^2 -m^2 \varphi^2 \right ]

</math>.

Therefore the energy reduces to the potential energy for the Coulomb force and the coefficients <math>a_1 </math> and <math>a_2 </math> are proportional to the electric charge. Unlike the Yukawa case, like bodies, in this electrostatic case, repel each other.

The equation of motion for this Lagrangian is the [[Klein-Gordon equation]]

==== Coulomb potential in a simple plasma or electron gas ====

:<math>

\partial^2 \varphi + m^2 \varphi =0

</math>.

===== Plasma waves =====

If we add a disturbance the probability amplitude becomes

The [[dispersion relation]] for [[plasma wave]]s is<ref name="Chen">{{cite book | first = Francis F. | last = Chen | title=Introduction to Plasma Physics| publisher= Plenum Press| year=1974 | isbn=0-306-30755-3}}</ref>{{rp|pp=75–82}}

:<math>

<math display="block">\omega^2 = \omega_p^2 + \gamma\left( \omega \right) \frac{T_\text{e}}{m} \mathbf k^2.</math>

Z =

where <math>\omega </math> is the angular frequency of the wave,

\int D\varphi \; \exp \left \{ i \int d^4x\; \left [ {1\over 2} \left ( \left ( \partial \varphi \right )^2 - m^2\varphi^2 \right ) + J\varphi \right ] \right \}

<math display="block">\omega_p^2 = \frac{4\pi n e^2}{m}</math>

</math>.

is the [[plasma frequency]], <math>e </math> is the magnitude of the [[electron charge]], <math>m </math> is the [[electron mass]], <math>T_\text{e} </math> is the electron [[temperature]] (the [[Boltzmann constant]] equal to one), and <math>\gamma\left( \omega \right) </math> is a factor that varies with frequency from one to three. At high frequencies, on the order of the plasma frequency, the compression of the electron fluid is an [[adiabatic process]] and <math>\gamma\left( \omega \right) </math> is equal to three. At low frequencies, the compression is an [[isothermal process]] and <math>\gamma\left( \omega \right) </math> is equal to one. [[Retarded potential|Retardation]] effects have been neglected in obtaining the plasma-wave dispersion relation.

For low frequencies, the dispersion relation becomes

If we integrate by parts and neglect boundary terms at infinity the probability amplitude becomes

<math display="block"> \mathbf k^2 + \mathbf k_\text{D}^2 = 0</math>

where

<math display="block"> k_\text{D}^2= \frac{4\pi n e^2}{T_e}</math>

is the Debye number, which is the inverse of the [[Debye length]]. This suggests that the propagator is

<math display="block">D\left ( k \right )\mid_{k_0=0} \; = \; \frac{1}{ k^2 + k_\text{D}^2}.</math>

In fact, if the retardation effects are not neglected, then the dispersion relation is

:<math>

<math display="block"> -k_0^2 + k^2 + k_\text{D}^2 -\frac{m}{T_\text{e}} k_0^2 = 0,</math>

Z =

which does indeed yield the guessed propagator. This propagator is the same as the massive Coulomb propagator with the mass equal to the inverse Debye length. The interaction energy is therefore

\int D\varphi \; \exp \left \{ i \int d^4x\; \left [ -{1\over 2}\varphi \left ( \partial^2 + m^2\right )\varphi + J\varphi \right ] \right \}

<math display="block">E = \frac{a_1 a_2}{4 \pi r} \exp \left ( -k_\text{D} r \right ).</math>

</math>.

The Coulomb potential is screened on length scales of a Debye length.

===== Plasmons =====

With the amplitude in this form it can be seen that the propagator is the solution of

In a quantum [[Free electron model|electron gas]], plasma waves are known as [[plasmon]]s. Debye screening is replaced with [[Thomas–Fermi screening]] to yield<ref>{{cite book | author=C. Kittel | title=[[Introduction to Solid State Physics]]|edition=Fifth | publisher= John Wiley and Sons| year=1976 | isbn=0-471-49024-5}} pp. 296-299.</ref>

:<math>

<math display="block">E = -\left ( frac{a_1 a_2}{4 \partial^2pi r} + m^2\rightexp ) D\left ( x-y k_\righttext{s} ) = \delta^4\left ( x-yr \right ) </math>

where the inverse of the Thomas–Fermi screening length is

</math>.

<math display="block"> k_\text{s}^2 = \frac{6\pi n e^2}{\varepsilon_\text{F}}</math>

and <math>\varepsilon_\text{F}</math> is the [[Fermi energy]] <math display="inline">\varepsilon_\text{F} = \frac{\hbar^2}{2m} \left( {3 \pi^2 n} \right)^{2/3} .</math>

This expression can be derived from the [[chemical potential]] for an electron gas and from [[Poisson's equation]]. The chemical potential for an electron gas near equilibrium is constant and given by

From this it can be seen that

<math display="block"> \mu = -e\varphi + \varepsilon_\text{F}</math>

where <math>\varphi</math> is the [[electric potential]]. Linearizing the Fermi energy to first order in the density fluctuation and combining with Poisson's equation yields the screening length. The force carrier is the quantum version of the [[plasma wave]].

===== Two line charges embedded in a plasma or electron gas =====

:<math>

D\left ( k \right )\mid_{k_0=0}\; = \;

-{1 \over \vec k^2 + m^2}

</math>.

We consider a line of charge with axis in the ''z'' direction embedded in an electron gas

The energy due to the static disturbances becomes (see [[Common integrals in quantum field theory]])

<math display="block"> J_1\left( x\right) = \frac{a_1}{L_B} \frac{1}{2 \pi r} \delta^2\left( r \right)</math>

where <math>r</math> is the distance in the ''xy''-plane from the line of charge, <math>L_B</math> is the width of the material in the z direction. The superscript 2 indicates that the [[Dirac delta function]] is in two dimensions. The propagator is

<math display="block">D\left ( k \right )\mid_{k_0=0}\; = \; \frac{1}{\mathbf k^2 + k_{Ds}^2}</math>

where <math>k_{Ds} </math> is either the inverse [[Debye–Hückel equation|Debye–Hückel screening length]] or the inverse [[Thomas–Fermi screening]] length.

The interaction energy is

:<math>

<math display="block"> E = \left( \frac{a_1\, a_2}{2 \pi L_B}\right) \int_0^{\infty} \frac{k\,dk}{k^2 + k_{Ds}^2} \mathcal J_0 ( kr_{12} )

E =

-= \left( \frac{a_1\, a_2 \over 4}{2 \pi r L_B} \expright) K_0 \left ( -mk_{Ds} rr_{12} \right )</math>

where <math> \mathcal J_n ( x ) </math> and <math> K_0 ( x )</math> are [[Bessel function]]s and <math> r_{12}</math> is the distance between the two line charges. In obtaining the interaction energy we made use of the integrals (see {{slink|Common integrals in quantum field theory#Integration of the cylindrical propagator with mass}})

</math>

<math display="block"> \int_0^{2 \pi} \frac{d\varphi}{2 \pi} \exp\left( i p \cos\left( \varphi \right) \right) = \mathcal J_0 ( p ) </math>

and

<math display="block"> \int_0^{\infty} \frac{k\,dk}{k^2 + m^2} \mathcal J_0 (kr) = K_0 (mr).</math>

For <math> k_{Ds} r_{12} \ll 1</math>, we have

with

<math display="block">K_0 \left( k_{Ds} r_{12} \right) \to -\ln \left(\frac{k_{Ds} r_{12}}{2}\right) + 0.5772.</math>

==== Coulomb potential between two current loops embedded in a magnetic field ====

:<math>

r^2 =

\left (\vec x_1 - \vec x_2 \right )^2

</math>

===== Interaction energy for vortices =====

which is attractive and has a range of

We consider a charge density in tube with axis along a magnetic field embedded in an electron gas

:<math>

<math display="block"> J_1\left( x\right) = \frac{a_1}{L_b} \frac{1}{2 \pi r} \delta^2{\left( r - r_{B1}\right)}</math>

{1 \over m}

where <math>r</math> is the distance from the [[guiding center]], <math>L_B</math> is the width of the material in the direction of the magnetic field

</math>.

<math display="block"> r_{B1} = \frac{\sqrt{4 \pi} m_1 v_1}{a_1 B} = \sqrt{\frac{2 \hbar}{m_1 \omega_c}} </math>

where the [[cyclotron frequency]] is ([[Gaussian units]])

<math display="block">\omega_c = \frac{a_1 B}{\sqrt{4 \pi} m_1 c} </math>

and

<math display="block"> v_1 = \sqrt {\frac{2 \hbar \omega_c}{m_1}} </math>

is the speed of the particle about the magnetic field, and B is the magnitude of the magnetic field. The speed formula comes from setting the classical kinetic energy equal to the spacing between [[Landau levels]] in the quantum treatment of a charged particle in a magnetic field.

In this geometry, the interaction energy can be written

[[Yukawa]] proposed that this field describes the force between two [[nucleon]]s in an atomic nucleus. It allowed him to predict both the range and the mass of the particle, now known as the [[pion]], associated with this field.

<math display="block"> E =

\left( \frac{a_1\, a_2}{2 \pi L_B}\right) \int_0^{\infty} {k\;dk \;} D\left( k \right) \mid_{k_0=k_B=0}

\mathcal J_0 \left ( kr_{B1} \right) \mathcal J_0 \left ( kr_{B2} \right) \mathcal J_0 \left ( kr_{12} \right)</math>

where <math>r_{12}</math> is the distance between the centers of the current loops and <math> \mathcal J_n ( x ) </math> is a [[Bessel function]] of the first kind. In obtaining the interaction energy we made use of the integral

<math display="block">

\int_0^{2 \pi} \frac{d\varphi}{2 \pi} \exp\left( i p \cos(\varphi) \right) = \mathcal J_0 ( p ) . </math>

===== Electric field due to a density perturbation =====

==Electrostatics: The Coulomb force==

The [[chemical potential]] near equilibrium, is given by

Consider the [[Spin (physics) | spin]]-1 [[Proca action | Proca Lagrangian]] with a disturbance<ref> Zee, pp. 30-31 </ref>

<math display="block"> \mu = -e\varphi + N\hbar \omega_c = N_0\hbar \omega_c</math>

where <math>-e\varphi</math> is the [[potential energy]] of an electron in an [[electric potential]] and <math>N_0</math> and <math>N</math> are the number of particles in the electron gas in the absence of and in the presence of an electrostatic potential, respectively.

The density fluctuation is then

:<math>

<math display="block"> \delta n = \frac{e \varphi}{\hbar \omega_c A_\text{M} L_B}</math>

\mathcal{L} [\varphi (x)] =

where <math> A_\text{M} </math> is the area of the material in the plane perpendicular to the magnetic field.

-{1\over 4} F_{\mu \nu} F^{\mu \nu} + {1\over 2} m^2 A_{\mu} A^{\mu} + A_{\mu} J^{\mu}

</math>

[[Poisson's equation]] yields

<math display="block">\left( k^2 + k_B^2 \right) \varphi = 0</math>

where

<math display="block"> k_B^2 = \frac{4 \pi e^2}{\hbar \omega_c A_\text{M} L_B}.</math>

The propagator is then

:<math>

<math display="block"> D\left( k \right) \mid_{k_0=k_B=0} = \frac{1}{k^2 + k_B^2}</math>

F_{\mu \nu} =

and the interaction energy becomes

\partial_{\mu} A_{\nu} - \partial_{\nu} A_{\mu}

</math display="block">, E =

\left( \frac{a_1\, a_2}{2 \pi L_B}\right) \int_0^{\infty} \frac{k\;dk \;}{k^2 + k_B^2}

\mathcal J_0 \left ( kr_{B1} \right) \mathcal J_0 \left ( kr_{B2} \right) \mathcal J_0 \left ( kr_{12} \right)

=

\left( \frac{2 e^2}{L_B}\right) \int_0^{\infty} \frac{k\;dk \;}{k^2 + k_B^2 r_B^2}

\mathcal J_0^2 \left ( k \right) \mathcal J_0 \left ( k\frac{r_{12}}{r_B} \right)

</math>

where in the second equality ([[Gaussian units]]) we assume that the vortices had the same energy and the electron charge.

In analogy with [[plasmons]], the [[force carrier]] is the quantum version of the [[upper hybrid oscillation]] which is a longitudinal [[plasma wave]] that propagates perpendicular to the magnetic field.

charge is conserved

===== Currents with angular momentum =====

:<math>

\partial_{\mu} J^{\mu} = 0

</math>,

====== Delta function currents ======

and we choose the [[Lorenz gauge]]

[[Image:100927c Angular momentum 11.jpg|thumb|250px|right|Figure 1. Interaction energy vs. ''r'' for angular momentum states of value one. The curves are identical to these for any values of <math>\ell = \ell'</math>. Lengths are in units are in <math>r_\ell</math>, and the energy is in units of <math display="inline"> \frac{e^2}{L_B}</math>. Here <math>r = r_{12}</math>. Note that there are local minima for large values of <math> k_{B}</math>.]]

:<math>

[[Image:100927 Angular momentum 15.jpg|thumb|250px|right|Figure 2. Interaction energy vs. r for angular momentum states of value one and five.]]

\partial_{\mu} A^{\mu} = 0

[[Image:101005 energy by theta.jpg|thumb|250px|right|Figure 3. Interaction energy vs. r for various values of theta. The lowest energy is for <math display="inline">\theta = \frac{\pi}{4}</math> or <math> \frac{\ell}{\ell'} = 1 </math>. The highest energy plotted is for <math display="inline">\theta = 0.90\frac{\pi}{4}</math>. Lengths are in units of <math>r_{\ell \ell'}</math>.]]

</math>.

[[Image:101011 energy picture.jpg|thumb|250px|right|Figure 4. Ground state energies for even and odd values of angular momenta. Energy is plotted on the vertical axis and r is plotted on the horizontal. When the total angular momentum is even, the energy minimum occurs when <math> \ell = \ell' </math> or <math display="inline"> \frac{\ell}{\ell^*} = \frac{1}{2} </math>. When the total angular momentum is odd, there are no integer values of angular momenta that will lie in the energy minimum. Therefore, there are two states that lie on either side of the minimum. Because <math> \ell \ne \ell' </math>, the total energy is higher than the case when <math> \ell = \ell' </math> for a given value of <math> \ell^* </math>.]]

Unlike classical currents, quantum current loops can have various values of the [[Larmor radius]] for a given energy.<ref name="Ezewa">{{cite book | first = Zyun F. | last = Ezewa | title=Quantum Hall Effects: Field Theoretical Approach And Related Topics | edition = Second | publisher= World Scientific| year=2008 | isbn=978-981-270-032-2}}</ref>{{rp|pp=187–190}} [[Landau level]]s, the energy states of a charged particle in the presence of a magnetic field, are multiply [[Degenerate energy level|degenerate]]. The current loops correspond to [[angular momentum]] states of the charged particle that may have the same energy. Specifically, the charge density is peaked around radii of

<math display="block">r_{\ell} = \sqrt{\ell}\;r_B\; \; \; \ell=0,1,2, \ldots</math>

where <math>\ell</math> is the angular momentum [[quantum number]]. When <math> \ell = 1</math> we recover the classical situation in which the electron orbits the magnetic field at the [[Larmor radius]]. If currents of two angular momentum <math> \ell > 0 </math> and <math>\ell' \ge \ell </math> interact, and we assume the charge densities are delta functions at radius <math>r_{\ell}</math>, then the interaction energy is

<math display="block"> E =

\left( \frac{2 e^2}{L_B}\right) \int_0^{\infty} \frac{k\;dk \;}{k^2 + k_B^2 r_{\ell}^2}

\;\mathcal J_0 \left ( k \right) \;\mathcal J_0 \left ( \sqrt{\frac{\ell'}{\ell}} \;k \right) \;\mathcal J_0 \left ( k \frac{r_{12}}{r_{\ell}} \right).</math>

The interaction energy for <math> \ell = \ell'</math> is given in Figure 1 for various values of <math>k_B r_\ell</math>. The energy for two different values is given in Figure 2.

Moreover, we assume that there is only a time-like component <math>J^{0} </math> to the disturbance. In ordinary language, this means that there is a charge at the points of disturbance, but there are no electric currents.

====== Quasiparticles ======

If we follow the same procedure as we did with the Yukawa potential we find that

For large values of angular momentum, the energy can have local minima at distances other than zero and infinity. It can be numerically verified that the minima occur at

:<math>

<math display="block">r_{12} = r_{\ell \ell'} = \sqrt{\ell + \ell'} \; r_B.</math>

-{1\over 4} \int d^4x F_{\mu \nu}F^{\mu \nu}

= -{1\over 4}\int d^4x \left( \partial_{\mu} A_{\nu} - \partial_{\nu} A_{\mu} \right)\left( \partial^{\mu} A^{\nu} - \partial^{\nu} A^{\mu} \right)

</math>

This suggests that the pair of particles that are bound and separated by a distance <math>r_{\ell \ell'} </math> act as a single [[quasiparticle]] with angular momentum <math> \ell + \ell'</math>.

If we scale the lengths as <math> r_{\ell \ell'} </math>, then the interaction energy becomes

:<math>

<math display="block"> E = \frac{2 e^2}{L_B} \int_0^{\infty} \frac{k\,dk}{k^2 + k_B^2 r_{\ell \ell'}^2}

= {1\over 2}\int d^4x \; A_{\nu} \left( \partial^{2} A^{\nu} - \partial^{\nu} \partial_{\mu} A^{\mu} \right)

\;\mathcal J_0 \left ( \cos \theta \, k \right) \;\mathcal J_0 ( \sin \theta \,k ) \;\mathcal J_0{\left( k \frac{r_{12}}{r_{\ell \ell'}} \right)}</math>

= {1\over 2}\int d^4x \; A^{\mu} \left( \eta_{\mu \nu} \partial^{2} \right) A^{\nu}

where

,</math>

<math display="block">\tan \theta = \sqrt{\frac{\ell}{\ell'}}.</math>

The value of the <math> r_{12} </math> at which the energy is minimum, <math>r_{12} = r_{\ell \ell'} </math>, is independent of the ratio <math display="inline"> \tan \theta = \sqrt{{\ell}/{\ell'}}</math>. However the value of the energy at the minimum depends on the ratio. The lowest energy minimum occurs when

which implies

<math display="block"> \frac{\ell}{\ell'} = 1.</math>

When the ratio differs from 1, then the energy minimum is higher (Figure 3). Therefore, for even values of total momentum, the lowest energy occurs when (Figure 4)

:<math>

<math display="block"> \ell = \ell' = 1</math>

\eta_{\mu \alpha} \left ( \partial^2 + m^2\right ) D^{\alpha \nu}\left ( x-y \right ) = \delta_{\mu }^{ \nu} \delta^4\left ( x-y \right )

or

</math>

<math display="block"> \frac{\ell}{\ell^*} = \frac{1}{2} </math>

where the total angular momentum is written as

<math display="block"> \ell^* = \ell + \ell'. </math>

When the total angular momentum is odd, the minima cannot occur for <math> \ell = \ell' . </math> The lowest energy states for odd total angular momentum occur when

<math display="block"> \frac{\ell}{\ell^*} = \; \frac{\ell^*\pm 1}{2\ell^*}</math>

or

<math display="block">\frac{\ell}{\ell^*} = \frac{1}{3}, \frac{2}{5}, \frac{3}{7}, \text{etc.,} </math>

and

<math display="block">\frac{\ell}{\ell^*} = \frac{2}{3}, \frac{3}{5}, \frac{4}{7}, \text{etc.,} </math>

which also appear as series for the filling factor in the [[fractional quantum Hall effect]].

====== Charge density spread over a wave function ======

:<math>

D_{\mu \nu}\left ( k \right )\mid_{k_0=0}\; = \;

\eta_{\mu \nu}{1 \over - k^2 + m^2}

.</math>

The charge density is not actually concentrated in a delta function. The charge is spread over a wave function. In that case the electron density is<ref name="Ezewa"/>{{rp|189}}

This yields

<math display="block">

\frac{1}{\pi r_B^2 L_B}

\frac{1}{n!}

\left( \frac{r}{r_B} \right)^{2l}

\exp \left( -\frac{r^2}{r_B^2} \right).</math>

The interaction energy becomes

:<math>

<math display="block"> E =

D\left ( k \right )\mid_{k_0=0}\; = \;

\left( \frac{2 e^2}{L_B}\right) \int_0^{\infty} \frac{k\;dk \;}{k^2 + k_B^2 r_{B}^2}

{1 \over \vec k^2 + m^2}

\; M {\left ( \ell + 1, 1, -\frac{k^2}{4} \right)} \;M {\left ( \ell' + 1, 1, -\frac{k^2}{4} \right)} \;\mathcal J_0 {\left ( k \frac{r_{12}}{r_{B}} \right)}

</math>

where <math>M</math> is a [[confluent hypergeometric function]] or [[Kummer function]]. In obtaining the interaction energy we have used the integral (see {{slink|Common integrals in quantum field theory#Integration over a magnetic wave function}})

<math display="block">

for the [[timelike]] propagator and

\frac{2}{n!} \int_0^{\infty} dr \; r^{2n+1} e^{-r^2} J_0(kr)

= M\left( n+1, 1, -\frac{k^2}{4}\right). </math>

As with delta function charges, the value of <math>r_{12}</math> in which the energy is a local minimum only depends on the total angular momentum, not on the angular momenta of the individual currents. Also, as with the delta function charges, the energy at the minimum increases as the ratio of angular momenta varies from one. Therefore, the series

:<math>

<math display="block">\frac{\ell}{\ell^*} = \frac{1}{3}, \frac{2}{5}, \frac{3}{7}, \text{etc.,} </math>

E =

and

{a_1 a_2 \over 4 \pi r } \exp \left ( -m r \right )

<math display="block">\frac{\ell}{\ell^*} = \frac{2}{3}, \frac{3}{5}, \frac{4}{7}, \text{etc.,} </math>

</math>

appear as well in the case of charges spread by the wave function.

The [[Laughlin wavefunction]] is an [[ansatz]] for the quasiparticle wavefunction. If the expectation value of the interaction energy is taken over a [[Laughlin wavefunction]], these series are also preserved.

which has the opposite sign to the Yukawa case.

=== Magnetostatics ===

In the limit of zero [[photon]] mass, the Lagrangian reduces to the Lagrangian for [[electromagnetism]]

==== Darwin interaction in a vacuum ====

:<math>

E =

{a_1 a_2 \over 4 \pi r }

.</math>

Therefore the energy reduces to the potential energy for the Coulomb force and the coefficients <math>a_1 </math> and <math>a_2 </math> are proportional to the electric charge. Unlike the Yukawa case, like bodies, in this electrostatic case, repel each other.

==Magnetostatics==

===The Darwin interaction in a vacuum===

A charged moving particle can generate a magnetic field that affects the motion of another charged particle. The static version of this effect is called the [[Darwin Lagrangian | Darwin interaction]]. To calculate this, consider the electrical currents in space generated by a moving charge

:<math>

\vec J_1\left( \vec x \right) = a_1 \vec v_1 \delta^3 \left( \vec x - \vec x_1 \right)

</math>

A charged moving particle can generate a magnetic field that affects the motion of another charged particle. The static version of this effect is called the [[Darwin Lagrangian|Darwin interaction]]. To calculate this, consider the electrical currents in space generated by a moving charge

with a comparable expression for <math> \vec J_2 </math>.

<math display="block">\mathbf J_1{\left( \mathbf x \right)} = a_1 \mathbf v_1 \delta^3 {\left( \mathbf x - \mathbf x_1 \right)}</math>

with a comparable expression for <math> \mathbf J_2 </math>.

The Fourier transform of this current is

<math display="block">\mathbf J_1{\left( \mathbf k \right)} = a_1 \mathbf v_1 \exp\left( i \mathbf k \cdot \mathbf x_1 \right).</math>

The current can be decomposed into a transverse and a longitudinal part (see [[Helmholtz decomposition]]).

:<math>

<math display="block">\vecmathbf J_1{\left( \vecmathbf k \right)} = a_1 \vecleft[ 1 - \hat\mathbf k \hat\mathbf k \right ] \cdot \mathbf v_1 \exp\left( i \vecmathbf k \cdot \mathbf x_1 \right) + a_1 \left[ \hat\mathbf k \hat\mathbf k \right ] \cdot \mathbf v_1 \exp\left( i \mathbf k \cdot \vecmathbf x_1 \right).</math>

.</math>

The current can be decomposed into a transverse and and a longitudinal part (see [[Helmholtz decomposition]]).

:<math>

\vec J_1\left( \vec k \right) = a_1 \left[ 1 - \hat k \hat k \right ] \cdot \vec v_1 \exp\left( i \vec k \cdot \vec x_1 \right)

+ a_1 \left[ \hat k \hat k \right ] \cdot \vec v_1 \exp\left( i \vec k \cdot \vec x_1 \right)

.</math>

The hat indicates a [[unit vector]]. The last term disappears because

<math display="block">\mathbf k \cdot \mathbf J = -k_0 J^0 \to 0,</math>

:<math>

\vec k \cdot \vec J = -k_0 J^0 \rightarrow 0

,</math>

which results from charge conservation. Here <math>k_0 </math> vanishes because we are considering static forces.

With the current in this form the energy of interaction can be written

<math display="block"> E = a_1 a_2\int \frac{d^3\mathbf{k}}{(2 \pi )^3} \; \; D\left ( k \right )\mid_{k_0=0} \;

\mathbf v_1 \cdot \left[ 1 - \hat\mathbf k \hat\mathbf k \right ] \cdot \mathbf v_2 \; \exp\left ( i \mathbf k \cdot \left ( \mathbf x_1 - \mathbf x_2 \right ) \right ) .</math>

:<math> E =

a_1 a_2\int {d^3k \over (2 \pi )^3 } \; \; D\left ( k \right )\mid_{k_0=0} \;

\vec v_1 \cdot \left[ 1 - \hat k \hat k \right ] \cdot \vec v_2 \; \exp\left ( i \vec k \cdot \left ( x_1 - x_2 \right ) \right )

</math>.

The propagator equation for the Proca Lagrangian is

<math display="block"> \eta_{\mu \alpha} \left ( \partial^2 + m^2\right ) D^{\alpha \nu}\left ( x-y \right ) = \delta_{\mu}^{\nu} \delta^4\left ( x-y \right ).</math>

:<math>

\eta_{\mu \alpha} \left ( \partial^2 + m^2\right ) D^{\alpha \nu}\left ( x-y \right ) = \delta_{\mu }^{ \nu} \delta^4\left ( x-y \right )

.</math>

The [[spacelike]] solution is

<math display="block">D\left ( k \right )\mid_{k_0=0}\; = \; -\frac{1}{k^2 + m^2},</math>

:<math>

D\left ( k \right )\mid_{k_0=0}\; = \;

-{1 \over \vec k^2 + m^2}

,</math>

which yields

<math display="block"> E =

- a_1 a_2 \int \frac{d^3\mathbf{k}}{(2 \pi )^3} \; \;

:<math> E =

\frac{\mathbf v_1 \cdot \left[ 1 - \hat\mathbf k \hat\mathbf k \right ] \cdot \mathbf v_2}{k^2 + m^2} \; \exp\left ( i \mathbf k \cdot \left (\mathbf x_1 - \mathbf x_2 \right ) \right ),</math>

- a_1 a_2\int {d^3k \over (2 \pi )^3 } \; \;

where <math display="inline">k = |\mathbf k|</math>. The integral evaluates to (see {{slink|Common integrals in quantum field theory#Transverse potential with mass}})

{\vec v_1 \cdot \left[ 1 - \hat k \hat k \right ] \cdot \vec v_2 \over \vec k^2 + m^2 } \; \exp\left ( i \vec k \cdot \left ( x_1 - x_2 \right ) \right )

<math display="block"> E =

- \frac{1}{2} \frac{a_1 a_2}{4 \pi r} e^{ - m r} \left\{

\frac{2}{\left( mr \right)^2} \left( e^{mr} -1 \right) - \frac{2}{mr} \right \}

\mathbf v_1 \cdot \left[ 1 + {\hat\mathbf r} {\hat\mathbf r}\right]\cdot \mathbf v_2

</math>

which evaluates to (see [[Common integrals in quantum field theory]])

:<math> E =

- {1\over 2} {a_1 a_2 \over 4 \pi r } e^{ - m r } \left\{

{2 \over \left( mr \right)^2 } \left( e^{mr} -1 \right) - {2\over mr} \right \}

\vec v_1 \cdot \left[ 1 + {\hat r} {\hat r}\right]\cdot \vec v_2

</math>

which reduces to

<math display="block"> E = - \frac{1}{2} \frac{a_1 a_2}{4 \pi r} \mathbf v_1 \cdot \left[ 1 + {\hat\mathbf r} {\hat\mathbf r}\right] \cdot \mathbf v_2 </math>

in the limit of small {{mvar|m}}. The interaction energy is the negative of the interaction Lagrangian. For two like particles traveling in the same direction, the interaction is attractive, which is the opposite of the Coulomb interaction.

==== Darwin interaction in plasma ====

:<math> E =

- {1\over 2} {a_1 a_2 \over 4 \pi r }

\vec v_1 \cdot \left[ 1 + {\hat r} {\hat r}\right]\cdot \vec v_2

</math>

In a plasma, the [[dispersion relation]] for an [[electromagnetic wave]] is<ref name="Chen"/>{{rp|pp=100–103}} (<math>c = 1</math>)

in the limit of small m. The interaction energy is the negative of the interaction Lagrangian. For two like part particles traveling in the same direction, the interaction is attractive, which is the opposite of the Coulomb interaction.

<math display="block">k_0^2 = \omega_p^2 + k^2,</math>

which implies

<math display="block">D\left ( k \right )\mid_{k_0=0}\; = \; -\frac{1}{ k^2 + \omega_p^2}.</math>

Here <math>\omega_p</math> is the [[plasma frequency]]. The interaction energy is therefore

===Interaction between current loops in a simple plasma or electron gas===

<math display="block"> E =

- \frac{1}{2} \frac{a_1 a_2}{4 \pi r}

\mathbf v_1 \cdot \left[ 1 + {\hat\mathbf r} {\hat\mathbf r}\right]\cdot \mathbf v_2

\; e^{ - \omega_p r } \left\{

\frac{2}{\left( \omega_p r \right)^2} \left( e^{\omega_p r} -1 \right) - \frac{2}{\omega_p r} \right \}.</math>

====The Magnetic interaction energybetween current loops in a simple plasma or electron gas ====

===== Interaction energy =====

Consider a tube of current rotating in a magnetic field embedded in a simple [[Plasma (physics) | plasma]] or [[electron gas]]. The current, which lies in the plane perpendicular to the magnetic field, is defined as

:<math>

\vec J_1\left( \vec x \right) = a_1 v_1 {1\over 2 \pi r} \; \delta^ 2 \left( r - r_{B1} \right)\;

\left( {\hat b \times \hat r }\right)

</math>

Consider a tube of current rotating in a magnetic field embedded in a simple [[Plasma (physics)|plasma]] or electron gas. The current, which lies in the plane perpendicular to the magnetic field, is defined as

<math display="block">\mathbf J_1( \mathbf x) = a_1 v_1 \frac{1}{2 \pi r L_B} \; \delta^ 2 {\left( r - r_{B1} \right)}

\left( \hat\mathbf b \times \hat\mathbf r \right)</math>

where

<math display="block"> r_{B1} = \frac{\sqrt{4 \pi}m_1 v_1}{a_1 B}</math>

and <math>\hat\mathbf b</math> is the unit vector in the direction of the magnetic field. Here <math>L_B</math> indicates the dimension of the material in the direction of the magnetic field. The transverse current, perpendicular to the [[wave vector]], drives the [[transverse wave]].

:<math>

r_{B1}

=

{m_1v_1\over a_1 B}

</math>

and <math>

\hat b

</math> is the unit vector in the direction of the magnetic field.

The energy of interaction is

<math display="block"> E = \left( \frac{a_1\, a_2}{2 \pi L_B}\right) v_1\, v_2\, \int_0^{\infty} {k\;dk \;} D\left( k \right) \mid_{k_0=k_B=0} \mathcal J_1 {\left ( kr_{B1} \right)} \mathcal J_1 {\left ( kr_{B2} \right)} \mathcal J_0 {\left ( kr_{12} \right)}</math>

where <math>r_{12}</math> is the distance between the centers of the current loops and <math> \mathcal J_n ( x )</math> is a [[Bessel function]] of the first kind. In obtaining the interaction energy we made use of the integrals

<math display="block">

\int_0^{2 \pi} \frac{d\varphi}{2 \pi} \exp\left( i p \cos\left( \varphi \right) \right)

= \mathcal J_0 \left( p \right)</math>

and

<math display="block"> \int_0^{2 \pi} \frac{d\varphi}{2 \pi} \cos\left( \varphi \right) \exp\left( i p \cos\left( \varphi \right) \right)

= i\mathcal J_1 \left( p \right) . </math>

See ''{{slink|Common integrals in quantum field theory#Angular integration in cylindrical coordinates}}''.

:<math>

E=

L_B \, a_1\, a_2\, v_1\, v_2\, \int {d^2k \over 2 \pi} D\left( k \right) \mid_{k_0=k_B=0}

\mathcal J_1 \left ( kr_{B1} \right) \mathcal J_1 \left ( kr_{B2} \right)

</math>

A current in a plasma confined to the plane perpendicular to the magnetic field generates an [[Electromagnetic electron wave#X wave|extraordinary wave]].<ref name="Chen"/>{{rp|pp=110–112}} This wave generates [[Hall current]]s that interact and modify the electromagnetic field. The [[dispersion relation]] for extraordinary waves is<ref name="Chen"/>{{rp|112}}

<math display="block"> -k_0^2 + k^2 + \omega_p^2 \frac{k_0^2 - \omega_p^2}{k_0^2- \omega_H^2} =0,</math>

which gives for the propagator

<math display="block"> D\left( k \right) \mid_{k_0=k_B=0}\;= \;-\left( \frac{1}{ k^2 + k_X^2}\right)</math>

where

<math display="block">k_X \equiv \frac{\omega_p^2}{\omega_H}</math>

in analogy with the Darwin propagator. Here, the upper hybrid frequency is given by

:<math>

<math display="block"> \omega_H^2 = \omega_p^2 + \omega_c^2,</math>

\mathcal J_1 \left ( kr_{B1} \right)

</math>

is a [[Bessel function]] of the first kind. Here <math>L_B</math> indicates the dimension of the material in the direction of the magnetic field. In obtaining the interaction energy we made use of the integral

:<math>

\int_0^{2 \pi} {d\varphi \over 2 \pi} \cos\left( \varphi \right) \exp\left( i p \cos\left( \varphi \right) \right)

=

i\mathcal J_1 \left( p \right)

. </math>

See [[Common integrals in quantum field theory]].

A current in a plasma confined to the plane perpendicular to the magnetic field generates an extraordinary wave.<ref> {{cite book | author=F. F. Chen | title=Introduction to Plasma Physics| publisher= Plenum Press| year=1974 | id=ISBN 0-306-30755-3}} pp. 110-112 </ref> This wave generates [[Hall current]]s that interact and modify the electromagnetic field. The dispersion relation for extraordinary waves is

:<math>

-k_0^2 +\vec k^2 + {\omega_p^4\over \omega_H^2} =0

,</math>

which gives for the propagator

:<math>

D\left( k \right) \mid_{k_0=k_B=0}

=

{1\over \vec k^2 + {\omega_p^4\over \omega_H^2}}

.</math>

Here, the upper hybrid frequency is given by

:<math>

\omega_H^2 = \omega_p^2 + \omega_c^2

,</math>

the [[cyclotron frequency]] is given by ([[Gaussian units]])

<math display="block"> \omega_c = \frac{e B}{m c},</math>

:<math>

\omega_c = {e B \over m c}

,</math>

and the [[plasma frequency]] ([[Gaussian units]])

<math display="block"> \omega_p^2 = \frac{4\pi n e^2}{m}.</math>

Here {{mvar|n}} is the electron density, {{math|''e''}} is the magnitude of the electron charge, and {{mvar|m}} is the electron mass.

:<math>

\omega_p^2 = {4\pi n e^2 \over m}

.</math>

The interaction energy becomes, for like currents,

Here n is the electron density, e is the magnitude of the electron charge, and m is the electron mass.

<math display="block"> E =

- \left( \frac{a^2}{2 \pi L_B}\right) v^2\, \int_0^{\infty} \frac{k\;dk}{ k^2 + k_X^2}

\mathcal J_1^2 \left ( kr_{B} \right) \mathcal J_0 \left ( kr_{12} \right)</math>

===== Limit of small distance between current loops =====

The interaction energy becomes

:<math>

E=

L_B \, a^2 v^2\,

I_1 \left( k_Xr_B \right)K_1 \left( k_Xr_B \right)

</math>

In the limit that the distance between current loops is small,

<math display="block"> E = - E_0 \; I_1 {\left( \mu \right)} K_1 {\left( \mu \right)}</math>

where

<math display="block"> E_0 = \left( \frac{a^2}{2 \pi L_B}\right) v^2</math>

and

<math display="block">\mu =\frac{\omega_p^2 r_B}{\omega_H}= k_X \;r_B</math>

and {{math|''I''}} and {{math|''K''}} are modified Bessel functions. we have assumed that the two currents have the same charge and speed.

We have made use of the integral (see {{slink|Common integrals in quantum field theory#Integration of the cylindrical propagator with mass}})

:<math>

<math display="block">\int_o^{\infty} \frac{k\; dk}{k^2 +m^2} \mathcal J_1^2 \left( kr \right)

k_X =

= I_1 \left( mr \right)K_1 \left( mr \right) . </math>

{\omega_p^2\over \omega_H}

</math>

For small {{math|''mr''}} the integral becomes

and I and K are modified Bessel functions. we have assumed that the two currents have the same charge and speed.

<math display="block">

I_1 {\left( mr \right)} K_1 {\left( mr \right)}

\to

\frac{1}{2}\left[ 1- \frac{1}{8}\left( mr \right)^2 \right] . </math>

For large {{math|''mr''}} the integral becomes

We have made use of the integral (see [[Common integrals in quantum field theory]])

<math display="block">

:<math>

\int_o^{\infty} {k\; dk \over k^2 +m^2} \mathcal J_1^2 \left( kr \right)

=

I_1 \left( mr \right)K_1 \left( mr \right)

. </math>

For small m the integral becomes

:<math>

\int_o^{\infty} {k\; dk \over k^2 +m^2} \mathcal J_1^2 \left( kr \right)

\rightarrow

\frac{1}{2}\;\left( \frac{1}{mr}\right) . </math>

{1\over 2 }

. </math>

===== Relation to the quantum Hall effect =====

For large r the integral becomes

:<math>

\int_o^{\infty} {k\; dk \over k^2 +m^2} \mathcal J_1^2 \left( kr \right)

\rightarrow

{1\over 2}\;\left( {1\over mr}\right)

. </math>

The interaction between the current loops is repulsive. Moreover the screening is not monotonic. It has a maximum.

====Relation to the quantum Hall effect====

The screening [[wavenumber]] can be written [[Gaussian units]]

:<math>

k_X =

{\omega_p^2\over \omega_H}

= \left( {2e^2\over L_B \hbar c }\right) \nu

= \left( {2 \alpha\over L_B }\right) \nu

</math>

The screening [[wavenumber]] can be written ([[Gaussian units]])

<math display="block"> \mu = \frac{\omega_p^2 r_B}{\omega_H c}

= \left( \frac{2e^2r_B}{L_B \hbar c}\right) \frac{\nu}{\sqrt{1+\frac{\omega_p^2}{\omega_c^2}}}

= 2 \alpha \left( \frac{r_B}{L_B}\right) \left(\frac{1}{\sqrt{1+\frac{\omega_p^2}{\omega_c^2}}}\right) \nu</math>

where <math>\alpha</math> is the [[fine-structure constant]] and the filling factor is

<math display="block"> \nu = \frac{2\pi N \hbar c}{eBA}</math>

and {{mvar|N}} is the number of electrons in the material and {{mvar|A}} is the area of the material perpendicular to the magnetic field. This parameter is important in the [[quantum Hall effect]] and the [[fractional quantum Hall effect]]. The filling factor is the fraction of occupied [[Landau levels|Landau states]] at the ground state energy.

For cases of interest in the quantum Hall effect, <math>\mu</math> is small. In that case the interaction energy is

:<math>

<math display="block"> E = - \frac{E_0}{2} \left[ 1- \frac{1}{8}\mu^2\right]</math>

\nu =

where ([[Gaussian units]])

{2\pi N \hbar c \over eBA}

<math display="block"> E_0 = {4\pi}\frac{e^2}{L_B}\frac{v^2}{c^2}

</math>

= {8\pi}\frac{e^2}{L_B}\left( \frac{\hbar \omega_c}{m c^2}\right)</math>

is the interaction energy for zero filling factor. We have set the classical kinetic energy to the quantum energy

and N is the number of electrons in the material and A is the area of the material perpendicular to the magnetic field. This parameter is important in the [[quantum Hall effect]] and the [[fractional quantum Hall effect]]. The filling factor is the fraction of occupied [[Landau levels | Landau states]] at the ground state energy.

<math display="block">\frac{1}{2} m v^2 = \hbar \omega_c.</math>

==Gravitation==

The Lagrangian for the gravitational field, which we will not write down explicitly, is [[Spin (physics) | spin]]-2. The disturbance is generated by the [[stress-energy tensor]] <math> T^{\mu \nu} </math>. If the disturbances are at rest, then the only component of the stress-energy tensor that survives is the <math> 00 </math> component. If we use the same trick of giving the [[graviton]] some mass and then taking the mass to zero at the end of the calculation the propagator becomes

:<math>

D\left ( k \right )\mid_{k_0=0}\; = \; - {4\over 3}

{1 \over \vec k^2 + m^2}

</math>

=== Gravitation ===

A gravitational disturbance is generated by the [[stress–energy tensor]] <math> T^{\mu \nu} </math>; consequently, the Lagrangian for the gravitational field is [[Spin (physics)|spin]]-2. If the disturbances are at rest, then the only component of the stress–energy tensor that persists is the <math> 00 </math> component. If we use the same trick of giving the [[graviton]] some mass and then taking the mass to zero at the end of the calculation the propagator becomes

<math display="block">D\left ( k \right )\mid_{k_0=0}\; = \; - \frac{4}{3} \frac{1}{ k^2 + m^2}</math>

and

<math display="block">E = -\frac{4}{3}\frac{a_1 a_2}{4 \pi r} \exp \left ( -m r \right ),</math>

which is once again attractive rather than repulsive. The coefficients are proportional to the masses of the disturbances. In the limit of small graviton mass, we recover the inverse-square behavior of Newton's Law.<ref name="Zee"/>{{rp|pp=32–37}}

Unlike the electrostatic case, however, taking the small-mass limit of the boson does not yield the correct result. A more rigorous treatment yields a factor of one in the energy rather than 4/3.<ref name="Zee"/>{{rp|35}}

:<math>

E =

-{4\over 3}{a_1 a_2 \over 4 \pi r } \exp \left ( -m r \right )

</math>,

which is once again attractive rather than repulsive. The coefficients are proportional to the masses of the disturbances. In the limit of small graviton mass, we recover the inverse-square behavior of Newton's Law.<ref> Zee, pp. 32-37 </ref>

Unlike the electrostatic case, however, taking the small-mass limit of the boson does not yield the correct result. A more rigorous treatment yields a factor of one in the energy rather than 4/3.<ref> Zee, p. 35 </ref>

== References ==

{{reflist}}

{{Reflist}}<!--added under references heading by script-assisted edit-->

[[Category:Quantum field theory]]