1. F. Károlyházy, NuovoCim. 52, 390 (1966).
2. F. Károlyházy, A. Frenkel and B. Lukács, in: Quantum concepts in space and time, eds. R. Penrose and C.J. Isham (Clarendon, London, 1986).
3. R. Penrose, ibid.
4. A.B. Komar, Int. J. Theor. Phys. 2, 257 (1969).
5. J. Bell, Phys.World 3, 33 (1990).