quantum electrodynamics by imran aziz

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Fundamentals of Quantum Electrodynamics MOHAMMAD IMRAN AZIZ Assistant Professor PHYSICS DEPARTMENT SHIBLI NATIONAL COLLEGE, AZAMGARH (India). pdfMachine A pdf writer that produces quality PDF files with ease! Produce quality PDF files in seconds and preserve the integrity of your original documents. Compatible across nearly all Windows platforms, simply open the document you want to convert, click “print”, select the “Broadgun pdfMachine printer” and that’s it! Get yours now!

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Quantum electrodynamics

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Page 1: Quantum electrodynamics by imran aziz

Fundamentals of Quantum Electrodynamics

MOHAMMAD IMRAN AZIZAssistant Professor

PHYSICS DEPARTMENTSHIBLI NATIONAL COLLEGE, AZAMGARH (India).

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Page 2: Quantum electrodynamics by imran aziz

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Coordinates of Fields

A wave field is specified by its amplitudes at allpoints of space and the dependence of theseamplitudes on the time, in much the same way asa system of particles is specified by thepositional coordinates q and their dependence onthe time. The field evidently has an infinitenumber of degrees of freedom, and is analogousto a system that consists of an infinite number ofparticles. It is natural, then, to use theamplitudes ø(r,t) at all points r as coordinates inanalogy with the particle coordinates q .

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Page 3: Quantum electrodynamics by imran aziz

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PROPERTIES OF THE FIELDS:1. In analogy with any physical system, the field is carrier of energy, momentum, angular momentum and other observable dynamical quantities.2. It is to be ideal regarding the definition of the field that if the values of ø are ordinary or complex numbers then it is called a classical field, and if the values of ø are operators in some Hilbert space, then the fields in question is called a quantum field.3. The degrees of freedom of the field is represented by amplitudes ø(r,t) which are infinite in number corresponding to finite number of points in space.

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Page 4: Quantum electrodynamics by imran aziz

Lagrangian Equations

� The Lagrangian L is a function of the time and a func onal of the possible paths q(t) of the

system. The actual paths are derived from the variational principle

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Page 5: Quantum electrodynamics by imran aziz

By analogy, we expect the fieldLagrangian to be a functional of thefield amplitude ø(r,t). It can usuallybe expressed as the integral over allspace of a Lagrangian density L:

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Page 6: Quantum electrodynamics by imran aziz

� ǁŚĞƌĞ�ƚŚĞ�ŝŶĮŶŝƚĞƐŝŵĂů�ǀĂƌŝĂƟŽŶ� of is subject to the restrictions

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Page 7: Quantum electrodynamics by imran aziz

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This is classical field equation derived from the Lagrangian density L ( ø, grad ø , ,t).

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Page 8: Quantum electrodynamics by imran aziz

Functional DerivativesIn order to pursue further the analogy with particle mechanics, it isdesirable to rewrite Lagrangian equation in terms of L

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Page 9: Quantum electrodynamics by imran aziz

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which closely resembles the Lagrangian equations .

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Page 10: Quantum electrodynamics by imran aziz

Hamiltonian Equations

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and

The Hamiltonian

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Page 11: Quantum electrodynamics by imran aziz

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We write H as the volume integral of a Hamiltonian density H, and assume that the cells aresmall enough so that the difference between a Volume integral and the corresponding cellsummation can be ignored; we then have

It follows from the earlierdiscussion of functionalderivatives that

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Page 12: Quantum electrodynamics by imran aziz

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The classical field equations in Hamiltonian form

This equation also serves to define the Poisson-bracket expression for two fuctionals of the field variables. In quantum mechanics Poission-bracket is

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Page 13: Quantum electrodynamics by imran aziz

Quantum equations for fieldsThe

quantum conditions for the canonical field variables then become

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The quantum conditions for the canonical field variables then become

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Page 14: Quantum electrodynamics by imran aziz

QUANTIZATION OF THE DIRAC EQUATION

� Our procedure consists in treating the one-particle equation as though it were a classical field equation. The resulting quantized field theory represents the motion of a number of non interacting free electrons.

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Page 15: Quantum electrodynamics by imran aziz

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Page 16: Quantum electrodynamics by imran aziz

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Then the Hamiltonian density is

Quantum Equations. Aswith the nonrelativisticSchrodinger equation, it isconvenient to rewrite theHamiltonian

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Page 17: Quantum electrodynamics by imran aziz

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We therefore quantize the field by imposing anticommutation relations on thecomponents of ø

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Page 18: Quantum electrodynamics by imran aziz

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Thus the four equa ons are equivalent to the Dirac equa on and follows:

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Page 19: Quantum electrodynamics by imran aziz

QUANTIZATION OF ELECTROMAGNETIC FIELDS

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Lagrangian Equations:

Maxwell's equations in empty space are obtained by setting and equal to zero

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Page 20: Quantum electrodynamics by imran aziz

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Page 21: Quantum electrodynamics by imran aziz

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Hamiltonian Equations:

The momentum canonically conjugate to A, is found

The Hamiltonian equations of motion are

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Page 22: Quantum electrodynamics by imran aziz

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The Hamiltonian is then

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Page 23: Quantum electrodynamics by imran aziz

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Quantum equations:The classical electromagnetic field is converted into a quantum field in the following way

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Page 24: Quantum electrodynamics by imran aziz

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Commutation Relations for E and H:The electric and magnetic fields are defined by the equations

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Page 25: Quantum electrodynamics by imran aziz

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Page 26: Quantum electrodynamics by imran aziz

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Quantization of Schrodinger equations

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Page 27: Quantum electrodynamics by imran aziz

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Page 28: Quantum electrodynamics by imran aziz

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Quantum equations:

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Page 29: Quantum electrodynamics by imran aziz

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