Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session C16: Loop Quantum Gravity and History of Physics 
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Sponsoring Units: DGRAV Chair: Ivan Agullo, Louisiana State University Room: Sheraton Grand Ballroom I 
Saturday, April 13, 2019 1:30PM  1:42PM 
C16.00001: Equivalence of models in loop quantum cosmology and group field theory Sean T Crowe, Martin Bojowald, Bekir Baytas The paradigmatic models often used to highlight cosmological features of loop quantum gravity and group field theory are shown to be equivalent, in the sense that they are different realizations of the same model given by harmonic cosmology. The loop version of harmonic cosmology is a canonical realization, while the group field version is a bosonic realization. The existence of a large number of bosonic realizations suggests generalizations of models in group field cosmology. 
Saturday, April 13, 2019 1:42PM  1:54PM 
C16.00002: Towards the Understanding of Quantum Cosmology from Loop Quantum Gravity BaoFei Li, Parampreet Singh, Anzhong Wang Loop quantum cosmology (LQC) provides an elegant resolution of the big bang singularity by a quantum bounce in the deep Planck era. Now an important issue that has remained open is its connection with loop quantum gravity (LQG). In this talk, I shall first give a brief review on various different approaches proposed so far, and then present our recent results of the cosmological dynamics resulting from an effective Hamiltonian, lately derived in LQG using Thiemann's regularization. Although the resulted quantum difference equation is of the fourthorder, a nonsingular bounce occurs generically. The corresponding dynamics can be described by either the Hamilton's or the FriedmannRaychaudhuri equations, but the map between the two descriptions is not onetoone. A careful analysis resolves the tension on symmetric versus asymmetric bounce in this model, showing that the bounce must be asymmetric and symmetric bounce is physically inconsistent, in contrast to the standard LQC. In addition, the current observations only allow a scenario where the prebounce branch is asymptotically de Sitter and the postbounce branch yields the classical general relativity. 
Saturday, April 13, 2019 1:54PM  2:06PM 
C16.00003: Emergent Dimension in BackgroundIndependent Quantum Gravity Kassahun Betre Nonperturbative quantum gravity has to be a theory independent of any underlying background spacetime geometry. In general, a background independent quantum theory will have to be defined fully relationally without any reference to external auxiliary or background fields. Understanding the mechanism by which geometric notions such as dimension and curvature could emerge as low energy effective degrees of freedom from a backgroundindependent quantum gravity will shed light on nonperturbative quantum gravity. In Quantum Graphity, a fully relational, backgroundindependent quantum system is defined on graphs and low energy Einstein geometry is expected to emerge through statistical mechanics. However, the mechanism by which low energy spacetime geometry emerges is unclear due to two main challenges. First, the graphtheoretic notions that correspond to geometric concepts of dimension and curvature are not clearly understood. Second, the Hamiltonian producing the statistical mechanical dynamics that can lead to low energy Einstein geometry is unknown. In this talk, we demonstrate that an Isingmodellike Hamiltonian dynamics with recursively defined graph dimension (Knill dimension), can produce a geometric dimension and uniform Knill curvature in the low energy limit. 
Saturday, April 13, 2019 2:06PM  2:18PM 
C16.00004: Classical axisymmetric gravity in real Ashtekar variables Jorge Pullin We set up axisymmetric general relativity in terms of real 
Saturday, April 13, 2019 2:18PM  2:30PM 
C16.00005: Loop Quantum Gravity on lattice: toward effective dynamics Andrea Dapor A derivation of the classical limit of LQG  leading to the emergence of GR  is still missing. In this talk, I will consider a family of LQG coherent states that represent discrete spatial geometries. I will investigate the quantum timeevolution of one such state, and discuss that it can be approximated by an effective timeevolution of the corresponding spatial geometry. In this sense, a discrete spacetime geometry emerges dynamically within LQG. Finally, I will discuss some examples: isotropic and anisotropic (Bianchi I) cosmologies; interior of a spherical black hole; gravitational waves on Minkowski background. 
Saturday, April 13, 2019 2:30PM  2:42PM 
C16.00006: Syracuse 196061 – A burgeoning international relativity community Donald C Salisbury There gathered under Peter G. Bergmann’s leadership at Syracuse University in the years 1960 – 61 a cohort of innovative relativists who played crucial roles in the development of general relativity theory in the latter half of the twentieth century. Perhaps the most significant breakthroughs occurred in the analysis of gravitational radiation and associated procedures for constructing solutions of Einstein’s equations. Included in this category are complex techniques which are most naturally described using spinors. Related techniques, based on assumed metric symmetries, established the foundations that led to the discovery of exact solutions representing rotating and electrically charged black holes. Participants included J. Goldberg, Kerr, Komar, Newman, Penrose, Robinson, Sachs, Schücking, and Trautman – with direct links to England, Germany, New Zealand, and Poland – and indirect connections to Belgium and France.

Saturday, April 13, 2019 2:42PM  2:54PM 
C16.00007: Wheeler and the two Japanese avatars Dieter R Brill The 1950's were a period of important changes in concepts and attitudes on spacetime physics, culminating in the socalled Revival (Renaissance) of General Relativity. This was the time when J. A. Wheeler burst on the scene, seemingly without previous credits in the field, but with a multitude of new and fresh research ideas. These involved a radical change of thinking, from particles to fields as the fundamental building blocks of nature. There are no publications documenting this transition, when he was of two minds between particles and fields, except one littleknown lecture he gave in connection with the 1953 International Conference of Theoretical Physics in Tokyo, a lecture that was published only in Japanese translation[1]. How could Wheeler present the two contrasting views to an audience accustomed to hear unique answers rather than Wheelerian speculation? He mentioned great scientists like Mach and Einstein, but most importantly he called upon two heroes of Japanese history whom he could question and from whom he could learn. We will see how they responded to the challenge, and also comment on the similarly challenging problems encountered in translating Wheeler's paper back into English. [1] J. Wheeler, Proceedings of the Physical Society of Japan 9, 36 (1954). 
Saturday, April 13, 2019 2:54PM  3:06PM 
C16.00008: Quantization Conditions, 1900–1927: From Ambiguity to Uncertainty Michel H P Janssen I give a brief overview of the development of quantization conditions from Planck to Heisenberg. It is widely accepted by now that Planck did not quantize the energy of his resonators when he introduced the constant named after him. Planck’s work, however, did sow the seeds for Sommerfeld's later phaseintegral approach to generalizing Bohr's model of the hydrogen atom. Schwarzschild connected this approach to some powerful techniques from celestial mechanics. The resulting theory, now remembered as the old quantum theory, was less than a decade old when it ran into insurmountable difficulties. In his famous "Umdeutung" [= reinterpretation] paper, Heisenberg showed that the way out of the impasse was to replace singlecomponent quantities occurring in the classical laws by manycomponent ones, which Born and Jordan soon recognized to be matrices. Following this "Umdeutung" procedure, Heisenberg, Born and Jordan rewrote the basic quantization condition of the old quantum theory in the form of the now familiar commutation relations for position and momentum. In 1927, drawing on Jordan's unification of matrix mechanics and Schrödinger's wave mechanics, Heisenberg showed that these commutation relations express what we now know as the uncertainty principle. 
Saturday, April 13, 2019 3:06PM  3:18PM 
C16.00009: Using the SLAC 8 GeV Spectrometer to Probe Nucleon Structure, 1968–1986 Michael Riordan The year 2019 can be viewed as the 50th anniversary of the discovery of quarks, as two pivotal papers on deepinelastic electronproton scattering were published in Physical Review Letters that October. But it would take another five years or more before the physics community became fully convinced that quarks existed. A pivotal detector facility involved in this discovery process was the SLAC 8 GeV Spectrometer, on which I performed my MIT Ph.D. and postdoctoral research. Unlike the 20 GeV Spectrometer used in the initial deepinelastic scattering experiments, it could readily roll out to large angles and detect electrons that had scattered at high momentum transfers Q^{2}, enabling experimenters to test and confirm the structurefunction scaling predictions of Bjorken and Feynman, which proved crucial in verifying the suggested pointlike nucleon substructure. This highly flexible detector allowed physicists their first detailed look at the new “hardscattering” regime discussed by Andrew Pickering in his 1984 book Constructing Quarks. If time permits, I will discuss the use of this spectrometer in separating the two nucleon structure functions W_{1 }and W_{2 }and, equivalently, determining the ratio R = σ_{L}/σ_{T}, which was the subject of my Ph.D. thesis and later research. 
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