Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

The Montonen-Olive Duality Conjecture concerns a field theory in which these particles exist as solitons; that theory was developed by 't Hooft and Polyakov. The original hypothesis of these particles' existence requires the use of two separate vector potentials and a singularity, and was developed by Paul Dirac. Gauss's Law of Magnetism, which states that the divergence of a magnetic field is zero, implies that these particles cannot exist. For 10 points, name these hypothetical particles that can only have an isolated North or South pole.
Science