I am currently attending a course on Quantum Field Theory and I got into thinking how valid these theories are. As the theory attempts to describe reality only far above the Planck (length) scale, this has to be taken into account.
Another problem that occurs is, how can we measure the validity of theory? For example the new particle found at LHC in 2012 has been detected with more than 5 sigma. But such a standard deviation can not be given for a whole theory.
I remember that one my lecturers once said that QED has been validated more than QCD. As both theories describe the same scope of energies, is that a statement one can make? Is there a way to measure the validity of a theory?