scholarly journals Unitarity and Information in Quantum Gravity: A Simple Example

Author(s):  
Lautaro Amadei ◽  
Hongguang Liu ◽  
Alejandro Perez

In approaches to quantum gravity, where smooth spacetime is an emergent approximation of a discrete Planckian fundamental structure, any effective smooth field theoretical description would miss part of the fundamental degrees of freedom and thus break unitarity. This is applicable also to trivial gravitational field (low energy) idealizations realized by the use of Minkowski background geometry which, as with any other spacetime geometry, corresponds, in the fundamental description, to infinitely many different and closely degenerate discrete microstates. The existence of such microstates provides a large reservoir q-bit for information to be coded at the end of black hole evaporation and thus opens the way to a natural resolution of the black hole evaporation information puzzle. In this paper we show that these expectations can be made precise in a simple quantum gravity model for cosmology motivated by loop quantum gravity. Concretely, even when the model is fundamentally unitary, when microscopic degrees of freedom irrelevant to low-energy cosmological observers are suitably ignored, pure states in the effective description evolve into mixed states due to decoherence with the Planckian microscopic structure. Moreover, in the relevant physical regime these hidden degrees of freedom do not carry any “energy” and thus realize, in a fully quantum gravitational context, the idea (emphasized before by Unruh and Wald) that decoherence can take place without dissipation, now in a concrete gravitational model strongly motivated by quantum gravity. All this strengthens the perspective of a quite conservative and natural resolution of the black hole evaporation puzzle where information is not destroyed but simply degraded (made unavailable to low-energy observers) into correlations with the microscopic structure of the quantum geometry at the Planck scale.

1999 ◽  
Vol 14 (24) ◽  
pp. 1667-1672 ◽  
Author(s):  
T. PADMANABHAN

The question of how tightly one can constrain the microscopic theory of quantum gravity from the known features of low energy gravity is addressed. To begin with, from the very fact that our universe made a transition from a quantum regime to classical one, it is possible to conclude that infinite number of degrees of freedom had to be integrated out from the fundamental theory to obtain the low energy Einstein Lagrangian. Further constraints can be imposed from the fact that the quantum state describing a black hole has to possess certain universal form of density of states, in any microscopic description of space–time, which can be ascertained from general considerations. Since a black hole can be formed from the collapse of any physical system with a low energy (E ≪ Ep) Hamiltonian H, it is possible to obtain the form the effective high energy (E ≫ Ep) Hamiltonian from general consideration. These results provide the physical reasons for some of the mathematical features underlying string theories and other models for quantum gravity.


2013 ◽  
Vol 28 (07) ◽  
pp. 1350022 ◽  
Author(s):  
ROBERTO ONOFRIO

We conjecture that weak interactions are peculiar manifestations of quantum gravity at the Fermi scale, and that the Fermi constant is related to the Newtonian constant of gravitation. In this framework one may understand the violations of fundamental symmetries by the weak interactions, in particular parity violations, as due to fluctuations of the spacetime geometry at a Planck scale coinciding with the Fermi scale. As a consequence, gravitational phenomena should play a more important role in the microworld, and experimental settings are suggested to test this hypothesis.


2008 ◽  
Vol 2008 ◽  
pp. 1-12 ◽  
Author(s):  
Leonardo Modesto

We calculate modifications to the Schwarzschild solution by using a semiclassical analysis of loop quantum black hole. We obtain a metric inside the event horizon that coincides with the Schwarzschild solution near the horizon but that is substantially different at the Planck scale. In particular, we obtain a bounce of theS2sphere for a minimum value of the radius and that it is possible to have another event horizon close to ther=0point.


2005 ◽  
Vol 20 (26) ◽  
pp. 6039-6049 ◽  
Author(s):  
XIN ZHANG

A toy model based upon the q-deformation description for studying the radiation spectrum of black hole is proposed. The starting point is to make an attempt to consider the space–time noncommutativity in the vicinity of black hole horizon. We use a trick that all the space–time noncommutative effects are ascribed to the modification of the behavior of the radiation field of black hole and a kind of q-deformed degrees of freedom are postulated to mimic the radiation particles that live on the noncommutative space–time, meanwhile the background metric is preserved as usual. We calculate the radiation spectrum of Schwarzschild black hole in this framework. The new distribution deviates from the standard thermal spectrum evidently. The result indicates that some correlation effect will be introduced to the system if the noncommutativity is taken into account. In addition, an infrared cutoff of the spectrum is the prediction of the model.


2005 ◽  
Vol 14 (12) ◽  
pp. 2301-2305
Author(s):  
JOHN SWAIN

Black hole thermodynamics suggests that the maximum entropy that can be contained in a region of space is proportional to the area enclosing it rather than its volume. We argue that this follows naturally from loop quantum gravity and a result of Kolmogorov and Bardzin' on the the realizability of networks in three dimensions. This represents an alternative to other approaches in which some sort of correlation between field configurations helps limit the degrees of freedom within a region. It also provides an approach to thinking about black hole entropy in terms of states inside rather than on its surface. Intuitively, a spin network complicated enough to imbue a region with volume only lets that volume grow as quickly as the area bounding it.


2014 ◽  
Vol 2014 ◽  
pp. 1-9 ◽  
Author(s):  
Deyou Chen ◽  
Zhonghua Li

Hawking’s calculation is unable to predict the final stage of the black hole evaporation. When effects of quantum gravity are taken into account, there is a minimal observable length. In this paper, we investigate fermions’ tunnelling from the charged and rotating black strings. With the influence of the generalized uncertainty principle, the Hawking temperatures are not only determined by the rings, but also affected by the quantum numbers of the emitted fermions. Quantum gravity corrections slow down the increases of the temperatures, which naturally leads to remnants left in the evaporation.


2020 ◽  
Vol 29 (14) ◽  
pp. 2043012
Author(s):  
Tejinder P. Singh

We start from classical general relativity coupled to matter fields. Each configuration variable and its conjugate momentum, as also spacetime points are raised to the status of matrices [equivalently operators]. These matrices obey a deterministic Lagrangian dynamics at the Planck scale. By coarse-graining this matrix dynamics over time intervals much larger than Planck time, one derives quantum theory as a low energy emergent approximation. If a sufficiently large number of degrees of freedom get entangled, spontaneous localisation takes place, leading to the emergence of classical spacetime geometry and a classical universe. In our theory, dark energy is shown to be a large-scale quantum gravitational phenomenon. Quantum indeterminism is not fundamental, but results from our not probing physics at the Planck scale.


2018 ◽  
Vol 27 (14) ◽  
pp. 1846002 ◽  
Author(s):  
Alejandro Perez ◽  
Daniel Sudarsky ◽  
James D. Bjorken

The value of the cosmological constant is explained in terms of a noisy diffusion of energy from the low energy particle physics degrees of freedom to the fundamental Planckian granularity which is expected from general arguments in quantum gravity. The quantitative success of our phenomenological model is encouraging and provides possibly useful insights about physics at the scale of quantum gravity.


2017 ◽  
Vol 26 (12) ◽  
pp. 1742002 ◽  
Author(s):  
Samir D. Mathur

Suppose we assume that (a) information about a black hole is encoded in its Hawking radiation and (b) causality is not violated to leading order in gently curved spacetime. Then, we argue that spacetime cannot just be described as a manifold with a shape; it must be given an additional attribute which we call “thickness.” This thickness characterizes the spread of the quantum gravity wave functional in superspace — the space of all three-geometries. Low energy particles travel on spacetime without noticing the thickness parameter, so they just see an effective manifold. Objects with energy large enough to create a horizon do note the finite thickness; this modifies the semiclassical evolution in such a way that we avoid horizon formation and the consequent violation of causality.


Science ◽  
2020 ◽  
Vol 370 (6513) ◽  
pp. 198-202
Author(s):  
Mark Van Raamsdonk

In the anti–de Sitter/conformal field theory approach to quantum gravity, the spacetime geometry and gravitational physics of states in some quantum theory of gravity are encoded in the quantum states of an ordinary nongravitational system. Here, I demonstrate that this nongravitational system can be replaced with an arbitrarily large collection of noninteracting systems (“bits”) placed in a highly entangled state. This construction makes manifest the idea that spacetime geometry emerges from entanglement between the fundamental degrees of freedom of quantum gravity and that removing this entanglement is tantamount to disintegrating spacetime. This setup also reveals that the entangled states encoding spacetimes may be well represented by a certain type of tensor network in which the individual tensors are associated with states of small numbers of bits.


Sign in / Sign up

Export Citation Format

Share Document