Most recent edit on 2008-08-13 03:49:45 by CharlesFrancis

Additions:

←  The Photon Field Operator  ↑  →

Deletions:

←  The Photon Field Operator  →

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Edited on 2008-03-13 08:18:39 by CharlesFrancis

Additions:
At this point we have a non-interacting Fock space and we have Dirac particles. We want to introduce interactions between particles, such that the interaction operator has an invariant form. Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). The possibilities are severely restricted. The natural and simplest thing to try is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.

Deletions:
At this point we have a non-interacting Fock space and we have Dirac particles . We want to introduce interactions between particles, such that the interaction operator has an invariant form. Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). The possibilities are severely restricted. The natural and simplest thing to try is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.

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Edited on 2008-03-13 08:14:49 by CharlesFrancis

Additions:
At this point we have a non-interacting Fock space and we have Dirac particles . We want to introduce interactions between particles, such that the interaction operator has an invariant form. Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). The possibilities are severely restricted. The natural and simplest thing to try is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.

Deletions:
Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). One way to do this is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.

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Edited on 2008-03-09 04:14:44 by CharlesFrancis

Additions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance. The most straightforward interaction, known as the minimal interaction, is with a vector particle, the photon. I introduce its properties and extend quantum logic to describe the behaviour of a particle whose position cannot be determined directly because it is only created or annihilated in interaction.

Deletions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance. The most straightforward interaction, known as the minimal interaction, is with a vector particle, the photon. I introduce its properties and extend quantum logic to describe the behaviour of a particle whose position cannot be determined directly because it is only created or annihilated in interaction.

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Edited on 2008-03-09 04:10:49 by CharlesFrancis

Additions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance. The most straightforward interaction, known as the minimal interaction, is with a vector particle, the photon. I introduce its properties and extend quantum logic to describe the behaviour of a particle whose position cannot be determined directly because it is only created or annihilated in interaction. Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). One way to do this is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.

Deletions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance. The most straightforward interaction, known as the minimal interaction., is with a vector particle, the photon. I introduce its properties and extend quantum logic to describe the behaviour of a particle whose position cannot be determined directly because it is only created or annihilated in interaction.
Since the current density observable, <span class=math><i>j^a</i>(<i>x</i>), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, <span class=math><i>A^a</I>(<i>x</i>)"". One way to do this is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.

---

Edited on 2008-03-09 04:10:05 by CharlesFrancis

Additions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance. The most straightforward interaction, known as the minimal interaction., is with a vector particle, the photon. I introduce its properties and extend quantum logic to describe the behaviour of a particle whose position cannot be determined directly because it is only created or annihilated in interaction.

Deletions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance.The most straightforward interaction, known as the minimal interaction., is with a vector particle, the photon. I introduce its properties.

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Edited on 2008-03-09 04:08:29 by CharlesFrancis

Additions:
The possibilities for interactions between Dirac particles and other matter are limited by covariance.The most straightforward interaction, known as the minimal interaction., is with a vector particle, the photon. I introduce its properties.
Since the current density observable, <span class=math><i>j^a</i>(<i>x</i>), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, <span class=math><i>A^a</I>(<i>x</i>). One way to do this is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the [[http://en.wikipedia.org/wiki/Photon photon]] at the limit of experimental accuracy. Zero mass is assumed. Because the photon commutator vanishes, the time evolution of the expectation of the photon field is trivial. Physical laws depend on derivatives of the photon field, not directly on <span class=math><i>A</i>. It is required that <a href=http://www.teleconnection.info/rqg/Interactions#TheLocalityCondition>locality</a>"»" is obeyed.

Deletions:
Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). One way to do this is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.
The value of the expectation of the photon field is hidden by gauge invariance. As a result, physical laws depend on derivatives of the photon field, not directly on A. It is required that locality is obeyed.

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Edited on 2008-03-09 03:33:57 by CharlesFrancis

Additions:

Definition:  The photon field operator is

Deletions:

Definition:  The photon field operator is

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Edited on 2008-03-09 03:32:54 by CharlesFrancis

Additions:

←  The Photon Field Operator  →

The Photon Field Operator ↑ Classical Electromagnetism →

Deletions:

←  The Photon Field Operator  →

The Photon Field Operator ↑ Feynman Diagrams →

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Edited on 2008-03-09 03:31:50 by CharlesFrancis

Deletions:

The Classical Field

A(x) has the required properties of an observable that it is a Hermitian operator appearing in the Hamiltonian density.

Definition:  The classical field, , is the expectation of the photon field operator, A(x).
It follows from Ehrenfest’s theorem that

Theorem:  The classical field, , satisfies the Lorenz gauge condition^»,

Proof:  Since the equal time commutator is zero, and using the Gupta-Bleuler gauge condition,

The Lorenz gauge condition fixes gauge up to the unobservable light-like polarisation. In classical electrodynamics one may choose a different gauge without affecting predictions, but here Lorenz gauge is fixed by the requirement of a first order covariant equation.

Classical Electromagnetism

In keeping with the idea that particles are the fundamental building blocks of matter, and have behaviour constrained by quantum theory and relativity, classical electromagnetism has not been assumed in this account. To show that classical electromagnetism is the consequence of particle interactions we need to show that is a conserved current density, and that Maxwell’s equations follow from the interaction in which a photon is emitted from or absorbed by the a Dirac particle.

Theorem:  The classical electromagnetic field, satisfies Maxwell’s Equations in Lorenz gauge:

Proof:
Differentiating the expectation of the photon field twice, using Ehrenfest’s theorem

Using the Hamiltonian density for qed, I(x) = ej(x) · A(x),

The equal time commutor for photons is

Maxwell’s equations in Lorenz gauge follow immediately.

Corollary:  is a classical conserved current density,

Proof: Partial derivatives commute (Clairaut’s Theorem). Differentiate Maxwell’s equations and use the Lorenz gauge condition. This can also be proved directly by calculating the commutator of the Hamiltonian with the current density operator, and using properties of Dirac spinors.
I have given Maxwell’s equations in terms of the classical vector field, . More commonly these are expressed in terms of the components of the Faraday tensor.

Definition:  The Faraday Tensor,

has components

The standard form of Maxwell’s equations follow.

Maxwell’s Equations:

Gauss’s law:		electrostatic
Ampère-Maxwell law:		electrodynamic
Faraday’s law:		magnetodynamic
Gauss’s law:		magnetostatic

(Proof). To convert to SI units», divide space derivatives (i.e. and B) by c, put , , and use ε₀μ₀ = c.
At no point has electromagnetism been assumed. Maxwell’s equations have been found as the consequence of an underlying structure consisting of particles and simple particle interactions. The general requirements of a theory of measurement within such a structure have lead to quantum theory and relativity. In turn, this has lead to spin-½ Dirac particles and the vector photon. The most straightforward interaction between these particles has yielded Maxwell’s equations. To complete the demonstration that classical electromagnetism is due to the transmission of photons between charged particles we also need to show the Lorentz force law.

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Edited on 2008-03-09 03:16:45 by CharlesFrancis

Additions:
The Photon Field Operator ↑ Feynman Diagrams →

Deletions:
The Photon Field Operator ↑ Feynman Diagrams →

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Oldest known version of this page was edited on 2008-03-09 03:15:47 by CharlesFrancis []

←  The Photon Field Operator  →

Photons

Since the current density observable, j^a(x), is a vector, a covariant theory can be found by contracting it with another Hermitian vector operator, A^a(x). One way to do this is to introduce a particle with a spin index which transforms as a vector, and which is its own antiparticle, i.e. its creation and annihilation operators appear in the same field operator. Vector particles may have non-zero mass, but empirical evidence is that this is not so for the photon» at the limit of experimental accuracy. Zero mass is assumed.


With this Hamiltonian density photons are always either created or destroyed in interaction. We cannot, therefore, talk of the measurements of the position of a photon, but only of measurement of the position at which it was annihilated, or the position at which it was created. x is not the position of a photon, but rather the position at which a charged particle would be found to have emitted or absorbed a photon, if a measurement were carried out.

Plane Wave Photon States

Since momentum is a conserved quantity, it is possible to talk about the measured momentum of a photon state. A photon created with a given momentum will be annihilated with the same momentum. So, it will be required that plane wave states are an orthogonal basis. First define a basis for spin states.


The scalar, λ, is required because the states refer to the hypothetical measurement of position of the electron which emits a photon, not the position at which a photon can be measured. Photons are always created or annihilated in interaction, and cannot be in eigenstates of a position operator. It is not meaningful to annihilate a photon at the instant of its creation. We do not require that states, , are orthogonal. Direction is determined by the distribution of matter, not by fundamental assumption, so λ depends only on the magnitude of p.

We require that is a delta function,
where η(0) = −1 and η(r) = 1 for r = 1, 2, 3. The minus sign from η(0) does not alter the expansion of the inner product for an orthonormal basis. The braket for the photon is,
The resolution of unity takes the form,
We do not have ; the braket is not positive definite, in conflict with the calculation of probabilities. In practice, we only need to generate probabilities for observations. We impose the condition that, in observations on the photon, there is no polarisation between time-like and longitudinal states,
With this restriction, probabilities for the observation time-like and longitudinal states are zero, and the braket reduces to
which is positive semidefinite». It will be seen that all four polarisation states are required for the derivation of Maxwell’s equations». We can conclude that the unobservable states have a real effect, and represent real particles, but the probability interpretation allows only the observation of a subspace containing the two transverse polarisation states, on which the inner product is positive definite. The braket is invariant under the addition of a light-like polarisation state, from which it follows that light-like polarisation cannot be determined from experimental results.

We require that the probability for the creation of a photon at x and its annihilation at y is covariant. Observe that
Then, setting
gives
which is covariant, as required.

Evolution of Photon States

We may expand using a basis of plane wave states,
Then the wave function for the state is
Since p is the momentum vector for a zero mass particle, the wave function satisfies a Klein-Gordon Equation,
Conservation of probability applies to the creation and annihilation of particles. Differentiating gives a first order equation as required by Stone’s theorem,

The Photon Field Operator

The creation operators for a plane wave state is given by . Substituting gives the photon field operator,


Proof: Differentiate directly.


Proof: Differentiate and use absence of polarisation between light-like and longitudinal states.

Photons are Bosons, obeying commutation relations,
Substituting p → −p in the second term gives the equal time commutator,
Since the integral is invariant, the commutator is zero outside the light cone, satisfying locality.

The Locality Condition for Photons

The value of the expectation of the photon field is hidden by gauge invariance. As a result, physical laws depend on derivatives of the photon field, not directly on A. It is required that locality is obeyed.


Proof:   Differentiating,
and
Substitute p → −p at x⁰ = y⁰. Then, for i = 1, 2, 3,
and, for the time component,
The integrals are invariant, so they are zero outside the light cone.

The Classical Field

A(x) has the required properties of an observable that it is a Hermitian operator appearing in the Hamiltonian density.


It follows from Ehrenfest’s theorem that


Proof:  Since the equal time commutator is zero, and using the Gupta-Bleuler gauge condition,
The Lorenz gauge condition fixes gauge up to the unobservable light-like polarisation. In classical electrodynamics one may choose a different gauge without affecting predictions, but here Lorenz gauge is fixed by the requirement of a first order covariant equation.

Classical Electromagnetism

In keeping with the idea that particles are the fundamental building blocks of matter, and have behaviour constrained by quantum theory and relativity, classical electromagnetism has not been assumed in this account. To show that classical electromagnetism is the consequence of particle interactions we need to show that is a conserved current density, and that Maxwell’s equations follow from the interaction in which a photon is emitted from or absorbed by the a Dirac particle.


Proof:
Differentiating the expectation of the photon field twice, using Ehrenfest’s theorem
Using the Hamiltonian density for qed, I(x) = ej(x) · A(x),
The equal time commutor for photons is
Maxwell’s equations in Lorenz gauge follow immediately.


Proof: Partial derivatives commute (Clairaut’s Theorem). Differentiate Maxwell’s equations and use the Lorenz gauge condition. This can also be proved directly by calculating the commutator of the Hamiltonian with the current density operator, and using properties of Dirac spinors.

I have given Maxwell’s equations in terms of the classical vector field, . More commonly these are expressed in terms of the components of the Faraday tensor.


The standard form of Maxwell’s equations follow.


(Proof). To convert to SI units», divide space derivatives (i.e. and B) by c, put , , and use ε₀μ₀ = c.

At no point has electromagnetism been assumed. Maxwell’s equations have been found as the consequence of an underlying structure consisting of particles and simple particle interactions. The general requirements of a theory of measurement within such a structure have lead to quantum theory and relativity. In turn, this has lead to spin-½ Dirac particles and the vector photon. The most straightforward interaction between these particles has yielded Maxwell’s equations. To complete the demonstration that classical electromagnetism is due to the transmission of photons between charged particles we also need to show the Lorentz force law.

The Photon Field Operator ↑ Feynman Diagrams →