Approaches to Quantum Gravity

574 Questions and answersinsight into what a single plane wave is and how it could be measured andthis is what we did for the time of flight experiment in ref. [1] using a normalordering prescription, as explained in Section 24.5.2. I agree that some suchjustification was needed to have any valid prediction and that this is a problemthat has plagued and still plagues much of the literature on this model.Also, a general point made in the article is that noncommutative spacetimeis most likely an effective description of some limit of a deeper QuantumGravity theory. In an effective description one isolates the relevant quantitiesand their approximate behaviour without necessarily understanding the wholeof the full theory. There is more than one way that one might do this and its anarea that definitely needs more attention. Section 1.5 aims to bring out someof the issues here.• Q - D. Sudarsky - to J. Kowalski-Glikman:1. In the second paragraph below eq. (25.1) you state that one could thinkof scales in terms of synchronization. That “in SR the velocity of light isindispensable for synchronization, as it provides the only meaningful way ofsynchronizing different observers”. I do not see why. Consider two inertialobservers A and B who want to synchronize their clocks, first of all they mustfind out if they are at rest relative to each other. To do this A sends a proton (nophoton) with a given energy and asks B to return another proton with the sameenergy as the one he received. Then A compares the energy of the proton hereceives with the one that he sends, if they are the same A and B are at relativerest. To synchronize the clocks A tells B to set his clock to zero at the time itreceives the above mentioned proton, while A sets his clock to zero at midtimebetween the moments he sends the proton and he receives a proton back. Notethat there are no photons involved. So do you stand by your claim?2. Referring to that same paragraph: In the above we see that one can use thingsthat travel to synchronize clocks, and photons are certainly useful in this way,precisely because they travel, but how can one talk of using a scale – related towhat physical aspect of nature – to synchronize anything? In fact what is themeaning of momentum space synchronization? What is being synchronized?3. Is the modification of SR the only option to explain the GZK anomaly (if it isconfirmed), or are there are other alternatives?4) You have acknowledged in Section 25.6 that there are serious problems interpretingthe formalism of DSR, we do not know what to make of the orderdependence of the addition law for momenta, we do not know what is thequantity we must identify with the measured momentum, we have the spectatorproblem, etc., etc. The question is: how can we consider doing phenomenology,using a formalism that we do not know how to interpret?