What is Particle Theory?

The 21st century has seen tremendous progress towards understanding the elementary constituents of matter, the forces that bind them, and the organizing principles that unite them. Concerted experimental and theoretical efforts have culminated in the discovery of the Higgs boson and the observation of gravitational waves, while also excluding countless proposed new phenomena. Today the central questions of particle physics are sharper than ever – from the nature of dark matter and dark energy, to the patterns of flavor and unification, to the origin of mass and the mechanism of electroweak symmetry breaking -- but answering them will require both the energetic synthesis of existing methods and the genesis of entirely new ones.