1.1. Classical mechanics. In classical mechanics, we study the motion of a particle. This motion is described by a (vector) function
of one variable,
, representing the position
of the particle as a function of time. This function must satisfy the Newton equation of motion,
where
(the difference of kinetic and potential energy), and the action functional
(for some fixed
Remark 1. The name “least action principle” comes from the fact that in some cases (for example when
) the action is not only extremized but also minimized at the solution
. In general, however, it is not the case, and the trajectory of the particle may not be a minimum, but only a saddle point of the action. Therefore, the law of motion is better formulated as the “extremal (or stationary) action principle” ; this is the way we will think of it in the future.
Remark 2. Physicists often consider solutions of Newton's equation on the whole line rather than on a fixed interval
. In this case, the naive definition of an extremal does not make sense, since the action integral
is improper and in general diverges. Instead, one makes the following “correct” definition: a function
on
is an extremal of
if the expression
where
1.2. Classical field theory. In classical field theory, the situation is similar. In this case, we should think not of a single particle, but of a “continuum of particles” (e.g. a string, a membrane, a jet of fluid); so the motion is described by a classical field-a (vector) function
depending on both space and time coordinates
. Consequently, the equation of motion is a partial differential equation. For example, for a string or a membrane the equation of motion is the wave equation
, where
is the D'Alembertian
(here
is the Laplacian
with respect to the space coordinates, and
the velocity of wave
propagation).
As in mechanics, in classical field theory there is a Lagrangian
(a differential polynomial in
), whose integral
over a region
in space and time is called the action. The law of motion can be expressed as the condition that the action must be extremized over any closed region
and fixed boundary
conditions; so the equations of motion (also called the field equations) are the Euler-Lagrange equations for this variational problem. For example, in the case of string or membrane, the Lagrangian is
Remark. Like in mechanics, solutions of the field equations on the whole space (rather than a closed region
where
1.3. Brownian motion. One of the main differences between classical and quantum mechanics is, roughly speaking, that quantum particles do not have to obey the classical equations of motion, but can randomly deviate from their classical trajectories. Therefore, given the position and velocity of the particle at a given time, we cannot determine its position at a later time, but can only determine the density of probability that at this later time the particle will be found at a given point. In this sense quantum particles are similar to random (Brownian) particles. Brownian particles are a bit easier to understand conceptually, so let us begin with them.
The motion of a Brownian particle in
in a potential field
is described by astochastic process
. That is, for each real
we have a random variable
(position of the particle at a time
), such that the dependence of
is regular in some sense. The random dynamics
of the particle is “defined” as follows: 1 if
:
is a continuously differentiable function, then the density of probability that
for
is proportional to
, where
is the action for the corresponding classical mechanical system, and
is the diffusion coefficient. Thus, for given
and
, the likeliest
is the one that minimizes
(in particular, solves the classical equations of motion
, while the likelihood of the other paths decays exponentially with the deviation of the action of these paths from the minimal possible.
Remark. This discussion assumes that the extremum of
at
is actually a minimum, which we know is not always the case.
All the information we can hope to get about such a process is contained in the correlation functions
, which by definition are the expectation values of the products of random variables
(more specifically, by Kolmogorov's theorem
the stochastic process
is completely determined by these functions). So such functions should be regarded as the output, or answer, of the theory of the Brownian particle.
So the main question is how to compute the correlation functions. The definition above obviously gives the following answer: given
, we have
where integration is carried out over the space of paths
It is clear, however, that such definition and answer are a priori not satisfactory from the mathematical viewpoint, since the infinite dimensional integration that we used requires justification. In this particular case, such justification is possible within the framework of Lebesgue measure theory, and the corresponding integration theory is called the theory of Wiener integrals. (To be more precise, one cannot define the measure
, but one can define the measure
for sufficiently nice potentials
.
1.4. Quantum mechanics. Now let us turn to a quantum particle. Quantum mechanics is notoriously difficult to visualize, and the randomness of the behavior of a quantum particle is less intuitive and more subtle than that of a Brownian particle; nevertheless, it was pointed out by Feynman that the behavior of a quantum particle in a potential field
is correctly described by the same model, with the real positive parameter
replaced by the imaginary number
, where
is the Planck constant. In other words, the dynamics of a quantum particle can be expressed via the correlation functions
where
1.5. Quantum field theory. The situation is the same in field theory. Namely, a useful theory of quantum fields (used in the study of interactions of elementary particles) is obtained when one considers correlation functions
where
Of course, from the mathematical point of view, this setting is a priori even less satisfactory than the one for the Brownian particle, since it involves integration with respect to the complex valued measure
, which nobody knows how to define. Nevertheless, physicists imagine that certain integrals of this type
exist and come to correct and interesting conclusions (both physical and mathematical). Therefore, making sense of such integrals is an interesting problem for mathematicians.
References
This is a derivative work from [1] a Creative Commons Attribution-Noncommercial-Share Alike 3.0 work
[1] MIT OpenCourseWare, 18.238 Geometry and Quantum Field Theory, Fall 2002
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