====== Haag's theorem ======



<tabbox Intuitive> 

<note tip>
Explanations in this section should contain no formulas, but instead colloquial things like you would hear them during a coffee break or at a cocktail party.
</note>
  
<tabbox Concrete> 

For the best explanation, see the section "How to stop worrying about Haag's theorem" in the book "The Conceptual Framework of Quantum Field Theory" by A. Duncan
 
<tabbox Abstract> 

<blockquote>In particular Haag's theorem, that one
cannot consistently define the vacuum in interacting theories, is now considered to have been the wrong way to look
at the problem, and largely disappears with considerations of RG flow.<cite>[[http://www.felixflicker.com/pdf/QFT_notes_F_Flicker.pdf|A Practical Introduction to Quantum Field Theory]] by Felix Flicker</cite></blockquote>
<tabbox Why is it interesting?> 

<blockquote>According to something called Haagís theorem there appears to be no known
consistent formalism within which interacting quantum field theory can be
expressed.

<cite>p. 115 in "An interpretative
introduction to quantum Field theory" by Teller (1995) </cite>
</blockquote>

<tabbox FAQ>

-->Why is there no problem in practical QFT calculations?#
see https://physics.stackexchange.com/questions/3983/haags-theorem-and-practical-qft-computations
<--

</tabbox>