Topological qauntum field theory in 3+1D and the emergence of quantum gravity.

Topological qauntum field theory(TQFT) is a very powerful theoretical tool
to study topological phases and phase transitions. In 2 + 1D, it is well
known that the Chern-Simons theory captures all the universal topological
data of topological phases, e.g., quasi-particle braiding statistics,
chiral central charge and even provides us a deep insight for the nature of
topological phase transitions. Recently, topological phases of quantum
matter are also intensively studied in 3 + 1D and it has been shown that
loop like excitation obeys the so-called three-loop-braiding statistics. In
this talk, I will discuss a TQFT framework to understand the quantum
statistics of loop like excitation in 3 + 1D. Most surprisingly, this new
class of 3+1D TQFT even provides us a new route towards a quantum theory of
gravity.