could someone explain the following sentence to me?(adsbygoogle = window.adsbygoogle || []).push({});

"First-order perturbation correction is often

precluded because of a symmetry principle operating

for the state under consideration."

"In second-order, on the other hand, the perturbation

has to connect the given state with some other state

and back, and therefore, its contribution is no longer

inhibited by symmetry."

I can comprehend the mathematical skills of the perturbation theory.

However, depending on the circumstance I do not understand the physical

basis how first-order perturbation can become zero due to symmetry.

2.

peopel say the cause of degeneracy is related with symmetry. then how is

the symmetry related with degeneracy ?

Since I am a Korean, please explain your answer in qualitative simple phrases.

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# [Q]perturbation theory

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