**Remaining Schedule**

Porter Williams (USC): March 14th.

Henrique Gomes (Perimeter Institute and Cambridge University): April 18th.

Sam Fletcher (UMN): May 9th

———–Upcoming Talk———–

14th March 2020, 3pm LPS Seminar room

**Porter Williams (USC), “Julian Schwinger, Renormalization, and Ineffective Field Theory”**

In the late 1940s and early 1950s, Julian Schwinger developed renormalization methods for extracting finite results from quantum electrodynamics and laid the initial foundation for postwar quantum field theory (QFT) itself. He continued to work within the framework of QFT through the mid-1960s, long after most particle theorists had shifted their hopes for modeling the strong interactions to S-matrix Theory or the methods of current algebra. During this period Schwinger developed a unique, and remarkably prescient, understanding of the physical significance of renormalization. He also adhered to methodological scruples remarkably similar to those that, in the 1970s, led particle theorists to develop the now-dominant understanding of QFTs as effective field theories. In Schwinger’s case, however, these methodological scruples led him to abandon QFT entirely in 1966 to devote himself to the development and elaboration of an alternative, largely fruitless theoretical framework for particle physics called Source Theory. In this talk, I argue that the reasons why Schwinger’s methodological scruples and understanding of renormalization led him to Source Theory, rather than effective field theory, offer insight into Schwinger’s approach to physics, the state of particle theory in the 1960s, and the meaning and significance of renormalization.

———–Previous Talks———–

15th February 2020, 3pm LPS Seminar room

**Eugene Chua and Craig Callender (UCSD), “No Time for Time from No-Time”**

Programs in quantum gravity often involve formalisms that are supposedly fundamentally timeless, with physicists claiming that time emerges from fundamentally timeless physics. In this paper, we restrict our attention to one popular approach, the semiclassical time program, which argues that time emerges from fundamentally timeless solutions to the Wheeler-DeWitt equation after applying a series of semi-classical approximations. We think there are inherent tensions in this approach: by focusing on three major components of the semiclassical approximations – the Born-Oppenheimer approximation, the WKB approximation, and decoherence – we argue that the physical justifications for applying them are laden with time. In a variety of ways, they require systems to be in time. Either we are unjustified in applying these approximations to timeless solutions (describing timeless systems) or we must assume time in the timeless solutions. The semiclassical time approach turns out to be either unjustified or circular in deriving time from no–time.

15th February 2020, 3pm LPS Seminar room

**Mike Schneider (UCI/ Notre Dame), “Stabs in the dark sector”.**

Abstract**:** In the context of ΛCDM, our current theory of large-scale cosmology, I argue that dark energy plausibly constitutes a signpost for future fundamental physics, whereas cold dark matter does not. But such an argument has several steps, beginning with getting clear on what it means for a feature within some current theory to constitute a signpost for future fundamental physics. The view I suggest we take on this front requires, for present purposes, that ΛCDM be interpreted in accordance with some or other criteria that is argued to carry normative force. I proceed to do just this, whereupon the conclusion of my argument ultimately follows fairly quickly from the interpretation of ΛCDM I will have just given. Time pending, I will then discuss a curious upshot of the particular route taken to my conclusion: that, in contrast with dark energy, it seems we may stand to genuinely learn something about cold dark matter in virtue of achieving next-generation fundamental theory, if, after all, cold dark matter turns out to have played an important role according to which that future theory comes to be developed.

7 December 2019, 3pm, LPS Seminar room

**Mario Hubert (Cal-tech), “Why the Wave-Function has to be Psi-Ontic”**

Abstract: The PBR-theorem aimed at proving that the wave-function has to represent objective features of a physical system. There have been many attempts to interpret the wave-function as not representing the objective physical state of a quantum system by abandoning one of the assumptions of the PBR-theorem. I argue that each theory that violates either of the assumptions meets unsurmountable problems. The most severe is to give up objective reality.

9 November 2019, 3pm, LPS seminar room

**Jeff Barrett (Irvine), “****Quantum Randomness and Underdetermination”**

Abstract: We will consider the nature of quantum randomness and how one might have empirical evidence for it. We will see why, depending on one’s computational resources, it may be impossible to determine whether a particular notion of randomness properly characterizes one’s empirical data. Indeed, we will see why an ideal observer with full empirical evidence may fail to have any empirical evidence whatsoever for believing that the results of her quantum-mechanical experiments are in fact randomly determined. This illustrates a radical sort of empirical underdetermination faced by fundamentally stochastic theories like quantum mechanics.