Sentence:  Some of the most beautiful and thus appealing physical <WSD>theories</WSD>, including quantum electrodynamics and quantum gravity, have been dogged for decades by infinities that erupt when theorists try to prod their calculations into new domains. Getting rid of these nagging infinities has probably occupied far more effort than was spent in originating the <WSD>theories</WSD>. 
Meaning of the word marked with <WSD> tags:  A coherent statement or set of ideas that explains observed facts or phenomena and correctly predicts new facts or phenomena not previously observed, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc. (from 17th c.)