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We show that the insulating states of magic-angle twisted bilayer graphenesupport a series of collective modes corresponding to local particle-holeexcitations on triangular lattice sites. Our theory is based on a continuummodel of the magic angle flat bands. When the system is insulating at moir\'eband filling $\nu=-3$, our calculations show that the ground state supportsseven low-energy modes that lie well below the charge gap throughout themoir\'e Brillouin zone, one of which couples strongly to THz photons. Thelow-energy collective modes are faithfully described by a model with a local$SU(8)$ degree of freedom in each moir\'e unit cell that we identify as thedirect product of spin, valley, and an orbital pseudospin. Apart from spin andvalley-wave modes, the collective mode spectrum includes a low-energyintra-flavor exciton mode associated with transitions between flat valence andconduction band orbitals.

Lattice Collective Modes from a Continuum Model of Magic-Angle Twisted Bilayer Graphene