The Maximum Tension Principle in General Relativity
  I suggest that classical General Relativity in four spacetime dimensions incorporates a Principal of Maximal Tension and give arguments to show that the value of the maximal tension is $c^4 \over 4 G$. The relation of this principle to other, possibly deeper, maximal principles is discussed, in particular the relation to the tension in string theory. In that case it leads to a purely classical relation between $G$ and the classical string coupling constant $\alpha ^\prime$ and the velocity of light $c$ which does not involve Planck's constant. 
G W Gibbons