Tomorrow we'll hear from Alexander Schmidhuber, who did a semester project with Esteban on  "A covariant extension of the Page-Wootters mechanism to spacetime". See the abstract below. 

We can try a hybrid meeting for those who are at ETH and interested; I've booked F31.1. The speaker will join us by zoom, so don't feel compelled to join in person. The zoom link is the usual one, https://ethz.zoom.us/j/362994444.

Best,

Joe

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Abstract:
Every observation in physics is made with respect to a frame of reference. In any concrete scenario, this reference frame is constituted by a physical system and thus subject to quantum phenomena such as entanglement and superposition. A relativistic description of the laws of physics hence needs to take into account such quantum reference frames. In this thesis, we investigate a mechanism for temporal quantum reference frames developed by Page and Wootters, and discuss its limitations. We show that two a-priori different approaches to overcoming Kuchar's criticisms, developed by Giovannetti, Lloyd & Maccone and Höhn, Smith & Lock, are equivalent. Building upon this result, we construct a covariant extension of the Page-Wootters mechanism to spacetime and derive the corresponding quantum reference frame transformations for states and operators.