Complex sentence: Professor Nambu had proposed a theory known as spontaneous symmetry breaking based on what was known to happen in superconductivity in condensed matter, however, the theory predicted massless particles (the Goldstone's theorem), a clearly incorrect prediction.
Split into simpler sentences: Professor Nambu had proposed a theory known as spontaneous symmetry breaking based on what was known to happen in superconductivity in condensed matter. However, the theory predicted massless particles (the Goldstone's theorem), a clearly incorrect prediction.