 If it is to remain fundamentally classical, then there is the simple question of what such a classical gravitational field would couple to: the quantum properties? That seems problematic for the reasons given above. Moreover, given the form of the Einstein field equations, with a classical c-number on the left hand side, that would mean equating a c-number with a q-number (i.e. a quantum operator). The standard way out of this problem is to couple the gravitational field instead to the expectation value of the stress-energy tensor of some quantized matter field. The expectation value is a c-number. There have been a variety of arguments and no-go theorems against this so-called semi-classical gravitational theory, most of which replay the kind of argument invoking violations of the uncertainty relations sketched above (see Eppley and Hannah 1977, and Page and Geilker 1981). Basically, the upshot of the Eppley and Hannah paper is that, given the coexistence of classical gravity and quantum fields, two things can happen upon a gravitational field measurement: on the one hand the quantum wavefunction could collapse, in which case there momentum non-conservation. On the other hand, the measurement could leave the quantum wavefunction in a coherent state, in which case signals can be sent faster than light. Mattingly (2006) argues that, when properly analyzed, the thought experiments employed by Eppley and Hannah violate basic physical principles involving the construction of the equipment that would be needed to make the necessary field measurements — however, while not viewing the original semi-classical approach as a viable option, Mattingly argues that an extension of the approach has the potential to reveal a viable theory of micro-gravity (see Mattingly 2010 and 2014). Adrian Kent has recently argued that general hybrid classical/quantum theories (including those involving gravity) need not allow superluminal signalling or violate relativity (Kent 2018).