Canonical quantization of scalar Þelds - IU Bdermisek/QFT_08/qft-I-2-1p.pdf · Canonical...
Transcript of Canonical quantization of scalar Þelds - IU Bdermisek/QFT_08/qft-I-2-1p.pdf · Canonical...
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Canonical quantization of scalar fieldsbased on S-3
Hamiltonian for free nonrelativistic particles:
Furier transform:
a(x) =!
d3p
(2!)3/2eip·x a(p)
we get:
can go back to x using:
!d3x
(2!)3eip·x = "3(p)
!d3p
(2!)3eip·x = "3(x)
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Canonical quantization of scalar fields
(Anti)commutation relations:
[A,B]! = AB !BA
Vacuum is annihilated by :
is a state of momentum , eigenstate of with
is eigenstate of with energy eigenvalue:
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Relativistic generalization
Hamiltonian for free relativistic particles:
spin zero, but can be either bosons or fermions
Is this theory Lorentz invariant?
Let’s prove it from a different direction, direction that we will use for any quantum field theory from now:
start from a Lorentz invariant lagrangian or action
derive equation of motion (for scalar fields it is K.-G. equation)
find solutions of equation of motion
show the Hamiltonian is the same as the one above
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A theory is described by an action:
where is the lagrangian.
Equations of motion should be local, and so
Thus:
where is the lagrangian density.
is Lorentz invariant:
For the action to be invariant we need:
the lagrangian density must be a Lorentz scalar!
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Let’s consider:
Any polynomial of a scalar field is a Lorentz scalar and so are products of derivatives with all indices contracted.
and let’s find the equation of motion, Euler-Lagrange equation:
(we find eq. of motion from variation of an action: making an infinitesimal variation in and requiring the variation of the action to vanish)
arbitrary constant
integration by parts, and at infinity in any direction (including time)
!"(x) = 0
is arbitrary function of x and so the equation of motion is Klein-Gordon equation
! = 1, c = 1
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Solutions of the Klein-Gordon equation:
one classical solution is a plane wave:
is arbitrary real wave vector and
The general classical solution of K-G equation:
where and are arbitrary functions of , and
where and are arbitrary functions of , and
is a function of (introduced for later convenience) |k|
if we tried to interpret as a quantum wave function, the second term would represent contributions with negative energy to the wave function!
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real solutions:
k !" !k
thus we get:
(such a is said to be on the mass shell)kµ
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Finally let’s choose so that is Lorentz invariant:
manifestly invariant under orthochronous Lorentz transformations
on the other hand
sum over zeros of g, in our case the only zero is k0 = !
for any the differential is Lorentz invariant
it is convenient to take for which the Lorentz invariant differential is:
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Finally we have a real classical solution of the K.-G. equation:
where again: , ,
For later use we can express in terms of :
where and we will call .
Note, is time independent.
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Constructing the hamiltonian:
Recall, in classical mechanics, starting with lagrangian as a function of coordinates and their time derivatives we define conjugate momenta and the hamiltonian is then given as:
In field theory:
hamiltonian density
and the hamiltonian is given as:
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In our case:
Inserting we get:
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!d3x
(2!)3eip·x = "3(p)
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