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[LSN/QFT-04004] Bosons and Fermions

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I Lesson Overview


Syllabus

Connection between second quantized Schrodinger theory and
nonrelativistic quantum mechanics. Hilbert space of second quantized
theory;

 Prerequisites

Quantum mechanics of identical particles;
Symmetrization postulate
Spin statistics connection; Bosons and fermions.

Objectives

  1. To recall and discuss quantum mechanics of identical particles; Symmetrization postulate.
  2. To show and establish connection of states in second quantized theory and wave function in quantum mechanics. To demonstrate that CCR quantization leads to correctly symmetrized wave function for a two particle state.
  3. To briefly describe the connection between description of assembly of many identical bosons in quantum mechanics and Hilbert space of the second quantized Schrodinger theory}. To describe briefly the changes needed in {quantization of Schrodinger field to describe system of identical fermions}.
  4. To give a brief about spin statistics connection

 II Recall and Discuss

QM of Identical Particles

In nonrelativistic quantum mechanics, when describing a system of
identical particles the total wave function must be symmetrized for bosons and
antisymmetrized for fermions. This is known as {\bf symmetrization
postulates} and has to be put in ``by hand'' in nonrelativistictheory.
The symmetrization (or antisymmetrization as the case may be)
is to be insisted under exchange of all space as well as spinvariables. 


 III Connection with Quantum Mechanics

\(\S1\) Connection with Quantum mechanics

\(\S2\) Second-quantized-theory-fermions 

The Hilbert space of the second quantized theory for fermions consists
of direct sum of anti-symmetric tensor products, instead of symmetric tensor products.


 IV Spin statistics connection 

In nonrelativistic quantum theory the spin symmetrization postulate is
an extra assumption. It is derived from experimental observations and
leads to Pauli exclusion principle for fermions.
The spin statistics connection states that the relativistic field for
integral spins must be quantized using CCR and field corresponding to
half integral spin must be quantized with CAR.
The spin statistics connection is a consequence of very
general requirements of relativistic invariance, causality and positive definiteness of energy. 


 V EndNotes

About second quantization

... The quantization of a wave field imparts
to it some particle properties; in the case of the electromagnetic field, a
theory of light quanta (photons) results. The field quantization technique
can also be applied to a field, such as that described by the
nonrelativistic Schrodinger equation (6.16) or by one of the relativistic
equations (42.4)or (43.3). As we shall see (Sec. 46), it then converts a
one-particle theory into a many-particle theory; in the non relativistic case,
this is equivalent to the transition from Eq. (6.16) to (16.1) or (32.1).
Because of this equivalence, it might seem that the quantization of fields
merely provides another formal approach to the many-particle problem. However,
the new formalism can also deal as well with processes that involve the
creation or destruction of material particles (radioactive beta
decay,meson-nucleon interaction). ...\\
L.I. Schiff, ``Quantum Mechanics'', McGraw Hill Book Co. New York (1949).

Who did the second quantization?

The equivalence of the operator (second-quantized) description and the
ordinary Schrodinger theory description of a system of n particles was
established by Jordan and Klein [Jordan (1927)] for particles obeying
Bose statistics and by Jordan and Wigner [Jordan (1928b)] for particles
obeying Fermi statistics. The operator formalism was subsequently re-
formulated in " Fock space" by Fock (1932) who gave the generalization
of the ordinary Schrodinger wave mechanics to systems for which the
number of particles is not a constant of the motion. Fock's work was
based on some previous investigations along similar lines by Landau and
Peierls [Landau (1930)] who gave a configuration space treatment of the
quantized electromagnetic field interacting with matter. Landau and
Peierls' treatment was in turn suggested earlier by Oppenheimer,
Heisenberg, and Pauli [Heisenberg (1930)] who sketched a configuration space
treatment of quantized field 'theories.
\paragraph*{Reference:}

Schweber S. S., An Introduction to Relativistic Quantum Field Theory,  Row, Peterson And Company (1961).

   

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