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 Apr 11 awarded Yearling Apr 11 revised Potential in Relativistic Scalar Field Theory added 908 characters in body Apr 9 answered Potential in Relativistic Scalar Field Theory Apr 8 comment Different approaches to calculating the Christoffel symbols @Gene - no problem. I am pretty sure I lost an afternoon of my life to these factors of 2 at one point, so I am happy to help. Apr 8 answered Different approaches to calculating the Christoffel symbols Apr 5 comment Quantum Electrodynamics Apr 4 comment exercise books for Feynman diagrams I am a little confused by what you mean when you say you know QFT but still need practice with Feynman diagrams. I would suggest just trying to work out the examples in textbooks like Scrednicki and Peskin, if you haven't already. Also you can peruse the web for classes people have taught where HW exercises and solutions have been posted. Better yet, just get involved with some research and you will learn what you need to know. Apr 4 answered Pauli matrices and the Levi-Civita symbol Apr 4 awarded Nice Question Apr 3 answered Density operator in second quantization Apr 1 comment B-field and Magnetic forces, speed of a particle Hint: Biot-Savart Law tells you how a moving charge creates a magnetic field. You need to find an equation that given a magnetic field, creates a force on a moving charged particle. The magnetic field in your problem is created by some other moving charges 'off stage'. Mar 30 comment Is Feynman's explanation of how the moon stays in orbit wrong? Strap the penny to a brick of C4. But carefully so that the penny only gets a horizontal velocity when you blow up the C4. In principle the penny can certainly escape the earth even though you are only giving it an horizontal velocity. Does this make anymore sense? Mar 30 answered Is Feynman's explanation of how the moon stays in orbit wrong? Mar 29 comment Calculate Capacitance in Series AC Circuits? You are trying to calculate the capacitance of an unknown resistor? I think you mistyped? Mar 29 comment Finite square well Pretty much. You can go a step further and actually try integrating $\int_{-L/2}^\infty \ dx \ e^{\kappa x}$ and you will see that you will get $\infty$, so there isn't any hope of normalizing this to anything finite. Mar 29 answered Finite square well Mar 28 comment Lorenz gauge fixing The fact that we can write down equations like this at all is because of the gauge redundancy of fields like $A^\mu$. That is, there is extra freedom in these variables as opposed to just writing things in terms of the electric and magnetic fields $\mathbf{E}$ and $\mathbf{B}$. If we choose to eliminate some of this redundancy is our choice, or the `gauge' we choose. This is, for an equation like the one you wrote above you are choosing to restrict the extra degrees of freedom in some way. Choosing a particule $\psi$ is part of this choice. Mar 28 comment Lorenz gauge fixing Gauge conditions are a constraint we choose to impose. That is, we don't solve gauge conditions for unknowns, we impose gauge conditions. Mar 20 answered A partial differential equation for kinetic energy Mar 19 comment A partial differential equation for kinetic energy I can't see where this has any utility at all. The point of having any equation, differential or algebraic, is to put constraints on a system. We then solve these equations to obtain an unknown. However, in this case, we already know the answer and the equation gives us no new information.