  We give a derivation of general relativity and the gauge principle that is
novel in presupposing neither spacetime nor the relativity principle. We
consider a class of actions defined on superspace with two key properties. The
first is 3-coordinate invariance. This is the only postulated symmetry, and it
leads to the standard momentum constraint. The second property is that the
Lagrangian is constructed from a `local' square root of an expression quadratic
in the velocities, `local' because it is taken before integration over 3-space.
It gives rise to quadratic constraints that do not correspond to any symmetry
and are not, in general, propagated by the Euler-Lagrange equations. Therefore
these actions are internally inconsistent. Only one action of this form is well
behaved: the Baierlein-Sharp-Wheeler reparametrisation-invariant action for GR.
  From this viewpoint, spacetime symmetry is emergent. It appears as a `hidden'
symmetry in the (underdetermined) solutions of the evolution equations, without
being manifestly coded into the action itself. In addition, propagation of the
constraints acts as a striking selection mechanism beyond pure gravity. If a
scalar field is included in the configuration space, it must have the same
characteristic speed as gravity. Thus Einstein causality emerges. Finally,
self-consistency requires that any 3-vector field must satisfy Einstein
causality, the equivalence principle and, in addition, the Gauss constraint.
Therefore we recover the standard (massless) Maxwell equations.
