The Lagrangian, L, of a dynamical system is a function that summarizes the dynamics of the system. It is named after Joseph Louis Lagrange. The concept of a Lagrangian was originally introduced in a reformulation of classical mechanics known as Lagrangian mechanics. In classical mechanics, the Lagrangian is defined as the kinetic energy, T, of the system minus its potential energy, V. In symbols,
Under conditions that are given in Lagrangian mechanics, if the Lagrangian of a system is known, then the equations of motion of the system may be obtained by a direct substitution of the expression for the Lagrangian into the Euler–Lagrange equation, a particular family of partial differential equations.
The Lagrange formulation
The Lagrange formulation of mechanics is important not just for its broad applications, but also for its role in advancing deep understanding of physics. Although Lagrange only sought to describe classical mechanics, the action principle that is used to derive the Lagrange equation is now recognized to be applicable to quantum mechanics.
Lagrangian mechanics and Noether's theorem together yield a natural formalism for first quantization by including commutators between certain terms of the Lagrangian equations of motion for a physical system.
Advantages over other methods
"Cyclic coordinates" and conservation laws
An important property of the Lagrangian is that conservation laws can easily be read-off from it. E.g., if the Lagrangian depends on the time-derivative of a generalized coordinate, but not on qi itself, then the generalized momentum is a conserved quantity. This is a special case of Noether's theorem, see below. Such coordinates are called "cyclic".
To repeat: the conservation of the generalized momentum , say, can be directly seen if the Lagrangian of the system is of the form
As an aside: If the time, t, does not appear in , then the conservation of the Hamiltonian follows. This is the energy conservation unless the potential energy depends on velocity, as in electrodynamics. More details can be found in any textbook on theoretical mechanics.
where the action, , is a functional of the dependent variables with their derivatives and s itself
The equations of motion obtained from this functional derivative are the Euler–Lagrange equations of this action. For example, in the classical mechanics of particles, the only independent variable is time, t. So the Euler-Lagrange equations are
Dynamical systems whose equations of motion are obtainable by means of an action principle on a suitably chosen Lagrangian are known as Lagrangian dynamical systems. Examples of Lagrangian dynamical systems range from the classical version of the Standard Model, to Newton's equations, to purely mathematical problems such as geodesic equations and Plateau's problem.
An example from classical mechanics
In the rectangular coordinate system
Suppose we have a three-dimensional space and the Lagrangian
Then, the Euler–Lagrange equation is:
where i = 1,2,3.
The derivation yields:
The Euler–Lagrange equations can therefore be written as:
where the time derivative is written conventionally as a dot above the quantity being differentiated, and is the del operator.
Using this result, it can easily be shown that the Lagrangian approach is equivalent to the Newtonian one.
If the force is written in terms of the potential ; the resulting equation is , which is exactly the same equation as in a Newtonian approach for a constant mass object.
A very similar deduction gives us the expression , which is Newton's Second Law in its general form.
In the spherical coordinate system
Suppose we have a three-dimensional space using spherical coordinates r,θ,φ with the Lagrangian
Then the Euler–Lagrange equations are:
Here the set of parameters si is just the time t, and the dynamical variables φi(s) are the trajectories of the particle.
Despite the use of standard variables such as x, the Lagrangian allows the use of any coordinates, which do not need to be orthogonal. These are "generalized coordinates".
Lagrangian of a test particle
A test particle is a particle whose mass and charge are assumed to be so small that its effect on external system is insignificant. It is often a hypothetical simplified point particle with no properties other than mass and charge. Real particles like electrons and up-quarks are more complex and have additional terms in their Lagrangians.
Classical test particle with Newtonian gravity
Suppose we are given a particle with mass kilograms, and position meters in a Newtonian gravitation field with potential joules per kilogram. The particle's world line is parameterized by time seconds. The particle's kinetic energy is:
and the particle's gravitational potential energy is:
Then its Lagrangian is joules where
Varying in the integral (equivalent to the Euler–Lagrange differential equation), we get
Integrate the first term by parts and discard the total integral. Then divide out the variation to get
is the equation of motion — two different expressions for the force.
Special relativistic test particle with electromagnetism
In special relativity, the form of the term which gives rise to the derivative of the momentum must be changed; it is no longer the kinetic energy. It becomes:
(In special relativity, the energy of a free test particle is )
where meters per second is the speed of light in vacuum, seconds is the proper time (i.e. time measured by a clock moving with the particle) and The second term in the series is just the classical kinetic energy. Suppose the particle has electrical charge coulombs and is in an electromagnetic field with scalar potential volts (a volt is a joule per coulomb) and vector potential volt seconds per meter. The Lagrangian of a special relativistic test particle in an electromagnetic field is:
Varying this with respect to , we get
which is the equation for the Lorentz force where
General relativistic test particle
In general relativity, the first term generalizes (includes) both the classical kinetic energy and interaction with the Newtonian gravitational potential. It becomes:
The Lagrangian of a general relativistic test particle in an electromagnetic field is:
If the four space-time coordinates are given in arbitrary units (i.e. unit-less), then meters squared is the rank 2 symmetric metric tensor which is also the gravitational potential. Also, volt seconds is the electromagnetic 4-vector potential. Notice that a factor of c has been absorbed into the square root because it is the equivalent of
Note that this notion has been directly generalized from special relativity
Lagrangians and Lagrangian densities in field theory
The time integral of the Lagrangian is called the action denoted by S.
and the Lagrangian density , which one integrates over all space-time to get the action:
The Lagrangian is then the spatial integral of the Lagrangian density. However, is also frequently simply called the Lagrangian, especially in modern use; it is far more useful in relativistic theories since it is a locally defined, Lorentz scalar field. Both definitions of the Lagrangian can be seen as special cases of the general form, depending on whether the spatial variable is incorporated into the index i or the parameters s in . Quantum field theories in particle physics, such as quantum electrodynamics, are usually described in terms of , and the terms in this form of the Lagrangian translate quickly to the rules used in evaluating Feynman diagrams.
To go with the section on test particles above, here are the equations for the fields in which they move. The equations below pertain to the fields in which the test particles described above move and allow the calculation of those fields. The equations below will not give you the equations of motion of a test particle in the field but will instead give you the potential (field) induced by quantities such as mass or charge density at any point . For example, in the case of Newtonian gravity, the Lagrangian density integrated over space-time gives you an equation which, if solved, would yield . This , when substituted back in equation (1), the Lagrangian equation for the test particle in a Newtonian gravitational field, provides the information needed to calculate the acceleration of the particle.
The Lagrangian (density) is joules per cubic meter. The interaction term is replaced by a term involving a continuous mass density kilograms per cubic meter. This is necessary because using a point source for a field would result in mathematical difficulties. The resulting Lagrangian for the classical gravitational field is:
where meters cubed per kilogram second squared is the gravitational constant. Variation of the integral with respect to gives:
Integrate by parts and discard the total integral. Then divide out by to get:
which yields Gauss's law for gravity.
Electromagnetism in special relativity
The interaction terms are replaced by terms involving a continuous charge density coulombs per cubic meter and current density amperes per square meter. The resulting Lagrangian for the electromagnetic field is:
Varying this with respect to , we get
which yields Gauss' law.
Varying instead with respect to , we get
which yields Ampère's law.
Electromagnetism in general relativity
For the Lagrangian of gravity in general relativity, see Einstein-Hilbert action. The Lagrangian of the electromagnetic field is:
If the four space-time coordinates are given in arbitrary units, then: joule seconds is the Lagrangian, a scalar density; coulombs is the current, a vector density; and volt seconds is the electromagnetic tensor, a covariant antisymmetric tensor of rank two. Notice that the determinant under the square root sign is applied to the matrix of components of the covariant metric tensor , and is its inverse. Notice that the units of the Lagrangian changed because we are integrating over which are unit-less rather than over which have units of seconds meters cubed. The electromagnetic field tensor is formed by anti-symmetrizing the partial derivative of the electromagnetic vector potential; so it is not an independent variable. The square root is needed to convert that term into a scalar density instead of just a scalar, and also to compensate for the change in the units of the variables of integration. The factor of inside the square root is needed to normalize it so that the square root will reduce to one in special relativity (since the determinant is in special relativity).
Lagrangians in quantum field theory
The Lagrangian density for a Dirac field is:
Quantum electrodynamic Lagrangian
The Lagrangian density for QED is:
where is the electromagnetic tensor.
Quantum chromodynamic Lagrangian
In order for the action to be local, we need additional restrictions on the action. If , we assume is the integral over M of a function of φ, its derivatives and the position called the Lagrangian, . In other words,
It is assumed below, in addition, that the Lagrangian depends on only the field value and its first derivative but not the higher derivatives.
Given boundary conditions, basically a specification of the value of φ at the boundary if M is compact or some limit on φ as x approaches (this will help in doing integration by parts), the subspace of consisting of functions, φ such that all functional derivatives of S at φ are zero and φ satisfies the given boundary conditions is the subspace of on shell solutions.
Published - July 2009
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