We show that the ghost-free, 5 degree of freedom, Wess-Zumino (1970) massive gravity model is acausal. By analyzing its characteristics, we demonstrate that shock wave solutions exhibit superluminal behavior. Ironically, this pathology arises from the very constraint that removes the Boulware-Deser ghost mode.
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INTRODUCTIONOver four decades ago, Isham, Salam and Strathdee proposed a 2-tensor "f -g" theory [1] by adding to the Einstein action that of a second vierbein, f µ m , plus a non-derivative coupling term, leaving a single common coordinate invariance. Of particular interest is the limit of non-dynamical (say flat) f , giving a finite range to the gravitons due to the coupling "mass" term. It was rapidly shown [2] however, that unlike their linearized massive spin 2 Fierz-Pauli (FP) limits, these models suffered from a ghost problem: generic non-linearities reinstate a 6th degree of freedom (DoF), beyond the linearized 2s+1 = 5 DoF, one of which is necessarily ghost-like. A final twist, also from that time, was Wess and Zumino's [3] discovery of a distinguished set of f -g mass terms of which at least one is immune from this disease, keeping 5 DoF. Because [3] was only published without detail in lecture notes, it remained unknown. Separately, other analyses showed that the linearized theory's matter coupling seemed to suffer a "vDVZ" discontinuity [4], as well as a failure of the Birkhoff theorem [5]. Hence, the subject remained moribund until the recent (independent) rediscovery [6] of the results [3] plus two new f -g models. This exhumation has, unsurprisingly, generated an immense industry (see the recent survey [7]). Our purpose is to re-inter f -g. We will show that the first, 5 DoF, Wess-Zumino, model is acausal. Our methods also show that of the two remaining such models [6], one is definitely acausal and the other likely so [8] [23]. Paradoxically, acausalities arise precisely because of the very constraint that removes the ghost. Note that there is no conflict between acausality and ghostlessness, as witness the old "charged" higher (s > 1) spin interactions with Maxwell and gravity, say those of s = (3/2, 2) [13][14][15], that are also invalidated only by acausality.Our results will be obtained by using the method of characteristics, analyzing the constraints' shock wave discontinuities, in particular that of the "5th" scalar one that results from combining the trace and double divergence of the field equations, just as is done in the linear FP model, to find a derivative-free constraint.
THE MODEL AND THE FIFTH CONSTRAINTOur concrete 5 DoF model iswhere all indices are moved by the dynamical metric g µν and its associated vierbein e µ m ; in particular f µν is the fixed background vierbein f µ m times e νm , and is manifestly symmetric on-shell. Vanishing of its antisymmetric part yields 6 conditions. Taking the reference f µ m field as the flat bein is a popular choice but is not physically required; in fact, our results, both for acausality and the absence of the sixth ghost mo...