Tim van Beek
Reputation
2,460
Next privilege 2,500 Rep.
Create tag synonyms
 Jan 27 comment Haag's theorem and practical QFT computations Dear Lubos, Haag's theorem assumes a certain mathematical framework, Renormalization is about subtracting an "infinite constant" from the Hamiltonian, which is not a mathematically well defined operation. In this sense Renormalization avoids Haag's theorem by employing a not (yet) well defined mathematical framework. You are of course entitled to state that Renormalization explains how everything works, but from a formal, mathematical point of view there is a gap in our understanding what Renormalization is, mathematically (ignoring Connes-Kreimer here). Jan 27 comment What is a maximal analytic extension? The last sentence seems to indicate that you are just confused by the wording "analytic extension", choosing a different chart that does not exhibit coordinate singularities where another one does, does not have a direct relationship to "extending" a complex function, although both topics are about removing "artificial singularities" coming from an inadequate chosen representation (a chart or a representation of a complex function). Jan 27 comment Newton's Bucket Sorry, I don't understand your question. In GR we have a spacetime, it is a solution to the Einstein field equations. Any mass distribution influences this solution, therefore any mass will influence the local gravitational field and therefore will influence what the local reference frame is (unless, of course, it does not because the mass and the point we are looking at are for ever spacelike separated, for example). Jan 27 answered Haag's theorem and practical QFT computations Jan 27 answered Newton's Bucket Jan 27 awarded Supporter Jan 27 comment Is a normal-ordered product of free fields at a point a Wightman field? Hi there, this software is unfit to host ongoing discussions, so I'll be brief: Making sense of $\phi(x) \phi(x)$ as a Wightman field would, that's my educated guess, result in the very first construction of an interacting Wightman theory. I doubt that it can be done. Products of distributions can be defined, however, under certain circumstances. Note that in the two point function for a Wightman field the product is actually a tensor product (you feed every factor one test function separately). Jan 26 awarded Editor Jan 26 revised Is a normal-ordered product of free fields at a point a Wightman field? added 771 characters in body Jan 26 answered Is a normal-ordered product of free fields at a point a Wightman field? Jan 26 comment Is causality a formalised concept in physics? @Nigel: No, as far as I know there is no deeper formalization of causality in physics, as Lubos already said. Maybe one could add that the time evolution of physical systems is described by hyperbolic evolution equations and that the causal relationship of events is preserved via the appropriate representations of the Poincare group, and that's it. Jan 25 answered Is causality a formalised concept in physics? Jan 24 awarded Teacher Jan 24 answered Why are von Neumann Algebras important in quantum physics?