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I'm a physics grad student just trying to figure out all this stuff like everyone else.


Apr
8
answered Different approaches to calculating the Christoffel symbols
Apr
5
comment Quantum Electrodynamics
Related: math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html
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
25
comment Regulator-scheme-independence in QFT
(2) I am not sure if things give the same results independent of scheme, but perhaps in the IR they will. The fact that when you look in the PDG they tell you 'we used MS bar here' indicates it matters what scheme you use. But I have never properly understood this.
Mar
25
comment Regulator-scheme-independence in QFT
This is murky water for me for indeed, so take everything I say with a grain of salt. I think there are 2 separate issues going on. (1) Whether the physics quantities are regulator independent. Like I said before, to my knowledge this is the content of the CS equation - we literally a physical quantity that we can measure and take a derivative with respect to mu and say this has got to be zero.
Mar
25
comment Regulator-scheme-independence in QFT
Assuming your regulator preserved the symmetries of the model, isn't this sort of what the Callan Symanzik equation does for us? It essentially says no physical quantity can't depend on how we did our regulation. Although I might be misreading your question.
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.
Mar
16
asked What is the status of Witten's and Vafa's argument that the QCD vacuum energy is a minimum for zero $\theta$ angle?