Approaches to Quantum Gravity

164 Questions and answersnotion of locality. What is your attitude to symmetries, for example, to localLorentz invariance, coming from these considerations? Do you expect the novelphenomenology you are searching for to be Lorentz violating (as in the conceptuallysimilar condensed state matter models I mentioned), or is this unclear? Ifso, what about the arguments against Lorentz violation coming from effectivefield theory?– A-F.Markopoulou:1. An example of microcausality and macrocausality that are what I called“directly related” is when the macro-system is obtained by coarse-grainingthe microscopic one. It is well-known that coarse-graining is a method withgreat limitations. For example, the classical 2d Ising system can be solved bycoarse graining, but we would not use this method to extract spin waves froma spin chain. The noiseless subsystem construction illustrates this and addsanother level of complication in the relationship between micro and macrothat can be traced to entanglement. This does not mean that there will beno relation whatsoever between the micro and macro systems and I agreethat your example should constitute a constraint for the effective theory. Themicrocausality is necessary simply because it is present in any dynamicalsystem. However, it does not need to have a geometric form and your exampleof the condensed matter system is exactly an instance of this.2. I expect violation of Lorentz invariance. As we have learned in recent years,there is a variety of ways to break Lorentz invariance with distinguishableexperimental signatures. We do not yet know what kind of violations ourscenario leads to. This is a question to be investigated in a specific modelimplementation of the mechanism outlined here, such as Quantum Graphity.As for effective field theory arguments, it is not clear that they are constraining.EFT relies on assumptions such as CPT invariance that may or may nothold in the quantum theory of gravity.• Q-D.Oriti-toF.Markopoulou:If the Einstein equations emerge as identities between the geometric degreesof freedom and the matter degrees of freedom, both identified with coherentexcitations of an underlying discrete and pre-geometric system, there is noroom in the theory (and in the world) for anything like “off-shell” propagationof gravity degrees of freedom, i.e. for purely ‘quantum’ or virtual propagatinggravitational fluctuations, or geometric fluctuations of spacetime. Is thiswhat you expect? Why? Or do you expect this to be true only if the underlyingquantum pre-geometric system is in some sense “in equilibrium”, so thatthe Einstein equations would represent something like an equation of state àla Jacobson, that are however violated when the system is even slightly out ofequilibrium?