## (In)definite Causal Structures in Quantum Theory and Gravity

Prof. Caslav Brukner, Quantum Optics and Quantum Information (IQOQI), Vienna

*This colloquium will be held at ***1 pm (note the change in time)**, 2nd November, in **Parnell 07-222**

The difficulties that arise when attempting to merge quantum theory and general relativity are so severe and have lasted for so long that one suspects they are not merely technical and mathematical but rather conceptual and fundamental in nature. One of the reasons that might have prevented the development of new ideas of how to use the theory of quantum gravity might be an insufficient discussion of thought experiments in the literature. We expect that, in a regime where both quantum theory and general relativity are relevant, the metric, and hence also the causal structure, fluctuate and become “indefinite”. But in that case, how are we to formulate a quantum theory with an “indefinite causal structure”? I will review the framework of “process matrices”, which makes it possible to describe “indefinite causal structures” and might provide a conceptual basis that is more conducive to a merger of quantum physics with general relativity. I will go on to show that “indefinite causal structures” can provide advantages in quantum communication and quantum computation and conclude with a discussion of a thought experiment that realises an instant of them by putting a massive gravitating object in a spatial superposition.