Non-commutativity of space-time and the hydrogen atom spectrum

There has been disagreement in the literature on whether the hydrogen atom spectrum receives any tree-level correction due to non-commutativity. Here we shall clarify this issue and show that indeed a general argument on the structure of the proton as a non-elementary particle leads to the appearance of such corrections. As a showcase, we evaluate the corrections in a simple non-relativistic quark model with a result in full agreement with the previous one we had obtained by considering the electron moving in the external electric field of proton. Thus the previously obtained bound on the non-commutativity parameter, θ<(104 GeV)-2, using the Lamb shift data, remains valid. (orig.)