  We demonstrate the tuning of the magnetic dipole-dipole interaction (DDI)
within a dysprosium Bose-Einstein condensate by rapidly rotating the
orientation of the atomic dipoles. The tunability of the dipolar mean-field
energy manifests as a modified gas aspect ratio after time-of-flight expansion.
We demonstrate that both the magnitude and the sign of the DDI can be tuned
using this technique. In particular, we show that a magic rotation angle exists
at which the mean-field DDI can be eliminated, and at this angle, we observe
that the expansion dynamics of the condensate is close to that predicted for a
non-dipolar gas. The ability to tune the strength of the DDI opens new avenues
toward the creation of exotic soliton and vortex states as well as unusual
quantum lattice phases and Weyl superfluids.
