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In non-relativistic quantum theory the Hamiltonian for a particle interacting with electromagnetic fields is $$H=\frac{(\mathbf{p}-\mathbf{A}*e/c)^2}{2m}+e\phi+\int\,d^3x \frac{\mathbf{E^2}+\mathbf{B^2}}{8\pi}.\tag{1}$$

According to Hamiltonian's equations, $$\dot{r_i}=\frac{\partial H}{\partial p_i}, \tag{2}$$ $$\dot{p_i}=-\frac{\partial H}{\partial r_i}.\tag{3}$$ They certainly can not produce the equations of motion of the particle as well as the electromagnetic fields. Where am I wrong? What are the coordinates and the canonical momentum for the fields?

Qmechanic
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yztsz
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3 Answers3

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I) The Hamiltonian for point charges and EM fields can certainly produce the EOMs of the particle(s) as well as the EM fields.

A full explanation is quite a long story. For pedagogical reasons, to see how this works, it is best to:

  1. Firstly, understand the corresponding Lagrangian formulation.

  2. Secondly, understand how the Hamiltonian formulations work for point charges and EM fields separately, see e.g. this & this Phys.SE posts.

  3. Thirdly, try to construct a Hamiltonian formulation for both point charges and EM fields together.

II) One correction: OP's Hamiltonian (1) yields the correct total energy, but OP asks how to produce Maxwell's equations. For the latter purpose, OP's Hamiltonian (1) is missing a Lagrange multiplier term that imposes Gauss' law.

III) Concretely, the minimal phase space is as follows:

  1. Particle position ${\bf r}(t)$ and particle momentum ${\bf p}(t)$: $$\{r^k(t), p_{\ell}(t)\}= \delta_{\ell}^k.$$

  2. (Minus$^1$) the electric field ${\bf E}(x)$ is the canonical conjugate variable to the magnetic gauge potential ${\bf A}(x)$: $$\{A_i({\bf x},t), E^j({\bf x}^{\prime},t)\}~=~ -\delta_i^j~\delta^3({\bf x}\!-\!{\bf x}^{\prime}).$$

  3. Lagrange multiplier $A^0(x)\equiv \phi(x)$.

IV) The equations come about as follows:

  1. The magnetic field ${\bf B}\equiv{\bf \nabla}\times {\bf A}$ is defined as the curl of the magnetic gauge potential ${\bf A}$.

  2. The Hamilton's equations for ${\bf r}$ and ${\bf p}$ yield (i) the Newton's 2nd law with a Lorentz force, and (ii) the relation between velocity $\dot{\bf r}$ and momentum ${\bf p}$.

  3. The Hamilton's equations for ${\bf A}$ and ${\bf E}$ yield (i) the Maxwell–Ampere's law, and (ii) the relation between the electric field ${\bf E}$ and the gauge potential $A_{\mu}$.

  4. The Lagrange multiplier $A^0\equiv\phi$ imposes Gauss' law.

  5. The source-free Maxwell equations follows from the existence of the gauge potential $A_{\mu}$.

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$^1$ We use $(-,+,+,+)$ Minkowski sign convention with $c=1$.

Qmechanic
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Remove the second term in the hamiltonian altogether. In the first term only consider the field of other particles.

my2cts
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Normally the EMF Hamiltonian must be written in terms of $Q$s and $P$s of the electromagnetic field which is done by representing it via harmonic oscillators. You may find decomposition of EMF into a set of harmonic oscillators in many textbooks.