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Calculating Eigenkets of Perturbed Matrix for Second-Order Correction

While the equation for the second-order correction \begin{equation} E_n^{(2)} = \langle n^0 | H^1 | n^1 \rangle \end{equation} Is correct, I found that it is more useful to put that equation in its ...
PineappleThursday's user avatar
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Solutions for nonrelativistic-matter perturbations

Deep in radiation domination ($y\ll 1$), the two solutions are $\delta=1$ and $\delta=\ln a$. Physically, these solutions describe the effects of particle drift in the complete absence of peculiar ...
Sten's user avatar
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What goes wrong with strongly coupled theories?

Yes, the problem with strong coupling is typically that you will need many terms to get a decent approximation. However, it is also true that the perturbative series often does not converge. Hence, if ...
Níckolas Alves's user avatar
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In a scalar QFT, how are Feynman diagrams drawn if the interaction Lagrangian has multiple terms?

Briefly speaking, since the interaction terms enter additively in the action $$S[\phi] ~:=~\underbrace{S_2[\phi]}_{\text{quadratic part}} + \underbrace{S_{\neq 2}[\phi]}_{\text{interactions}}, \tag{1}...
Qmechanic's user avatar
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