What do we mean by pure gravity

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SUMMARY

The discussion focuses on the concept of "pure gravity," distinguishing it from "gravity coupled to matter." Pure gravity is characterized by a central charge (c=0), which has significant implications in theoretical physics, particularly in string theory and the Bargmann algebra. Central charges arise from normal ordering issues or when Lagrangians transform into total derivatives under symmetry transformations. This understanding is crucial for grasping the foundational aspects of gravitational theories and their mathematical frameworks.

PREREQUISITES
  • Understanding of central charge in theoretical physics
  • Familiarity with Lagrangian mechanics
  • Knowledge of string theory principles
  • Concepts of symmetry transformations in physics
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  • Research the role of central charges in string theory
  • Explore the implications of Lagrangians transforming to total derivatives
  • Study the Bargmann algebra and its applications in physics
  • Investigate normal ordering issues in quantum field theory
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The discussion is beneficial for theoretical physicists, researchers in quantum gravity, and students studying advanced concepts in gravitational theories and string theory.

unrwar
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what do we mean by "pure gravity"

1-what we mean by "pure gravity" and "gravity coupled to matter"?
2- why pure gravity characterized by c=0 (c: central charge), and what is the conceptual meaning of central charge
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Central charges appear when one has normal ordering issues, or when Lagrangians transform to total derivatives under symmetry transformations. The first situation appears in e.g. string theory upon quantization. An example of the second situation is the Bargmann algebra, which is a centrally-extended Galilei algebra; the commutator between spatial translations and boosts is then centrally extended. This is related to the fact that the Lagrangian of a non-rel. point paricle transforms under boosts as a total time derivative.
 

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