Relativistic Lagrangian mechanics

Short description: Mathematical formulation of special and general relativity

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In theoretical physics, relativistic Lagrangian mechanics is Lagrangian mechanics applied in the context of special relativity and general relativity.

## Contents

* 1 Introduction
* 2 Lagrangian formulation in special relativity
* 2.1 Coordinate formulation
* 2.2 Covariant formulation
* 3 Examples in special relativity
* 3.1 Special relativistic 1d free particle
* 3.2 Special relativistic 1d harmonic oscillator
* 3.3 Special relativistic constant force
* 3.4 Special relativistic test particle in an electromagnetic field
* 4 Lagrangian formulation in general relativity
* 5 Examples in general relativity
* 5.1 General relativistic test particle in an electromagnetic field
* 6 See also
* 7 Footnotes
* 8 Citations
* 9 References

## Introduction

The relativistic Lagrangian can be derived in relativistic mechanics to be of the form:

L=−m0c2γ(𝐫˙)−V(𝐫,𝐫˙,t).

Although, unlike non-relativistic mechanics, the relativistic Lagrangian is not expressed as difference of kinetic energy with potential energy, the relativistic Hamiltonian corresponds to total energy in a similar manner but without including rest energy. The form of the Lagrangian also makes the relativistic action functional proportional to the proper time of the path in spacetime.

In covariant form, the Lagrangian is taken to be:[1][2][3]

Λ=e2[e−2gαβdxαdσdxβdσ−m2c2],

where σ is an affine parameter which parametrizes the spacetime curve.

## Lagrangian formulation in special relativity

Lagrangian mechanics can be formulated in special relativity as follows. Consider one particle (N particles are considered later).

### Coordinate formulation

If a system is described by a Lagrangian L, the Euler–Lagrange equations

ddt∂L∂𝐫˙=∂L∂𝐫

retain their form in special relativity, provided the Lagrangian generates equations of motion consistent with special relativity. Here r = (x, y, z) is the position vector of the particle as measured in some lab frame where Cartesian coordinates are used for simplicity, and

𝐯=𝐫˙=d𝐫dt=(dxdt,dydt,dzdt)

is the coordinate velocity, the derivative of position r with respect to coordinate time t. (Throughout this article, overdots are with respect to coordinate time, not proper time.) It is possible to transform the position coordinates to generalized coordinates exactly as in non-relativistic mechanics, r = r(q, t). Taking the total differential of r obtains the transformation of velocity v to the generalized coordinates, generalized velocities, and coordinate time

𝐯=∑j=1n∂𝐫∂qjq˙j+∂𝐫∂t,q˙j=dqjdt

remains the same. However, the energy of a moving particle is different from non-relativistic mechanics. It is instructive to look at the total relativistic energy of a free test particle. An observer in the lab frame defines events by coordinates r and coordinate time t, and measures the particle to have coordinate velocity v = dr/dt. By contrast, an observer moving with the particle will record a different time, this is the proper time, τ. Expanding in a power series, the first term is the particle's rest energy, plus its non-relativistic kinetic energy, followed by higher order relativistic corrections;

E=m0c2dtdτ=m0c21−𝐫˙2(t)c2=m0c2+12m0𝐫˙2(t)+38m0𝐫˙4(t)c2+⋯,

where c is the speed of light in vacuum. The differentials in t and τ are related by the Lorentz factor γ,[nb 1]

dt=γ(𝐫˙)dτ,γ(𝐫˙)=11−𝐫˙2c2,𝐫˙=d𝐫dt,𝐫˙2(t)=𝐫˙(t)⋅𝐫˙(t).

where · is the dot product. The relativistic kinetic energy for an uncharged particle of rest mass m0 is

T=(γ(𝐫˙)−1)m0c2

and we may naïvely guess the relativistic Lagrangian for a particle to be this relativistic kinetic energy minus the potential energy. However, even for a free particle for which V = 0, this is wrong. Following the non-relativistic approach, we expect the derivative of this seemingly correct Lagrangian with respect to the velocity to be the relativistic momentum, which it is not.

The definition of a generalized momentum can be retained, and the advantageous connection between cyclic coordinates and conserved quantities will continue to apply. The momenta can be used to "reverse-engineer" the Lagrangian. For the case of the free massive particle, in Cartesian coordinates, the x component of relativistic momentum is

px=∂L∂x˙=γ(𝐫˙)m0x˙,

and similarly for the y and z components. Integrating this equation with respect to dx/dt gives

L=−m0c2γ(𝐫˙)+X(y˙,z˙),

where X is an arbitrary function of dy/dt and dz/dt from the integration. Integrating py and pz obtains similarly

L=−m0c2γ(𝐫˙)+Y(x˙,z˙),L=−m0c2γ(𝐫˙)+Z(x˙,y˙),

where Y and Z are arbitrary functions of their indicated variables. Since the functions X, Y, Z are arbitrary, without loss of generality we can conclude the common solution to these integrals, a possible Lagrangian that will correctly generate all the components of relativistic momentum, is

L=−m0c2γ(𝐫˙),

where X = Y = Z = 0.

Alternatively, since we wish to build a Lagrangian out of relativistically invariant quantities, take the action as proportional to the integral of the Lorentz invariant line element in spacetime, the length of the particle's world line between proper times τ1 and τ2,[nb 1]

S=ε∫τ1τ2dτ=ε∫t1t2dtγ(𝐫˙),L=εγ(𝐫˙)=ε1−𝐫˙2c2,

where ε is a constant to be found, and after converting the proper time of the particle to the coordinate time as measured in the lab frame, the integrand is the Lagrangian by definition. The momentum must be the relativistic momentum,

𝐩=∂L∂𝐫˙=(−εc2)γ(𝐫˙)𝐫˙=m0γ(𝐫˙)𝐫˙,

which requires ε = −m0c2, in agreement with the previously obtained Lagrangian.

Either way, the position vector r is absent from the Lagrangian and therefore cyclic, so the Euler–Lagrange equations are consistent with the constancy of relativistic momentum,

ddt∂L∂𝐫˙=∂L∂𝐫⇒ddt(m0γ(𝐫˙)𝐫˙)=0,

which must be the case for a free particle. Also, expanding the relativistic free particle Lagrangian in a power series to first order in (v/c)2,

L=−m0c2[1+12(−𝐫˙2c2)+⋯]≈−m0c2+m02𝐫˙2,

in the non-relativistic limit when v is small, the higher order terms not shown are negligible, and the Lagrangian is the non-relativistic kinetic energy as it should be. The remaining term is the negative of the particle's rest energy, a constant term which can be ignored in the Lagrangian.

For the case of an interacting particle subject to a potential V, which may be non-conservative, it is possible for a number of interesting cases to simply subtract this potential from the free particle Lagrangian,

L=−m0c2γ(𝐫˙)−V(𝐫,𝐫˙,t).

and the Euler–Lagrange equations lead to the relativistic version of Newton's second law. The derivative of relativistic momentum with respect to the time coordinate is equal to the force acting on the particle:

𝐅=ddt∂V∂𝐫˙−∂V∂𝐫=ddt(m0γ(𝐫˙)𝐫˙),

assuming the potential V can generate the corresponding force F in this way. If the potential cannot obtain the force as shown, then the Lagrangian would need modification to obtain the correct equations of motion.

Although this has been shown by taking Cartesian coordinates, it follows due to invariance of Euler Lagrange equations, that it is also satisfied in any arbitrary co-ordinate system as it physically corresponds to action minimization being independent of the co-ordinate system used to describe it. In a similar manner, several properties in Lagrangian mechanics are preserved whenever they are also independent of the specific form of the Lagrangian or the laws of motion governing the particles. For example, it is also true that if the Lagrangian is explicitly independent of time and the potential V(r) independent of velocities, then the total relativistic energy

E=∂L∂𝐫˙⋅𝐫˙−L=γ(𝐫˙)m0c2+V(𝐫)

is conserved, although the identification is less obvious since the first term is the relativistic energy of the particle which includes the rest mass of the particle, not merely the relativistic kinetic energy. Also, the argument for homogeneous functions does not apply to relativistic Lagrangians.

The extension to N particles is straightforward, the relativistic Lagrangian is just a sum of the "free particle" terms, minus the potential energy of their interaction;

L=−c2∑k=1Nm0kγ(𝐫˙k)−V(𝐫1,𝐫2,…,𝐫˙1,𝐫˙2,…,t),

where all the positions and velocities are measured in the same lab frame, including the time.

The advantage of this coordinate formulation is that it can be applied to a variety of systems, including multiparticle systems. The disadvantage is that some lab frame has been singled out as a preferred frame, and none of the equations are manifestly covariant (in other words, they do not take the same form in all frames of reference). For an observer moving relative to the lab frame, everything must be recalculated; the position r, the momentum p, total energy E, potential energy, etc. In particular, if this other observer moves with constant relative velocity then Lorentz transformations must be used. However, the action will remain the same since it is Lorentz invariant by construction.

A seemingly different but completely equivalent form of the Lagrangian for a free massive particle, which will readily extend to general relativity as shown below, can be obtained by inserting[nb 1]

dτ=1cηαβdxαdtdxβdtdt,

into the Lorentz invariant action so that

S=ε∫t1t21cηαβdxαdtdxβdtdt⇒L=εcηαβdxαdtdxβdt

where ε = −m0c2 is retained for simplicity. Although the line element and action are Lorentz invariant, the Lagrangian is not, because it has explicit dependence on the lab coordinate time. Still, the equations of motion follow from Hamilton's principle

δS=0.

Since the action is proportional to the length of the particle's worldline (in other words its trajectory in spacetime), this route illustrates that finding the stationary action is asking to find the trajectory of shortest or largest length in spacetime. Correspondingly, the equations of motion of the particle are akin to the equations describing the trajectories of shortest or largest length in spacetime, geodesics.

For the case of an interacting particle in a potential V, the Lagrangian is still

L=εcηαβdxαdtdxβdt−V,

which can also extend to many particles as shown above, each particle has its own set of position coordinates to define its position.

### Covariant formulation

In the covariant formulation, time is placed on equal footing with space, so the coordinate time as measured in some frame is part of the configuration space alongside the spatial coordinates (and other generalized coordinates).[4] For a particle, either massless or massive, the Lorentz invariant action is (abusing notation)[5]

S=∫σ1σ2Λ(xν(σ),uν(σ),σ)dσ,

where lower and upper indices are used according to covariance and contravariance of vectors, σ is an affine parameter, and uμ = dxμ/dσ is the four-velocity of the particle.

For massive particles, σ can be the arc length s, or proper time τ, along the particle's world line,

ds2=c2dτ2=gαβdxαdxβ.

For massless particles, it cannot because the proper time of a massless particle is always zero;

gαβdxαdxβ=0.

For a free particle, the Lagrangian has the form[1][2]

Λ=gαβdxαdσdxβdσ

where the irrelevant factor of 1/2 is allowed to be scaled away by the scaling property of Lagrangians. No inclusion of mass is necessary since this also applies to massless particles. The Euler–Lagrange equations in the spacetime coordinates are

ddσ∂Λ∂uα−∂Λ∂xα=d2xαdσ2+Γβγαdxβdσdxγdσ=0,

which is the geodesic equation for affinely parameterized geodesics in spacetime. In other words, the free particle follows geodesics. Geodesics for massless particles are called "null geodesics", since they lie in a "light cone" or "null cone" of spacetime (the null comes about because their inner product via the metric is equal to 0), massive particles follow "timelike geodesics", and hypothetical particles that travel faster than light known as tachyons follow "spacelike geodesics".

This manifestly covariant formulation does not extend to an N-particle system, since then the affine parameter of any one particle cannot be defined as a common parameter for all the other particles.

## Examples in special relativity

### Special relativistic 1d free particle

For a 1d relativistic free particle, the Lagrangian is[6]

L=−m0c21−x˙2(t)c2.

This results in the following equation of motion:

m0x¨1(1−x˙2c2)32=0.

Derivation

ddt∂L∂x˙−∂L∂x=0
⇒ddt∂∂x˙(−m0c21−x˙2c2)=0
⇒−m0c2ddt(−x˙c211−x˙2c2)=0
⇒m0ddtx˙1−x˙2c2=0
⇒m0(x¨1−x˙2c2+x˙2c2x¨(1−x˙2c2)32)=0
⇒m0x¨(11−x˙2c2+x˙2c2(1−x˙2c2)32)=0
⇒mx¨(1−x˙2c2(1−x˙2c2)32+x˙2c2(1−x˙2c2)32)=0
⇒mx¨1(1−x˙2c2)32=0

### Special relativistic 1d harmonic oscillator

For a 1d relativistic simple harmonic oscillator, the Lagrangian is[7][8]

L=−mc21−x˙2(t)c2−k2x2.

where k is the spring constant.

### Special relativistic constant force

For a particle under a constant force, the Lagrangian is[9]

L=−mc21−x˙2(t)c2−mgx,

where g is the force per unit mass.

This results in the following equation of motion:

1(1−x˙2c2)32x¨=−g.

Which, given initial conditions of

x(t=0)=x0x˙(t=0)=v0

results in the position of the particle as a function of time being

x(t)=x0+c2g[11−v02c2−11−v02c2+g2t2c2−2v0gtc21−v02c2].

Derivation of equation of motion

ddt∂L∂x˙−∂L∂x=0
⇒ddt∂∂x˙(−mc21−x˙2c2)−∂∂x(−mgx)=0
⇒−mc2ddt(−x˙c211−x˙2c2)=−mg
⇒ddtx˙1−x˙2c2=−g
⇒x¨1−x˙2c2+x˙2c2x¨(1−x˙2c2)32=−g
⇒(11−x˙2c2+x˙2c2(1−x˙2c2)32)x¨=−g
⇒(1−x˙2c2(1−x˙2c2)32+x˙2c2(1−x˙2c2)32)x¨=−g
⇒1(1−x˙2c2)32x¨=−g

Derivation of solution

From Euler-Lagrange equation we have

ddtx˙1−x˙2c2=−g

Integrating with respect to time:

⇒x˙1−x˙2c2=−gt+A

Where A is an undetermined constant.

Solving this equation for x˙:

x˙(t)=A−gt1+(A−gt)2c2

Then, using x˙(t=0)=v0,

A1+A2c2=v0

This implies that

A=v01−v02c2≡γ0v0

Thus

x˙(t)=γ0v0−gt1+(γ0v0−gt)2c2

Note that for a large value of gt, we have 1<<(γ0v0−gt)2⇒1+(γ0v0−gt)2c2≈γ0v0−gtc and see that x˙(t->∞)=c.

Then, given that

dxdt=γ0v0−gt1+(γ0v0−gt)2c2

we have

x(t)=∫γ0v0−gt1+(γ0v0−gt)2c2dt

Picking u≡1+(γ0v0−gt)2c2⇒du=−2gc2(γ0v0−gt)dt, we have

x(u)=−c22g∫1udu

Then note that ∫u−12du=2u12+C for some undetermined constant C so that

x(u)=−c2gu+B

Using u=1+(γ0v0−gt)2c2:

x(t)=−c2g1+(γ0v0−gt)2c2+B

Recalling that x(t=0)=x0:

B−c2g1+(γ0v0)2c2=x0

Since γ0=11−v02c2, we have 1+(γ0v0)2c2=γ02 and come to

B=x0+c2gγ0

Therefore

x(t)=x0+c2g[γ0−1+(γ0v0−gt)2c2]

Plugging in the definition of γ0=11−v02c2 and using γ02=1+(γ0v0)2c2 brings the solution to

x(t)=x0+c2g[11−v02c2−11−v02c2+g2t2c2−2v0gtc21−v02c2]

The Newtonian limit of this solution can be obtained by making the following approximations, which are equivalent to stating that x˙(t)<<c:

γ0=11−v02c2=1v0−gtc<<1

This simplifies the solution to

x(t)≈x0+c2g[1−1+g2t2c2−2v0gtc2]

Then using the approximation that α<<1⇒1+α≈1+α2:

x(t)≈x0+c2g[1−1−g2t22c2+v0gtc2]

Which simplifies to

x(t)≈−12gt2+v0t+x0

This is expected solution to the equation of motion to the Newtonian particle subject to a constant force: x¨=−g

### Special relativistic test particle in an electromagnetic field

Main article: Physics:Covariant formulation of classical electromagnetism

In special relativity, the Lagrangian of a massive charged test particle in an electromagnetic field modifies to[10][11]

L=−mc21−v2c2−qϕ+q𝐫˙⋅𝐀.

The Lagrangian equations in r lead to the Lorentz force law, in terms of the relativistic momentum

ddt(m𝐫˙1−v2c2)=q𝐄+q𝐫˙×𝐁.

In the language of four-vectors and tensor index notation, the Lagrangian takes the form

L(τ)=12muμ(τ)uμ(τ)+quμ(τ)Aμ(x),

where uμ = dxμ/dτ is the four-velocity of the test particle, and Aμ the electromagnetic four-potential.

The Euler–Lagrange equations are (notice the total derivative with respect to proper time instead of coordinate time)

∂L∂xν−ddτ∂L∂uν=0

obtains

quμ∂Aμ∂xν=ddτ(muν+qAν).

Under the total derivative with respect to proper time, the first term is the relativistic momentum, the second term is

dAνdτ=∂Aν∂xμdxμdτ=∂Aν∂xμuμ,

then rearranging, and using the definition of the antisymmetric electromagnetic tensor, gives the covariant form of the Lorentz force law in the more familiar form,

ddτ(muν)=quμFνμ,Fνμ=∂Aμ∂xν−∂Aν∂xμ.

## Lagrangian formulation in general relativity

The Lagrangian is that of a single particle plus an interaction term LI

L=−mc2dτdt+LI.

Varying this with respect to the position of the particle xα as a function of time t gives

δL=mdt2dτδ(gμνdxμdtdxνdt)+δLI=mdt2dτ(gμν,αδxαdxμdtdxνdt+2gανdδxαdtdxνdt)+∂LI∂xαδxα+∂LI∂dxαdtdδxαdt=12mgμν,αδxαdxμdτdxνdt−ddt(mgανdxνdτ)δxα+∂LI∂xαδxα−ddt(∂LI∂dxαdt)δxα+d(⋯)dt.

This gives the equation of motion

0=12mgμν,αdxμdτdxνdt−ddt(mgανdxνdτ)+fα

where

fα=∂LI∂xα−ddt(∂LI∂dxαdt)

is the non-gravitational force on the particle. (For m to be independent of time, we must have fαdxα/dt = 0.)

Rearranging gets the force equation

ddt(mdxνdτ)=−mΓμσνdxμdτdxσdt+gναfα

where Γ is the Christoffel symbol, which describes the gravitational field.

If we let

pν=mdxνdτ

be the (kinetic) linear momentum for a particle with mass, then

dpνdt=−Γμσνpμdxσdt+gναfα

and

dxνdt=pνp0

hold even for a massless particle.

## Examples in general relativity

### General relativistic test particle in an electromagnetic field

In general relativity, the first term generalizes (includes) both the classical kinetic energy and the interaction with the gravitational field. For a charged particle in an electromagnetic field, the Lagrangian is given by

L(x,x˙)=−mc2−c−2gμν(x(τ))dxμ(τ)dτdxν(τ)dτ+qdxμ(τ)dτAμ(x(τ)).

If the four spacetime coordinates xμ are given in arbitrary units (i.e. unitless), then gμν is the rank 2 symmetric metric tensor, which is also the gravitational potential. Also, Aμ is the electromagnetic 4-vector potential.

There exists an equivalent formulation of the relativistic Lagrangian, which has two advantages:

* it allows for a generalization to massless particles and tachyons;
* it is based on an energy functional instead of a length functional, such that it does not contain a square root.

In this alternative formulation, the Lagrangian is given by

L(x,x˙,e)=12e(λ)gμν(x(λ))dxμ(λ)dλdxν(λ)dλ−e(λ)m2c22+qAμ(x(λ))dxμ(λ)dλ,

where λ is an arbitrary affine parameter and e is an auxiliary parameter that can be viewed as an einbein field along the worldline. In the original Lagrangian with the square root the energy-momentum relation appears as a primary constraint that is also a first class constraint. In this reformulation this is no longer the case. Instead, the energy-momentum relation appears as the equation of motion for the auxiliary field e. Therefore, the constraint is now a secondary constraint that is still a first class constraint, reflecting the invariance of the action under reparameterization of the affine parameter λ. After the equation of motion has been derived, one must gauge fix the auxiliary field e. The standard gauge choice is as follows:

* If m2>0, one fixes e=|m|−1. This choice automatically fixes λ=τ, i.e. the affine parameter is fixed to be the proper time.
* If m2<0, one fixes e=|mc|−1. This choice automatically fixes λ=s, i.e. the affine parameter is fixed to be the proper length.
* If m=0, there is no choice that fixes the affine parameter λ to a physical parameter. Consequently, there is some freedom in fixing the auxiliary field. The two common choices are:
* Fix e=1. In this case, e does not carry a dependence on the affine parameter λ, but the affine parameter is measured in units of time per unit of mass, i.e. [λ]=T/M.
* Fix e=c2E(λ), where E is the energy of the particle. In this case, the affine parameter is measured in units of time, i.e. [λ]=T, but e retains a dependence on the affine parameter λ.

## See also

* Relativistic mechanics
* Fundamental lemma of the calculus of variations
* Canonical coordinates
* Functional derivative
* Generalized coordinates
* Hamiltonian mechanics
* Hamiltonian optics
* Lagrangian analysis (applications of Lagrangian mechanics)
* Lagrangian point
* Lagrangian system
* Non-autonomous mechanics
* Restricted three-body problem
* Plateau's problem

## Footnotes

1. ↑ 1.0 1.1 1.2 The line element squared is the Lorentz invariant

c2dτ2=ηαβdxαdxβ=c2dt2−d𝐫2,
which takes the same values in all inertial frames of reference. Here ηαβ are the components of the Minkowski metric tensor, dxα = (cdt, dr) = (cdt, dx, dy, dz) are the components of the differential position four-vector, the summation convention over the covariant and contravariant spacetime indices α and β is used, each index takes the value 0 for timelike components, and 1, 2, 3 for spacelike components, and

d𝐫2≡d𝐫⋅d𝐫≡dx2+dy2+dz2
is a shorthand for the square differential of the particle's position coordinates. Dividing by c2dt2 allows the conversion to the lab coordinate time as follows,

dτ2dt2=1c2ηαβdxαdtdxβdt=1−1c2d𝐫2dt2=1γ(𝐫˙)2
so that

dτ=1cηαβdxαdtdxβdtdt=dtγ(𝐫˙).

## Citations

1. ↑ 1.0 1.1 Foster & Nightingale 1995, p. 62–63
2. ↑ 2.0 2.1 Hobson, Efstathiou & Lasenby 2006, p. 79–80
3. ↑ Polchinski 2005, p.10
4. ↑ Goldstein 1980, p. 328
5. ↑ Hobson, Efstathiou & Lasenby 2006, p. 79–80
6. ↑ Landau & Lifshitz 1975, p. 26
7. ↑ Goldstein 1980, p. 324
8. ↑ Hand & Finch 1998, p. 551
9. ↑ Goldstein 1980, p. 323
10. ↑ Goldstein, Poole & Safko 2002, p. 314
11. ↑ Hand & Finch 1998, p. 534

## References

* Penrose, Roger (2007). The Road to Reality. Vintage books. ISBN 978-0-679-77631-4.
* Landau, L. D.; Lifshitz, E. M. (15 January 1976). Mechanics (3rd ed.). Butterworth Heinemann. p. 134. ISBN 978-0-7506-2896-9. https://archive.org/details/mechanics01land_382.
* Landau, Lev; Lifshitz, Evgeny (1975). The Classical Theory of Fields. Elsevier Ltd.. ISBN 978-0-7506-2768-9.
* Hand, L. N.; Finch, J. D. (13 November 1998). Analytical Mechanics (2nd ed.). Cambridge University Press. p. 23. ISBN 978-0-521-57572-0. https://archive.org/details/analyticalmechan00hand.
* Louis N. Hand; Janet D. Finch (1998). Analytical mechanics. Cambridge University Press. pp. 140–141. ISBN 0-521-57572-9. https://books.google.com/books?id=1J2hzvX2Xh8C&pg=PA141.
* Goldstein, Herbert (1980). Classical Mechanics (2nd ed.). San Francisco, CA: Addison Wesley. pp. 352–353. ISBN 0-201-02918-9. https://archive.org/details/classicalmechani00gold_639.
* Goldstein, Herbert; Poole, Charles P. Jr.; Safko, John L. (2002). Classical Mechanics (3rd ed.). San Francisco, CA: Addison Wesley. pp. 347–349. ISBN 0-201-65702-3. http://www.pearsonhighered.com/educator/product/Classical-Mechanics/9780201657029.page.
* Lanczos, Cornelius (1986). "II §5 Auxiliary conditions: the Lagrangian λ-method". The variational principles of mechanics (Reprint of University of Toronto 1970 4th ed.). Courier Dover. p. 43. ISBN 0-486-65067-7. https://books.google.com/books?id=ZWoYYr8wk2IC&pg=PA43.
* Feynman, R. P.; Leighton, R. B.; Sands, M. (1964). The Feynman Lectures on Physics. 2. Addison Wesley. ISBN 0-201-02117-X. https://feynmanlectures.caltech.edu/II_toc.html.
* Foster, J; Nightingale, J.D. (1995). A Short Course in General Relativity (2nd ed.). Springer. ISBN 0-03-063366-4. https://books.google.com/books?id=K2fTBwAAQBAJ&q=foster+nightingale+general+relativity.
* Hobson, M. P.; Efstathiou, G. P.; Lasenby, A. N. (2006). General Relativity: An Introduction for Physicists. Cambridge University Press. pp. 79–80. ISBN 978-0-521-82951-9. https://books.google.com/books?id=xma1QuTJphYC&q=hobson+general+relativity+gravitomagnetic+field&pg=PA496.
* Joseph Polchinski (2005). String Theory, An Introduction to the Bosonic String. 1. Cambridge University Press. p. 10. ISBN 978-0-521-67227-6. https://www.cambridge.org/9780521672276.

* v
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Relativity

Special
relativity|

| Background|

* Principle of relativity (Galilean relativity
* Galilean transformation)
* Special relativity
* Doubly special relativity

|

Fundamental
concepts|

* Frame of reference
* Speed of light
* Hyperbolic orthogonality
* Rapidity
* Maxwell's equations
* Proper length
* Proper time
* Relativistic mass

Formulation|

* Lorentz transformation

Phenomena|

* Time dilation
* Mass–energy equivalence
* Length contraction
* Relativity of simultaneity
* Relativistic Doppler effect
* Thomas precession
* Ladder paradox
* Twin paradox

Spacetime|

* Light cone
* World line
* Minkowski diagram
* Biquaternions
* Minkowski space

General
relativity|

| Background|

* Introduction
* Mathematical formulation

|

Fundamental
concepts|

* Equivalence principle
* Riemannian geometry
* Penrose diagram
* Geodesics
* Mach's principle

Formulation|

* ADM formalism
* BSSN formalism
* Einstein field equations
* Linearized gravity
* Post-Newtonian formalism
* Raychaudhuri equation
* Hamilton–Jacobi–Einstein equation
* Ernst equation

Phenomena|

* Black hole
* Event horizon
* Singularity
* Two-body problem

* Gravitational waves: astronomy
* detectors (LIGO and collaboration
* Virgo
* LISA Pathfinder
* GEO)
* Hulse–Taylor binary

* Other tests: precession of Mercury
* lensing
* redshift
* Shapiro delay
* frame-dragging / geodetic effect (Lense–Thirring precession)
* pulsar timing arrays

Advanced
theories|

* Brans–Dicke theory
* Kaluza–Klein
* Quantum gravity

Solutions|

* Cosmological: Friedmann–Lemaître–Robertson–Walker (Friedmann equations)
* Kasner
* BKL singularity
* Gödel
* Milne

* Spherical: Schwarzschild (interior
* Tolman–Oppenheimer–Volkoff equation)
* Reissner–Nordström
* Lemaître–Tolman

* Axisymmetric: Kerr (Kerr–Newman)
* Weyl−Lewis−Papapetrou
* Taub–NUT
* van Stockum dust
* discs

* Others: pp-wave
* Ozsváth–Schücking metric

Scientists|

* Poincaré
* Lorentz
* Einstein
* Hilbert
* Schwarzschild
* de Sitter
* Weyl
* Eddington
* Friedmann
* Lemaître
* Milne
* Robertson
* Chandrasekhar
* Zwicky
* Wheeler
* Choquet-Bruhat
* Kerr
* Zel'dovich
* Novikov
* Ehlers
* Geroch
* Penrose
* Hawking
* Taylor
* Hulse
* Bondi
* Misner
* Yau
* Thorne
* Weiss
* others

Categories|

Theory of relativity

* v
* t
* e

Branches of physics

Divisions|

* Theoretical
* Phenomenology
* Computational
* Experimental
* Applied

Classical|

| Classical mechanics|

* Continuum
* Solid
* Fluid
* Acoustics

|

Electrodynamics|

* Electrostatics
* Magnetostatics
* Plasma physics
* Accelerator physics

Statistical mechanics|

* Thermodynamics
* Condensed matter
* Materials
* Mesoscopic
* Polymers
* Soft
* Solid-state

Modern|

| Quantum mechanics|

* Quantum electrodynamics
* Quantum field theory
* Quantum gravity
* Quantum information

|

Relativity|

* General
* Special

Particle physics|

* Astroparticle
* Nuclear
* Quantum chromodynamics

Atomic, molecular
and optical physics|

* Atomic physics
* Molecular physics
* Optics
* Photonics
* Quantum optics

Cosmology|

* Astrophysics
* Nuclear
* Celestial mechanics
* Solar
* Heliophysics
* Space physics

Interdisciplinary|

* Biophysics
* Agrophysics
* Medical
* Neurophysics
* Engineering
* Geophysics
* Atmospheric
* Cloud
* Materials science
* Mathematical physics
* Physical chemistry
* Chemical physics
* Physical oceanography
* Quantum computing
* Social physics
* Econophysics

See also|

* History of physics
* Nobel Prize in Physics
* Timeline of physics discoveries
* Theory of everything

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