Time and gravity
Pro Research Analysisby 
Searched over 200M research papers
The Interplay Between Time and Gravity: A Comprehensive Overview
Canonical Quantization and the Problem of Time in Quantum Gravity
In the realm of quantum gravity, the canonical quantization approach presents a significant challenge: the state functional appears to be independent of time, complicating the physical interpretation of quantum gravity . This issue has led to various attempts to reconcile the concept of time within this framework, though many of these efforts have notable shortcomings .
Internal Time in Gravity Theories
A promising approach to addressing the time problem in canonical gravity involves the concept of internal time. This method treats reparametrization-invariant theories, described by singular Hamiltonians, as equivalent to a set of canonical evolution equations with respect to an internal time variable . This internal time, often derived from the dynamical fields of the theory, replaces the traditional parametric time evolution. For instance, in the context of gravitation, the Jordan-Brans-Dicke (JBD) action has been proposed as a means to achieve internal time evolution, with the JBD scalar interpreted as the internal time variable 23.
Time Evolution and Nonlinear Gauge Approaches
The nonlinear gauge approach to gravity offers another perspective on time evolution in the presence of gravity. This approach suggests that physical time evolution requires the breaking of internal time-translational symmetry, which can be achieved through mechanisms such as the Jordan-Brans-Dicke action . This framework allows for nontrivial time evolution, aligning with special relativity in the absence of gravity .
Cosmological Time in Unimodular Gravity
Unimodular gravity introduces a cosmological time variable through its Hamiltonian formulation, which measures the evolution of dynamical observables. This approach identifies constants of motion on null hypersurfaces, with evolution measured by a volume clock introduced by the cosmological time variable . This method provides a unique way to incorporate time into the gravitational framework.
Time-Space Duality in Quantum Gravity
In 2D quantum gravity, the concept of time-space duality is crucial for understanding superpositions and quantum uncertainties of spacetime causal relations. By identifying a global Z2 symmetry, it is shown that gravitational path integral configurations can have equal amplitude pairs with timelike and spacelike relations exchanged. This implies that any two points in a universe without boundary conditions are equally probable to be timelike or spacelike separated 56. This duality highlights the inherent uncertainties in causal relations within quantum gravity.
Gravitational Decoherence and Temporal Resolution
The interaction between clocks and gravity can be modeled using a classical information channel, known as classical-channel gravity (CCG). This model posits that gravity, as a classical force carrier, cannot entangle particles gravitationally. It focuses on the decoherence rates and temporal resolution of clock arrays, showing that gravitational redshift between a clock and a massive particle leads to a fundamental limitation in time accuracy . This model underscores the operational challenges in measuring time in the presence of gravity.
Unifying Gravity and Time
The relationship between gravity and time has been a longstanding question in theoretical physics. While Newton's and Einstein's formulations describe gravity in terms of mass and spacetime curvature, respectively, they do not fully unify gravity with time. However, by incorporating velocity or time into these equations, it is possible to achieve a unification of gravity with time, considering factors such as the mass of objects, their rotational speed, and spacetime curvature .
Hybrid Time Clock and Quantum Electrodynamics
The hybrid time clock concept extends the understanding of time as a function of gravity. This approach uses a standardized construct of measuring time, akin to Einstein's use of clocks in special relativity. The hybrid time clock aims to integrate time with gravity and quantum electrodynamics (QED), providing a new perspective on the relationship between these fundamental concepts .
Conditional Interpretation of Time in Quantum Gravity
The conditional interpretation of time in quantum gravity addresses the incompatibility between time in quantum mechanics (QM) and general relativity (GR). By introducing a dynamical time operator in relativistic quantum mechanics (RQM), it is possible to bridge this gap. This operator, derived from the canonical quantization of special relativity, offers a potential solution to the disjunction between QM and GR's notions of time .
Conclusion
The intricate relationship between time and gravity continues to be a central challenge in theoretical physics. Various approaches, from internal time concepts and nonlinear gauge theories to cosmological time variables and hybrid time clocks, offer promising avenues for integrating time into the gravitational framework. These efforts highlight the ongoing quest to unify our understanding of time and gravity, bridging the gap between quantum mechanics and general relativity.
Sources and full results
Most relevant research papers on this topic