The perturbation-theory tag has no wiki summary.
11
votes
2answers
420 views
Why is the second order perturbative correction to the ground state energy always down?
What is the physical/deeper reason for the second order shift of the ground state energy in time independent perturbation theory to be always down?
I know that it follows from the formula quite ...
16
votes
1answer
81 views
Asymptoticity of Pertubative Expansion of QFT
It seems to be lore that the perturbative expansion of quantum field theories is generally asymptotic. I have seen two arguments.
i)There is the Dyson instability argument as in QED, that is showing ...
1
vote
1answer
564 views
Fermi's Golden Rule and Density of States
I know Fermi's Golden Rule in the form
$$\Gamma_{fi} ~=~ \sum_{f}\frac{2\pi}{\hbar}\delta (E_f - E_i)|M_{fi}|^2$$
where $\Gamma_{fi}$ is the probability transition rate, $M_{fi}$ are the transition ...
4
votes
2answers
222 views
What does a non-perturbative theory mean?
I'm a science writer and I'm having difficulty understanding what a non-perturbative approach means. I thought I understood what perturbative meant, but in looking for explanations of ...
16
votes
4answers
589 views
Staying in orbit - but doesn't any perturbation start a positive feedback?
I am not a physicist; I am a software engineer. While trying to fall asleep recently, I started thinking about the following.
There are many explanations online of how any object stays in orbit. The ...
3
votes
0answers
186 views
Question about the perturbative renormalization group
I'm currently learning about the renormalization group (RG) in condensed matter physics and just want to clarify a couple of things:
When doing the RG transformation, there's a flow to a fixed point. ...
3
votes
3answers
323 views
References for ADM formalism and cosmological perturbation theory [closed]
What would you consider the best online resources for learning the 3+1 ADM formalism and gauge invariant perturbation theory in cosmology? (Assuming intermediate level GR and QFT familiarity)
