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Arrow of Causality and Quantum Gravity.

John F Donoghue¹, Gabriel Menezes²

¹Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, USA.

Physical Review Letters

|November 9, 2019

Summary

Quantum field theory conventions define causality direction. Mixing these conventions, seen in some quantum gravity theories, violates microcausality, with stable states then defining the arrow of causality.

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Area of Science:

Theoretical Physics
Quantum Field Theory
Quantum Gravity

Background:

Causality in quantum field theory relies on field commutators for spacelike separations.
Standard definitions do not inherently establish a direction for causal effects.
Quantization conventions implicitly define the arrow of causality (past vs. future).

Purpose of the Study:

To explore the connection between quantization conventions and the arrow of causality.
To investigate the implications of mixed quantization conventions within a single theory.
To examine the role of stable states in defining causality in such theories.

Main Methods:

Analysis of field commutators and quantization conventions.
Theoretical examination of microcausality violation.
Identification of theories exhibiting mixed causality conditions.

Main Results:

Mixing quantization conventions leads to a violation of microcausality.
In theories with mixed conventions, stable states determine the arrow of causality.
This mixed causality condition is relevant to quantum gravity theories like quadratic gravity and asymptotic safety.

Conclusions:

The choice of quantization conventions is crucial for defining the arrow of causality.
Theories with mixed conventions present unique challenges to understanding causality.
Implications for quantum gravity theories require further investigation.