On Tuesday 24 November 2020, FraMEPhys hosted a free one-day workshop on the topic of Density Matrix Realism via Zoom.

**SCHEDULE **(times GMT/UTC)

2.30-3.00pm: Katie Robertson (Birmingham)

‘An introduction to density matrix realism: what’s at stake?’

3.00-3.45pm: Owen Maroney (Oxford)

‘DMR, thermodynamic asymmetry, and the interpretation of quantum theory’

10-min break

3.55-4.40pm: Roderich Tumulka (Tuebingen)

‘Why Bohmian mechanics allows for different roles of density matrices’

20-min break

5.00-5.45pm: Eddy Keming Chen (UC San Diego)

‘The Wentaculus: Density Matrix Realism Meets the Arrow of Time’

10-min break

5.55-6.25pm: General discussion and wrap-up

**ABSTRACTS**

**Roderich Tumulka** (Tuebingen)

Why Bohmian mechanics allows for different roles of density matrices

As is well known, if you are doing a quantum experiment on a system with a random wave function Psi, then the statistics of outcomes can be expressed in terms of a density matrix rho; rho encodes information about the probability distribution mu of Psi. If you care only about computing these statistics, then mu and rho, or different mu’s with the same rho, are interchangeable for you. But in Bohmian mechanics, you need to be clear about what is real, and then several possibilities become visible: There might be a random wave function Psi with distribution mu (and then there is a fact in the world about which mu is correct), or there might just be rho but no Psi and no mu (with a modified version of Bohm’s equation of motion). So, as I will explain in my talk, Bohmian mechanics can give clear meaning to density matrix realism. This trait is, in fact, not limited to Bohmian mechanics, but applies also to other theories with a clear ontology in 3-space, such as some versions of the GRW collapse theory.

**Eddy Keming Chen** (UC San Diego)

‘The Wentaculus: Density Matrix Realism Meets the Arrow of Time’

In this talk, I explain my understanding of how to be a realist about the fundamental density matrix of the universe, and discuss the ramifications of such realism for understanding the arrow of time. Since the resultant theory is inspired by the “Mentaculus Vision” of David Albert and Barry Loewer, I call the overall pacakge “The Wentaculus,” where “W” stands for the fundamental density matrix. Unlike the Mentaculus, in the Wentaculus there is a uniquely possible initial quantum state of the universe. In this way, the Wentaculus does away with the need for a probability distribution over initial quantum states. I bring the Wentaculus to bear on several problems in the foundations of physics and the philosophy of science, including the nature of the quantum state and the status of the Past Hypothesis.