  Freely decaying relativistic force-free turbulence is studied for the first
time. We initiate the magnetic field at a short wavelength and simulate its
relaxation toward equilibrium on two and three dimensional periodic domains, in
both helical and non-helical settings. Force-free turbulent relaxation is found
to exhibit an inverse cascade in all settings, and in 3D to have a magnetic
energy spectrum consistent with the Kolmogorov $5/3$ power law. 3D relaxations
also obey the Taylor hypothesis; they settle promptly into the lowest energy
configuration allowed by conservation of the total magnetic helicity. But in
2D, the relaxed state is a force-free equilibrium whose energy greatly exceeds
the Taylor minimum, and which contains persistent force-free current layers and
isolated flux tubes. We explain this behavior in terms of additional
topological invariants that exist only in two dimensions, namely the helicity
enclosed within each level surface of the magnetic potential function. The
speed and completeness of turbulent magnetic free energy discharge could help
account for rapidly variable gamma-ray emission from the Crab Nebula, gamma-ray
bursts, blazars, and radio galaxies.
