Complex sentence: Greene's area of research is string theory, a candidate for a theory of quantum gravity, attempts to explain the different particle species of the standard model of particle physics as different aspects of a single type of one-dimensional, vibrating string.
Split into simpler sentences: Greene's area of research is string theory, a candidate for a theory of quantum gravity. String theory attempts to explain the different particle species of the standard model of particle physics as different aspects of a single type of one-dimensional, vibrating string.