The Paradox of Virtual Dipoles in the Einstein Action
  The functional integral of pure Einstein 4D quantum gravity admits abnormally large and long-lasting "dipolar fluctuations", generated by virtual sources with the property Int d^4x Sqrt{g(x)} Tr T(x) = 0. These fluctuations would exist also at macroscopic scales, with paradoxical consequences. We set out their general features and give numerical estimates of possible suppression processes. 
G. Modanese (C.I.P.A., Palo Alto and Univ. Bolzano)