Last week, from September 11 to September 17, I was at the DICE 2016 conference in Castiglioncello (Tuscanny). It is a very nice conference which brings together researchers from usually non-overlapping communities. It thus gives a pretty good overview of the different routes physicists are exploring to understand quantum mechanics and possibly unify it with gravity.
Although String Theory was sometimes mentioned (G. Veneziano was a keynote speaker), most of the talks were about other approaches. This probably comes from a deliberate choice of the organizers, whose objective was to give a platform to heterodoxy, and from the fact that String Theory (or Loop Quantum Gravity for that matter) is not a very good tool to understand the subtle effects that could arise in the low energy sector of Newtonian quantum gravity. In this domain, the principal signature of the interplay between quantum mechanics and gravity is decoherence. Although the high energy characteristics of gravity such as a discrete spacetime or higher hidden dimensions would presumably have only dim cosmological signatures (and most likely never any testable consequences in accelerators), the decoherence at low energy could soon be probed in experiments (see e.g. the proposals of the group of Oriol Romero-Isart in Innsbruck arxiv, that of the group of Yanbein Chen in Caltech arxiv or that of the group of Angelo Bassi in Trieste arxiv).
Interestingly, the Newtonian limit doesn’t seem to depend much on how one chooses to address the difficulties of the UV completion of an elusive quantum theory of gravity. Rather, it is what we mean by a quantum theory of gravity that is important. Do we really want to “quantize” gravity or would a peaceful coexistence between a classical spacetime and quantum matter be enough? What meaning do we give to the quantum formalism? Are we ready to break unitarity if it makes the coupling with gravity simpler? I find these questions very interesting but I have to disclose that I am very biased having worked on the last option myself.
Having registered too late, I could only present a poster based on a previous article with Lajos Diosi. I’ll likely write a friendly introduction to the ideas developed in this article later but, in a nutshell, we essentially showed that continuous dynamical reduction models, which are an ad hoc way to solve the measurement problem, can surprisingly also be used to solve the problem of coupling a classical spacetime and quantum matter in a mathematically consistent way. If we consider that we have already paid the price of their non unitarity to kill Schrödinger’s cat, then dynamical reduction models offer a solution to semiclassical Newtonian gravity that comes for free.