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"Yet nature is made better by no mean
But nature makes that mean : so, over that art
Which you say adds to nature, is an art,
That nature makes."
William Shakespear, A winters tale.
"Energy makes it go and entropy tells where to go"

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scattering matrix

In physics, the s matrix or scattering matrix correlates the initial state and the final state of a physical system evolving a scattering process. It is utilized in quantum mechanics, scattering theory and quantum field theory (QFT). More formally, in the language of QFT, the s matrix is defined as the unitary matrix connecting sets of asymptotically free particle states (the in-states and the out-states) in the Hilbert space of quantum physical states. A multi-particle state is assumed to be free (non-interacting) if it transforms under Lorentz transformations as a tensorial product, or direct product in physics parlance, of one-particle states as prescribed by equation (1) below. Asymptotically free then implies that the state has this appearance in either the distant past or the distant future.
While the s-matrix may be defined for any background (spacetime) that is asymptotically solvable and has no event horizons, it has a simplistic structure in the case of the Minkowski spacetime. In this special case, the Hilbert space is a space of irreducible unitary representations of the inhomogeneous Lorentz group (the Poincaré group); the s matrix is the evolution operator between time equal to minus infinity (the distant past), and time equal to plus infinity (the distant future). It is defined only in the limit of zero energy density (or infinite particle separation distance).
It can be proven that if a quantum field theory in Minkowski spacetime has a mass gap, the state in the asymptotic past and in the asymptotic future are both described by Fock spaces.

Motivation

In high-energy particle physics we are very interested in computing the probability for distinct outcomes in scattering experiments. These experiments can be broken down into three separate stages:
a)collide together a collection of incoming particles (usually two particles with high energies).
b)Allowing the incoming particles to interact with themselves. These interactions may change the types of particles present (e.g. if an electron and a positron (anto-electron) annihilate they may produce two photons).
c)Measuring the mass of resulting outgoing particles.
The process by which the incoming particles are converted (through their interaction) into the outgoing particles is called scattering process. For particle physics, a physical theory of these processes must be able to systematically compute the probability for different outgoing particles when different incoming particles collide with different energies.
The S-matrix in quantum field theory achieves exactly this goal. It is assumed that the small-energy-density approximation is valid in these cases.

s matrix

An operator (a matrix) describing the process of transference of a quantum-mechanical system from one state into another under their interactions (scattering).
Under process of scattering, the system moves from one quantum state, the initial one (one may relate it to the time t=−∞), into another, the final one (related to t=+∞). If one denotes the set of quantum numbers describing the initial (final) state by i (j), then the scattering amplitude (the square of whose modulus defines the probability of a given scattering) can be written as S(ij). The collection or set of all scattering amplitudes forms a table with two inputs, and is called the scattering matrix S.
Determining scattering matrices is a core problem in quantum mechanics and quantum field theory. The scattering matrix contains complete information about the behaviour of a system, provided that one knows not only the numerical values, but also the analytical properties of its elements. In particular, its poles specify the bound states of the system (and thus the discrete energy levels). The most important property of a scattering matrix follows from the basic principles of quantum theory: it must be unitary.

Explanation

The difference between Heisenberg picture and Schrodinger picture of atom is the time dependency of observables and operators. Fundamentally two pictures developed by Heisenberg and Schrodinger is equivalent. They both describe same phenomena in two different ways. A little comparison can be made with a chart.
atomic pictures
The interaction part , as seen , is the representation of two states that interact through Hamiltonian H(0,s) which is time dependent. S-matrix is developed based on this interaction Hamiltonian H(0,s). S-matrix or scattering matrix is the transformation matrix between incoming and outgoing states of particles waves. This is the intermediate representation between Schrodinger picture and Heisenberg picture. In Heisenberg picture the state vector is constant or time-independent but in Schrodinger's picture this is not the case. In interaction picture both the state vector and operator carry time dependence of the observables. The interaction part of the Hamiltonian is dependent of time. Total Hamiltonian is the sum of free part H(0) and an interaction part V. So the interaction picture is :

propagator
--- (1)

Let us give a short description of each step taken to reach the final form of S - matrix. The first step denotes the projection of the initial state Φ(i) on the final state Φ(f). This gives the elements of S matrix and defines an operator S. In the next step the evolution operator U(,) is used and comparing the two results we see S-operator is the evolution operator U(a,a). The final S(fi) matrix is the projection of initial state onto final state Ψ(i) by the operator S which is defined above. The s - matrix is similar to transition probability amplitude in quantum mechanics. The elements of S-matrix represents scattering amplitudes. Scattering amplitudes are various ways in which wave interact with themselves or with particles.

s-matrix, s-matrix, this page is all about s matrix

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Notes

CROSS SECTION IN CLASSICAL MECHANICS

To understand the concept of a cross section in classical mechanics first, consider the following situation: A beam of particles approaches the target under a certain impact parameter b. According to this impact parameter, dN of the particles are scattered into an angle interval [θ, θ+dθ] per unit time. To normalise, the number n of particles passing a unit area orthogonal to the direction of the beam per unit time is introduced. Then,
dσ = dN/n
is the corresponding classical cross section. Obviously it has dimensions of an area. It is completely determined by the scattering center and the incoming particles.
CROSS SECTION IN QUANTUM MECHANICS - Consider now the scattering of incoming particles on some fixed target, the classical configuration. The cross section, usually denoted by σ, is defined as the transition rate per scatter seed in the target, per normalised incident flux of scatterers. The transition rate is the transition probability per unit time.
A cross section is therefore a measure of the effective surface area seen by the impinging particles, and as such is expressed in units of area. The cross section of two particles (i.e. observed when the two particles are colliding with each other) is a measure of the interaction event between the two particles. The cross section is proportional to the probability that an interaction will occur; for example in a simple scattering experiment the number of particles scattered per unit of time (current of scattered particles Ir) depends only on the number of incident particles per unit of time (current of incident particles Ii), the characteristics of target (for example the number of particles per unit of surface N), and the type of interaction.


HERTZ’S DYNAMICS

We have seen that Newton’s Laws are wholly lacking in selfevidence— so much so, indeed, that they contradict the law of causation in a form which has usually been held to be indubitable. We have seen also that these laws are specially suggestive of the law of gravitation. In order to eliminate what, in elementary Dynamics, is specially Newtonian, from what is really essential to the subject, we shall do well to examine some attempts to re-state the fundamental principles in a form more applicable to such sciences as Electricity. For this purpose the most suitable work seems to be that of Hertz.
The fundamental principles of Hertz’s theory are so simple and so admirable that it seems worth while to expound them briefly. His object, like that of most recent writers, is to construct a system in which there are only three fundamental concepts, space, time and mass. The elimination of a fourth concept, such as force or energy, though evidently demanded by theory, is difficult to carry out mathematically. Hertz seems, however, to have overcome the difficulty in a satisfactory manner. There are, in his system, three stages in the specification of a motion. In the first stage, only the relations of space and time are considered: this stage is purely kinematical. Matter appears here merely as a means of establishing, through the motion of a particle, a oneone correlation between a series of points and a series of instants. At this stage a collection of n particles has 3n coordinates, all so far independent: the motions which result when all are regarded as independent are all the thinkable motions of the system. But before coming to kinetics, Hertz introduces an intermediate stage. Without introducing time, there are in any free material system direct relations between space and mass, which form the geometrical connections of the system. (These may introduce time in the sense of involving velocities, but they are independent of time in the sense that they are expressed at all times by the same equations, and that these do not contain the time explicitly.) Those among thinkable motions which satisfy the equations of connection are called possible motions. The connections among the parts of a system are assumed further to be continuous in a certain well-defined sense (p. 89). It then follows that they can be expressed by homogeneous linear differential equations of the first order among the coordinates. But now a further principle is needed to discriminate among possible motions, and here Hertz introduces his only law of motion, which is as follows:
“Every free system persists in its state of rest or of uniform motion in a straightest path.”
This law requires some explanation. In the first place, when there are in a system unequal particles, each is split into a number of particles proportional to its mass. By this means all particles become equal. If now there are n particles, their 3n coordinates are regarded as the coordinates of a point in space of 3n dimensions. The above law then asserts that, in a free system, the velocity of this representative point is constant, and its path from a given point to another neighbouring point in a given direction is that one, among the possible paths through these two points, which has the smallest curvature. Such a path is called a natural path, and motion in it is called a natural

Reference materials:

Feynman's lectures on physics (elementary)
Quantum electrodynamics and s matrix (springer)
s matrix and quantum eqlectrodynamics by Greiner W & Reinhardt J.
Quantum mechanics
Grand Design by Stephen Hawking
Higher Engineering Mathematics ( PDFDrive.com ).pdf
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