Abstract
It has long been thought that observing the effects of quantum gravity is effectively impossible, since gravity is so much weaker than other forces: consider, for instance the utterly insensible gravitational attraction of a magnet, compared with the very sensible magnetic force it exerts. But by drawing on ideas from 'quantum information theory' (QIT), and on recent experimental advances in quantum mechanics and in observing tiny gravitational fields, Bose et al (2017), and Marletto and Vedral (2017) have shown how, in principle, weak gravitational fields might have detectable quantum effects. This work has attracted a great deal of interest, as such 'BMV experiments' are tantalizingly close to current experimental physics; many experimentalists are working on the real possibility that the predicted effect could be measured in the next few years, even though the experiment would be one of the most delicate ever undertaken. But its significance would be a momentous advance in the study of quantum gravity. There are a number of conceptual issues to unpack regarding this proposal: First, in what sense would the BMV experiment count as an observation of the 'quantum nature' of gravity? First, there are theoretical issues: different, seemingly equally reasonable, theoretical commitments can lead to rather different understandings of the meaning and significance of the experiment. On the one hand, the BMV argument assumes that gravity should be modeled as a dynamical system. On the other, a gauge theoretic approach to quantizing gravity explains the effects in terms of a non-dynamical gauge constraint. Then, a further issue is the comparison with previous experiments which combined quantum and gravitational effects; what new information would we obtain from the BMV experiment, and in what senses would we gain greater practical control over quantum gravity? The philosophical issues thus concern the interpretation of physical theory, and the nature and role of experiment in science: both important topics within philosophy of physics and philosophy of science. Finally, moreover, QIT is in the first place a specific formulation of quantum mechanics, but it can involve further more specific assumptions. In the analysis of the BMV experiment in particular, one has to stipulate what it is for a system to be 'classical' rather than 'quantum'. Clearly this is an important, contentious, but undertheorized question in the philosophy of physics literature. This talk will outline the idea of the BMV experiment, and address the philosophical issues that it raises: (1) arguing that ultimately the *the* meaning of the 'quantum nature’ of a system is arbitrary to a certain extent; (2) explaining how the BMV experiment would both give an observation of the quantum nature of gravity in a deeper sense, and require more robust control over it, than previous experiments; (3) clarify the stakes in the assumptions of the QIT theorem.
