By: Luka Vinck, Sebastian De Haro
It has recently been claimed that algebraic methods can be used to give a background-independent description of semiclassical gravity. This paper aims to provide an entry point into recent algebraic approaches to quantum gravity and to analyse and critically assess the conception of background-independence here involved. We discuss an algebraic framework for formulating an algebra of observables along the worldline of an observer in a cosmolo... more
It has recently been claimed that algebraic methods can be used to give a background-independent description of semiclassical gravity. This paper aims to provide an entry point into recent algebraic approaches to quantum gravity and to analyse and critically assess the conception of background-independence here involved. We discuss an algebraic framework for formulating an algebra of observables along the worldline of an observer in a cosmological setting. By 'gravitationally dressing' the observables to the observer's worldline, the reference to the spacetime background can be set aside at the level of the abstract *-algebra. Thus this *-algebra is taken to be independent of any particular spacetime realisation, with different states on the algebra corresponding to different backgrounds. We compare this algebraic framework to three influential notions of background-independence in the literature and argue that none of these fully captures the sense in which the *-algebra is taken to be background-independent. Building on this, we extract a novel notion of algebraic background-independence tied to the representation-independence of the *-algebra. This algebraic conception motivates two further concepts of background-independence tailored to semiclassical theories: semiclassical background-independence and perturbative background-independence. We suggest that these provide useful tools for evaluating the scope and limitations of semiclassical approaches to quantum gravity. less
By: Juliusz Doboszewski, Jamee Elder
We investigate the epistemic opacity of computer simulations and machine learning methods in the context of black hole imaging. We argue that there are forms of opacity-including opacity resulting from the use of machine learning-which do not need to affect the reliability of an inference when it is seen as a part of a broader inferential framework. We propose conditions under which that can plausibly be the case, and discuss how opaque metho... more
We investigate the epistemic opacity of computer simulations and machine learning methods in the context of black hole imaging. We argue that there are forms of opacity-including opacity resulting from the use of machine learning-which do not need to affect the reliability of an inference when it is seen as a part of a broader inferential framework. We propose conditions under which that can plausibly be the case, and discuss how opaque methods can be useful in the context of the (next generation) Event Horizon Telescope. However, we also argue that at least one problematic form of opacity is currently present in black hole imaging: GRMHD models of Sagittarius A* are opaque. This form of opacity signals the limitations of current understanding of the models of this source, and constrains the potential uses of ML models in future observations. less
