The physical observer II: Gauge and diff anomalies
  In a companion paper we studied field theory in the presence of a physical observer with quantum dynamics. Here we describe the most striking consequence of this assumption: new gauge and diff anomalies arise. The relevant cocycles depend on the observer's spacetime trajectory and can hence not appear in QFT, where this quantity is never introduced. Diff anomalies necessarily arise in every locally nontrivial, non-holographic theory of quantum gravity. Cancellation of the divergent parts of the anomalies only works if spacetime has four dimensions. 
T. A. Larsson