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Lectures on the Infrared Structure of Gravity and Gauge Theory (1703.05448v2)

Published 16 Mar 2017 in hep-th, astro-ph.HE, gr-qc, hep-ph, math-ph, and math.MP

Abstract: This is a redacted transcript of a course given by the author at Harvard in spring semester 2016. It contains a pedagogical overview of recent developments connecting the subjects of soft theorems, the memory effect and asymptotic symmetries in four-dimensional QED, nonabelian gauge theory and gravity with applications to black holes. The lectures may be viewed online at https://goo.gl/3DJdOr. Please send typos or corrections to [email protected].

Citations (775)

Summary

  • The paper presents a novel framework linking soft theorems, asymptotic symmetries, and the memory effect as a unified infrared structure.
  • The paper details how infrared divergences in quantum electrodynamics and nonabelian gauge theories inform both gravitational waves and collider physics.
  • The paper outlines experimental implications, including gravitational memory detection and refining S-matrix approaches in complex quantum field processes.

An Essay on the Infrared Structure of Gravity and Gauge Theory

The lecture series presented by Andrew Strominger, titled "Lectures on the Infrared Structure of Gravity and Gauge Theory," provides a comprehensive overview of the intricate relationships between soft theorems, asymptotic symmetries, and the memory effect in four-dimensional quantum electrodynamics (QED), nonabelian gauge theory, and gravity. This discussion is centered around the profound equivalence that governs the infrared (IR) dynamics of theories involving massless particles, captured succinctly by the concept of the "Infrared Triangle."

Core Elements of the Infrared Triangle

1. Soft Theorems: These characterize the behavior of scattering amplitudes as a massless particle's energy approaches zero. Originally studied in the context of QED, these theorems were generalized to theories of gravity by Weinberg. Such results underscore the production of soft particles in a wide range of physical processes, providing an integral understanding of the consistency required by quantum field theories.

2. Asymptotic Symmetries: The work extends to studying symmetries or conserved charges associated with systems that possess asymptotic regions or boundaries. Pioneered in the studies of asymptotically flat spacetimes in general relativity, the BMS group, discovered by Bondi, van der Burg, Metzner, and Sachs, reveals an infinite-dimensional generalization of the Poincaré symmetry, fundamentally altering the understanding of symmetry in general relativity.

3. Memory Effect: Originally investigated in the context of gravitational physics, the memory effect describes a quantifiable permanent displacement caused by the passage of gravitational waves, an effect similarly proposed for detection at observatories like LIGO. This provides a tangible manifestation of the asymptotic symmetries tied to the IR structure.

Theoretical Implications and Developments

The insights from the infrared triangle have further implications in several domains:

  • Holography in Flat Space: The newly recognized symmetries bolster attempts to conceptualize the holographic nature of quantum gravity in asymptotically flat spacetimes, drawing parallels with Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence.
  • Collider Physics and Finite S-Matrix Concerns: Understanding IR divergences in gauge theories and their handling through symmetries can refine calculations in collider physics, where soft particles obscure direct measurements. The intricacies highlighted might guide strategies to formulate an IR finite S-matrix in theories like nonabelian gauge theories where such formulations are elusive.
  • Supergravity and Advanced Symmetry Concepts: For supersymmetric theories, the discussion extends to the implications of enhanced symmetries and potential anomalies at quantum levels, challenging traditional views of vacua uniqueness in quantum field theories and pointing towards a broader structure.

Conclusion and Outlook

Strominger's lectures delineate a fertile domain of research, revealing the unity underlying seemingly distinct physical phenomena through the lens of the infrared triangle. This unification not only revolutionizes the understanding of symmetries and fundamental processes in quantum field theories and general relativity but also paves the way for groundbreaking experimental confirmations and theoretical advancements in quantum gravity, helping demystify key aspects of the universe's underlying fabric. Future research will undoubtedly build upon these findings, further exploring the deeply intertwined principles of gauge and gravity theories and extending their impact across physics.