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"

# Quantum field theory

Path integral | S Matrix | Quantum electrodynamics# Theory of relativity

Special theory of relativity | General theory of relativity### s matrix and 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.

**"failure is the pillar of success.."**

### 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. Each of these scattering amplitudes or transition amplitudes represents a Feynman diagram which contributes to its corresponding scattering amplitude perturbatively. Feynman diagram tells how particles interact with each other. Thus it is the same thing as the initial assumption of s-matrix.

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. To switch into the interaction picture, we divide the Schrödinger picture Hamiltonian into two parts:

A little comparison can be made with a chart.

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). mIf the Hamiltonian has explicit time-dependence (for example, if the quantum system interacts with an applied external electric field that varies in time), it will usually be advantageous to include the explicitly time-dependent terms with H(1,S) leaving H(0,S) time-independent.
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 :

--- (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.

**"you can watch me, mock me and try to block me but you can not stop me.."**

### 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