We study a two-dimensional dilaton gravity model related by a conformal transformation of the metric to the Callan-Giddings-Harvey-Strominger model. We find that most of the features and problems of the latter can be simply understood in terms of the class

with negative mass(thus giving up the cosmic censorship conjecture).The dynamics can be now described in terms of the evolution of a timelike boundary in Minkowski space, it is therefore equivalent to the dynamical moving mirrors discussed in ref.[5].There the authors considered a boundary on the curve where e−2φ=C,where C is an arbitrary positive paring with the expression(10)one easily sees that this is equivalent to consider our solutions with negative mass given by M=−Cλwith a boundary on the curve where e−2φ=0.The runaway solutions describing a forever accelerating moving mirror found in[5]are therefore related to the infinite tower of black hole states with negative mass,i.e to spacetimes with a timelike boundary at e−2φ=0.Thus,unless one uses a cosmic censorship hypothesis to discard these states as unphysical,one has to deal with processes in which the black hole mass becomes arbitrarily negative.A similar point of view,stating the role of spacetime singularities acting as regulators forbidding the appearance of unphysical states in the spectrum,has been put forward recently by Horowitz and Myers[10].

References.

[1]C.G.Callan,S.B.Giddings,J.A.Harvey and A.Strominger,Phys.Rev.D45,

1005(1992);

[2]J.G.Russo,L.Susskind and L.Thorlacius Phys.Rev.D46,3445(1992);Phys.

Rev.D47,533(1993).

[3]D.Cangemi and R.Jackiw,Phys.Rev.D50,3913(1994);T.Fujiwara,Y.Igarashi

and J.Kubo,Phys.Lett.B316,66(1993).

[4]M.Cadoni and S.Mignemi,Phys.Rev.D(in press),hep-th9410041.

[5]T.D.Chung and H.Verlinde,Nucl.Phys.B418,305(1994).

[6]W.Rindler,Essential Relativity(Springer-Verlag,1969).

[7]R.B.Mann,Phys.Rev.D47,4438(1993);S.Mignemi,preprint INFNCA-TH-

94-28.

[8]R.B.Mann,A.Shiekh and I.Tarasov,Nucl.Phys.B341,134(1990);R.B.Mann,

Gen.Rel.Grav.24,433(1992).

[9]N.D.Birrell and P.C.W.Davies,Quantumfields in curved space(Cambridge Un.

Press,1982).

[10]G.T.Horowitz and R.C.Myers,gr-qc9503062.

6