Classical gravity
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Classical Gravity: Insights and Developments
Classical Gravity and Quantum Field Theory Integration
Recent research has explored the integration of classical gravity with quantum field theory, proposing a consistent theory that maintains the principles of general relativity while incorporating quantum mechanics. This theory is linear in the density matrix, completely positive, and trace-preserving, reducing to Einstein's equations in the classical limit. It introduces stochastic elements and probabilistic jumps in space-time and quantum fields, which inherently cause wave-function collapse without the need for a measurement postulate.
Gravity-Induced Decoherence and Classicality Emergence
The debate on whether gravity can induce decoherence and thus explain the quantum-to-classical transition has been contentious. Some researchers argue that gravity-induced decoherence, due to gravitational redshifts in superposition states, can account for this transition. However, others dispute this claim, suggesting that the observed decoherence effects are not universally valid and may not be solely attributable to gravity. This ongoing discussion highlights the complexity of understanding gravity's role in the emergence of classicality.
Loop Amplitudes and Classical Gravity Observables
Advancements in computational methods have enabled more efficient extraction of classical gravity observables from loop integrals in scattering amplitudes. By refining the soft-region method of integration, researchers have successfully computed integrals for black-hole scattering up to the second Post-Minkowskian order. These methods confirm the universality and high-energy behavior of gravitational interactions, providing deeper insights into classical general relativity.
Gravity as a Classical Channel and Dissipative Models
Models describing gravitational interaction through continuous measurement and feedback protocols have been proposed, treating gravity as a classical channel. These models can reconstruct quantum gravitational interactions at the statistical operator level but introduce decoherence effects leading to energy divergence. A dissipative generalization of these models has been suggested to address this issue, ensuring that systems thermalize to an effective temperature over time, thus avoiding energy divergence.
Universality in Massless Gravitational Scattering
Research has demonstrated the universality of the gravitational deflection angle for massless particles through high-energy limits of two-loop four-graviton scattering amplitudes. This work confirms long-standing theoretical predictions and provides a robust framework for understanding gravitational interactions in both Einstein gravity and supergravity.
Higher Derivative Terms in Classical Gravity
Incorporating higher derivative terms into the gravitational action results in multimass models of gravity, introducing massive spin-two and scalar excitations alongside the usual massless field excitations. These models yield static solutions combining Newtonian and Yukawa potentials, with observational evidence placing weak restrictions on the new masses. This approach offers a broader perspective on the dynamical content of gravitational fields.
Open Questions and Alternative Theories
Several open questions remain regarding the validity and uniqueness of the standard second-order Newton-Einstein classical gravitational theory. Observational data does not exclusively support the standard theory, and alternative theories, such as the conformal invariant fourth-order theory, also meet observational constraints without requiring dark matter. This ongoing inquiry underscores the need for a deeper understanding of the fundamental principles underlying classical gravity.
Semi-Classical Gravity and Bohmian Mechanics
An alternative approach to semi-classical gravity, based on Bohmian mechanics, has been proposed. This method treats the actual configuration of a quantum system as the matter source term in Einstein's field equations, potentially improving upon the traditional semi-classical approach. This novel perspective aims to bridge the gap between classical and quantum descriptions of gravity.
Conclusion
The study of classical gravity continues to evolve, integrating quantum mechanics, exploring decoherence effects, and refining computational methods. While significant progress has been made, many questions remain open, inviting further research and exploration into the fundamental nature of gravity and its interaction with quantum systems.
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