- D.37.1 Classical energy minimization
- D.37.2 Quantum energy minimization
- D.37.3 Rewriting the Lagrangian
- D.37.4 Coulomb potential energy

D.37 Forces by particle exchange derivations

D.37.1 Classical energy minimization

The energy minimization including a selecton is essentially the same as the one for only spoton and foton field. That one has been discussed in chapter A.22.1 and in detail in {A.2}. So only the key differences will be listed here.

The energy to minimize is now

So the only real difference in the variational analysis is

That means that the Poisson equation now becomes

Since the Poisson equation is linear, the solution is

The energy lowering is now

Multiplying out, you get, of course, the energy lowerings for the
spoton and selecton in isolation. But you also get two additional
interaction terms between these sarges. These two terms are equal;
the selecton field

The foton field energy is still half of the particle-field interaction energies and of opposite sign. That is why the energy change is half of what you would expect from the interaction of the particles with each other’s field: the other half is offset by changes in field energy.

D.37.2 Quantum energy minimization

This derivation includes the selecton in the spoton-fotons system analyzed in {A.22.3}. Since the analysis is essentially unchanged, only the key differences will be highlighted.

If an selecton is added to the system, the system wave function
becomes

The demon can hold the selecton in its other hand. The Hamiltonian will now of course include a term for the selecton in isolation, as well as an interaction with the foton field. These are completely analogous to the corresponding spoton terms.

So the energy to be minimized for the ground state becomes

If this is minimized as in {A.22.3}, the energy is

The square absolute value of a quantity can be found as the product of that quantity times its complex conjugate. That gives the same energy lowering as for the lone spoton, and a similar term for a lone selecton. However, there is an additional term

If you write out the inner product integrals over the selecton coordinates explicitly, this becomes

Summed over all

Except for the differences in notation, that is the same selecton-spoton interaction energy as found in {A.22.1}.

D.37.3 Rewriting the Lagrangian

The rules of engagement are as follows:

- The Cartesian axes are numbered using an index
, with1, 2, and 3 for , , andrespectively. - Also,
indicates the coordinate in the direction, , , or. - Derivatives with respect to a coordinate
are indicated by a simple subscript . - If the quantity being differentiated is a vector, a comma is used to separate the vector index from differentiation ones.
- Index
is the number immediately following in the cyclic sequence ...123123...and is the number immediately preceding . - If
is already been used for something else, can be used the same way. - Time derivatives are indicated by a subscript t.

Consider first the square magnetic field:

Expanding out the square, that is equivalent to

The summation indices can now be cyclically redefined to give an equivalent sum over

The terms can be combined in sets of three as

Here summation over

The square electric field is

All together, that gives

as can be verified by multiplying out and simplifying. The right hand side in the first line is the self-evident electromagnetic Lagrangian density, except for the factor

Pure derivatives do not produce changes in the action, as the changes in the potentials disappear on the boundaries of integration.

D.37.4 Coulomb potential energy

The Coulomb potential energy between charged particles is typically
derived in basic physics. But it can also easily be verified from the
conventional electromagnetic energy (A.143). In the steady
case, there is only the electric field, due to the Coulomb potential.
The energy may then be written as

where the first equality comes from the definition of the electric field, the second from integration by parts and the third one from the first Maxwell equation. Substitution of the Coulomb potential in terms of the charge distribution as given in {A.22.8},

now gives the Koulomb potential energy

For point charges, the charge distribution is by definition

Here

Recall that the delta function picks out the value at

That is the Coulomb potential energy

Note again that physically all the energy is inside the electromagnetic field. There is no energy of interaction of the charged particles with the field. If equal charges move closer together, they increase the energy in the electromagnetic field. That requires work.