Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 8 down vote favorite

What's so not-quantum about tree-level diagrams?

share|improve this question
From the tag of the question, you are already assuming "FIELD" theory, are you? So the queston is how classical the fields are? – user135 Dec 18 '11 at 16:34

2 Answers

up vote 7 down vote accepted

The reasons were given here. Essentially, at tree level you recover classical results. Loop corrections are proportional to powers of $\hbar$ and these are quantum terms.

share|improve this answer
It is a bit more obscure for particles with spin, because then the classical result has still an $\hbar$. – user135 Dec 18 '11 at 16:33
That is right, but classically spin does not exist and so, the result is consistent. – Jon Dec 18 '11 at 17:29
up vote 4 down vote

Tree level diagrams are one-particle relativistic quantum mechanics, but not quantum field theories.

The point is obscured by two reasonable details in modern QFT books: they avoid to speak of "2nd quantisation", and they set h=1 everywhere (so for instance it is not so clear how different the h->0 limit is for fermions than for scalar fields, or how bosons can add to build a classical electromagnetic field but fermions can not). It is expected that you go across a relativistic quantum mechanics textbook before jumping to QFT, but sometimes the career path is different.

share|improve this answer
There are relativistic quantum mechanics textbooks that are not quantum field theory texts? – Arun Nanduri Dec 20 '11 at 18:29

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.