>>11881891A quantum field theory is usually written in terms of ``bare'' quantities, like the bare electron mass, or the bare interaction constants. However, these quantities can never be measured (in an interacting theory), as every particle always carries a ``cloud'' of excited fields with it that. What one can do instead is to combine the bare particle and its cloud into an effective compound object with a new mass and new interaction constants. This process is called renormalization.
However, this is only easy for weakly interacting theories (like quantum electrodynamics). Things are more difficult for the strong force, for example.
