This week we will have talks by two visitors, Tamara Kohler from UCL and Esteban Castro Ruiz from Vienna. Also, due to a scheduling conflict, we will have to start late, at 16:45, and in a different room, H42. I hope this is not an inconvenience. 

So far I have an abstract from Esteban Castro Ruiz, which you can find below. I will send more information on Tamara Kohler's talk when it becomes available. 

Best,

-joe

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Title: Time reference frames and gravitating quantum clocks

Abstract: The standard formulation of quantum theory relies on the 
assumption of a fixed background notion of time, according to which events are localised and the order between them is well defined. In general relativity, physics is independent of any such background notion --spacetime points have no direct physical meaning. For this reason, the localisation of events in time is defined operationally, via the measurement of physical clocks. When the clocks are classical, the order between events is still well-defined. Here, we explore the problem of defining the localisation of events with respect to gravitating quantum clocks. I will present a quantum framework for “time reference frames,” describing quantum states, dynamics and measurements with respect to different quantum 
clocks, which might interact, gravitationally or otherwise, with other quantum systems. We will see that, with respect to interacting quantum clocks, the time localisability of events, as well as their order, becomes a relative concept, depending on the time reference frame. We discuss the consequences of this finding for the physical realisation of indefinite causal structures at the interplay between quantum physics and gravity.