Rigorous model of classical spacetime foam (Q1367662)

The authors consider a toy model of classical spacetime for a foam, which is described mathematically by the space consisting of four intersecting 3-spaces naturally spanned by the positive Minkowski coordinate half-axes. After developing some differential geometry for such spaces, they discuss consequences of the principle of general covariance and the equivalence principle for their toy model, and show that the Lorentz spacetime foam can be a solution of suitably generalized Einstein equations.