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The Laplace–Runge–Lenz vector describes the shape and orientation of the orbit of one astronomical body around another. In general, the LRL vector is conserved (it's a constant of the motion) in all problems in which two bodies interact by a central force that varies as the inverse square of the distance between them (Kepler problem). Its conservation is significant in the quantization of the Hydrogen atom.

3 votes

Index Manipulation and Angular Momentum Commutator Relations

It is possible to continue OP's calculation as follows $$ =~i\hbar\; \epsilon_{jmn}(\epsilon_{ink}p_m L_k+\epsilon_{imk}p_k L_n) ~=~-i\hbar( \epsilon_{jkm} \epsilon_{min}+\epsilon_{kim} \epsilon_{mj …
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37 votes
Accepted

What symmetry causes the Runge-Lenz vector to be conserved?

Hamiltonian Problem. The Kepler problem has Hamiltonian $$\begin{align} H~=~&T+V, \cr T~:=~& \frac{p^2}{2m}, \cr V~:=~& -\frac{k}{q}, \end{align}\tag{1} $$ where $m$ is the 2-body reduced mass. The …
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2 votes

Textbook proof error? Runge Lenz

They say that $\vec{r} \cdot \dot{\vec{r}} = |\vec{r}|\frac{d}{dt}|\vec{r}|,$ not that $\vec{r} \cdot \dot{\vec{r}} = |\vec{r}||\frac{d}{dt}\vec{r}|.$
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3 votes

How is angular momentum defined on symplectic space?

In the Kepler-problem, note that only 5 of the 6 integrals of motion $(\vec{L},\vec{B})$ are algebraically independent, since $\vec{L}\cdot \vec{B}\equiv 0$. They parametrize a 5-dimensional submanifo …
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23 votes

Why are hydrogen energy levels degenerate in $\ell$ and $m$?

The degeneracy of energy-levels can be traced to the fact that the hydrogen atom possesses an enhanced $SO(4)$ symmetry caused by (among other things) the conservation of the Laplace-Runge-Lenz vector …
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3 votes

Conserved Quantities in Kepler Problem?

He could (and should) count the mechanical energy as a conserved quantity. He also didn't mention that the total momentum of the 2 objects is 3 integrals of motion. However, not all quantities are in …
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