  Spin liquids are conventionally described by gauge theories with a vector
gauge field. However, there exists a wider class of spin liquids with higher
rank tensors as the gauge variable. In this work, we focus on (3+1)-dimensional
spin liquids described by U(1) symmetric tensor gauge theories, which have
recently been shown to be stable gapless spin liquids. We investigate the
particle structure of these tensor gauge theories and find that they have deep
connections with the "fracton" models recently discovered by Vijay, Haah, and
Fu. Tensor gauge theories have more conservation laws than the simple charge
conservation law of rank 1 theories. These conservation laws place severe
restrictions on the motion of particles. Particles in some models are fully
immobile (fractons), while other models have particles restricted to motion
along lower-dimensional subspaces.
