BHI Colloquium


Tuesday, March 10, 2020, 1:30pm to 2:30pm


BHI Meeting Room

Mina Himwich (BHI)

 Title: Universal Polarization of the Photon Ring

Abstract: Black hole images are dominated by a photon ring with universal features that are completely governed by general relativity. In this talk, I will discuss how the polarimetric image of a black hole also displays universal properties. In particular, the photon ring exhibits a self-similar pattern of polarization that encodes the black hole spin. The corresponding polarimetric signatures  on long interferometric baselines allow for measurements of the black hole spin using a sparse interferometric array. 



Tomasz Bigaj (University of Warsaw)

Title: Two concepts of quantum individuation

Abstract: The quantum theory of many particles imposes well-known restrictions on the available states of systems of “indistinguishable” particles (limiting these states to symmetric for bosons and antisymmetric for fermions). This restriction raises the question of how to refer to individual particles composing these systems. I distinguish two approaches to quantum individuation: the orthodox and the heterodox ones. According to orthodoxy, individual components are picked out by the unphysical labels (indices) attached to the identical copies of Hilbert spaces in an appropriate tensor product. Consequently, this approach leads to the thesis that particles of the same type can never by discerned by any measurable properties, as argued, among others, by Steven French, Michael Redhead, Paul Teller, Jeremy Butterfield, Nick Huggett. I follow an alternative approach, based on the work of Giancarlo Ghirardi and developed later by Adam Caulton, which uses certain symmetric projectors as a way to individuate the components of multiparticle systems. In the case of two same-type particles it may be argued that when the system’s state is within the range of projector P Ä (I – P) + (I – PÄ P, where P is a single-particle projector, one particle possesses property P while the other does not possess P. This means that discernibility by properties is available to same-type particles, contrary to orthodoxy. In my talk I would like to focus on some possible objections to the unorthodox approach which are based on the “folk” interpretation of measurements and their outcomes in entangled states (of which the singlet-spin state is the most famous example). I will spell out these objections and show that they can be repelled if we pay proper attention to the role the spatial degrees of freedom play in measurement preparations. 

See also: Physics