  Although it's widely believed that gravity should have a quantum nature like
every other force, the conceptual obstacles to constructing a quantum theory of
gravity compel us to explore other perspectives. Gravity is not like any other
force. It alone defines a universal space-time geometry, upon which quantum
fields evolve. We feel gravity because matter causes space-time to bend. Time
flows at unequal rates at different locations. The rate at which time flows,
and the causal structure it provides, may be required to have a classical
description in order for quantum theory to be well-formulated. I discuss
arguments for this proposition, but ultimately conclude that we must turn to
experiment to guide us.
