The authors Chirco, Haggard, Riello, and Rovelli (CHR&R) have just posted what I think is a major paper proposing to resolve a question raised by Jacobson almost 20 years ago, in 1995. https://www.physicsforums.com/showthread.php?p=4637856#post4637856 Jacobson (as many posting here are aware) managed to DERIVE the Einstein GR equation from Thermodynamic assumptions, as if (to put it fancifully) the GR equation were simply the equation of state of something like a "gas of little molecules of geometry" jittering wiggling dividing recombining etc. So one way of posing the outstanding question raised by J's paper would be "what after all are the underlying degrees of freedom, from which the Einstein Field Equation arises as the EoS?" And does it even make sense to quantize the GR equation? Well, do we quantize the pressure wave equation by which sound propagates in air? No. It describes a bulk large scale process quite different from the behavior of individual air molecules. QM applies at the level of individual air molecules, not to the equation for sound in air. To get Jacobson's landmark 1995 paper, google "Einstein equation of state" or "Jacobson Einstein equation" or any of several other choices of words from the title http://arxiv.org/abs/gr-qc/9504004 Chirco et al present an answer to the question of what underlies the Einstein Field Equation. To get the CHR&R paper, which coming after nearly 20 years is also itself something of a landmark, google "haggard riello spacetime" or "haggard riello thermodynamics". Either one will get you http://arxiv.org/abs/1401.5262 as the first hit.