68,816 research outputs found

### Stochastic Gravity

We give a summary of the status of current research in stochastic
semiclassical gravity and suggest directions for further investigations. This
theory generalizes the semiclassical Einstein equation to an Einstein-Langevin
equation with a stochastic source term arising from the fluctuations of the
energy-momentum tensor of quantum fields. We mention recent efforts in applying
this theory to the study of black hole fluctuations and backreaction problems,
linear response of hot flat space, and structure formation in inflationary
cosmology. To explore the physical meaning and implications of this stochastic
regime in relation to both classical and quantum gravity, we find it useful to
take the view that semiclassical gravity is mesoscopic physics and that general
relativity is the hydrodynamic limit of certain spacetime quantum
substructures. Three basic issues - stochasticity, collectivity, correlations-
and three processes - dissipation, fluctuations, decoherence- underscore the
transformation from quantum micro structure and interaction to the emergence of
classical macro structure and dynamics. We discuss ways to probe into the high
energy activity from below and make two suggestions: via effective field theory
and the correlation hierarchy. We discuss how stochastic behavior at low energy
in an effective theory and how correlation noise associated with coarse-grained
higher correlation functions in an interacting quantum field could carry
nontrivial information about the high energy sector. Finally we describe
processes deemed important at the Planck scale, including tunneling and pair
creation, wave scattering in random geometry, growth of fluctuations and forms,
Planck scale resonance states, and spacetime foams.Comment: Latex 35 pages, to be published in Int. J. Theor. Phys. (1999

### Nonequilibrium Quantum Fields in Cosmology: Comments on Selected Current Topics

Concepts of quantum open systems and ideas of correlation dynamics in
nonequilibrium statistical mechanics, as well as methods of closed-time-path
effective action and influence functional in quantum field theory can be
usefully applied for the analysis of quantum statistical processes in
gravitation and cosmology. We raise a few conceptual questions and suggest some
new directions of research on selected currrent topics on the physics of the
early universe, such as entropy generation in cosmological particle creation,
quantum theory of galaxy formation, and phase transition in inflationary
cosmology.Comment: IASSNS-94/78, UMDPP-95-051, LATEX 16 pages (Invited Talk given at the
Second Journee Cosmologie, Observatorie de Paris, June 2-4, 1994.

### Fluctuation, Dissipation and Irreversibility in Cosmology

We discuss the appearance of time-asymmetric behavior in physical processes
in cosmology and in the dynamics of the Universe itself. We begin with an
analysis of the nature and origin of irreversibility in well-known physical
processes such as dispersion, diffusion, dissipation and mixing, and make the
distinction between processes whose irreversibility arises from the stipulation
of special initial conditions, and those arising from the system's interaction
with a coarse-grained environment. We then study the irreversibility associated
with quantum fluctuations in cosmological processes like particle creation and
the `birth of the Universe'. We suggest that the backreaction effect of such
quantum processes can be understood as the manifestation of a
fluctuation-dissipation relation relating fluctuations of quantum fields to
dissipations in the dynamics of spacetime. For the same reason it is shown that
dissipation is bound to appear in the dynamics of minisuperspace cosmologies.
This provides a natural course for the emergence of a cosmological and
thermodynamic arrow of time and suggests a meaningful definition of
gravitational entropy. We conclude with a discussion on the criteria for the
choice of coarse-grainings and the stability of persistent physical structures.
Invited Talk given at the Conference on The Physical Origin of Time-Asymmetry
Huelva, Spain, Oct. 1991, Proceedings eds. J. J. Halliwell, J. Perez-Mercader
and W. H. Zurek, Cambridge University Press, 1993Comment: 31pp, UMDPP #93-5

### Can Spacetime be a Condensate?

We explore further the proposal that general relativity is the hydrodynamic
limit of some fundamental theories of the microscopic structure of spacetime
and matter, i.e., spacetime described by a differentiable manifold is an
emergent entity and the metric or connection forms are collective variables
valid only at the low energy, long wavelength limit of such micro-theories. In
this view it is more relevant to find ways to deduce the microscopic
ingredients of spacetime and matter from their macroscopic attributes than to
find ways to quantize general relativity because it would only give us the
equivalent of phonon physics, not the equivalents of atoms or quantum
electrodyanmics. It may turn out that spacetime is merely a representation of
collective state of matter in some limiting regime of interactions, which is
the view expressed by Sakharov. In this talk, working within the conceptual
framework of geometro-hydrodynamics, we suggest a new way to look at the nature
of spacetime inspired by Bose-Einstein Condensate (BEC) physics. We ask the
question whether spacetime could be a condensate, even without the knowledge of
what the `atom of spacetime' is. We begin with a summary of the main themes for
this new interpretation of cosmology and spacetime physics, and the `bottom-up'
approach to quantum gravity. We then describe the `Bosenova' experiment of
controlled collapse of a BEC and our cosmology-inspired interpretation of its
results. We discuss the meaning of a condensate in different context. We
explore how far this idea can sustain, its advantages and pitfalls, and its
implications on the basic tenets of physics and existing programs of quantum
gravity.Comment: 12 pages Latex. Added some references and footnotes pertaining to
work of authors on related theme

### Gravitational Decoherence, Alternative Quantum Theories and Semiclassical Gravity

In this report we discuss three aspects: 1) Semiclassical gravity theory
(SCG): 4 levels of theories describing the interaction of quantum matter with
classical gravity; 2) Alternative Quantum Theories: Discerning those which are
derivable from general relativity (GR) plus quantum field theory (QFT) from
those which are not; 3) Gravitational Decoherence: Derivation of a master
equation and examination of the assumptions which led to the claims of
observational possibilities. We list three sets of corresponding problems
worthy of pursuit: a) Newton-Schr\"odinger Equations in relation to SCG; b)
Master equation of gravity-induced effects serving as discriminator of 2); and
c) Role of gravity in macroscopic quantum phenomena.Comment: 18 pages. Invited talk at the Second International Conference on
Emergent Quantum Mechanics, Vienna, October 3-6, 2013. Note: this arXiv
version is more up to date than that which will appear in J. Phys. (Conf.
Ser.

### Statistical Mechanics and Quantum Cosmology

Statistical mechanical concepts and processes such as decoherence,
correlation, and dissipation can prove to be of basic importance to
understanding some fundamental issues of quantum cosmology and theoretical
physics such as the choice of initial states, quantum to classical transition
and the emergence of time. Here we summarize our effort in 1) constructing a
unified theoretical framework using techniques in interacting quantum field
theory such as influence functional and coarse-grained effective action to
discuss the interplay of noise, fluctuation, dissipation and decoherence; and
2) illustrating how these concepts when applied to quantum cosmology can alter
the conventional views on some basic issues. Two questions we address are 1)
the validity of minisuperspace truncation, which is usually assumed without
proof in most discussions, and 2) the relevance of specific initial conditions,
which is the prevailing view of the past decade. We also mention how some
current ideas in chaotic dynamics, dissipative collective dynamics and
complexity can alter our view of the quantum nature of the universe.Comment: Essay published in 1990 Conference Proceedings is reprinted here with
no alteration nor reference update. It is antecedent to related reviews in
gr-qc/9302025, gr-qc/9403061, gr-qc/951107

### General Relativity as Geometro-Hydrodynamics

In the spirit of Sakharov's `metric elasticity' proposal, we draw a loose
analogy between general relativity and the hydrodynamic state of a quantum gas.
In the `top-down' approach, we examine the various conditions which underlie
the transition from some candidate theory of quantum gravity to general
relativity. Our emphasis here is more on the `bottom-up' approach, where one
starts with the semiclassical theory of gravity and examines how it is modified
by graviton and quantum field excitations near and above the Planck scale. We
mention three aspects based on our recent findings: 1) Emergence of stochastic
behavior of spacetime and matter fields depicted by an Einstein-Langevin
equation. The backreaction of quantum fields on the classical background
spacetime manifests as a fluctuation-dissipation relation. 2) Manifestation of
stochastic behavior in effective theories below the threshold arising from
excitations above. The implication for general relativity is that such
Planckian effects, though exponentially suppressed, is in principle detectable
at sub-Planckian energies. 3) Decoherence of correlation histories and quantum
to classical transition. From Gell-Mann and Hartle's observation that the
hydrodynamic variables which obey conservation laws are most readily decohered,
one can, in the spirit of Wheeler, view the conserved Bianchi identity obeyed
by the Einstein tensor as an indication that general relativity is a
hydrodynamic theory of geometry. Many outstanding issues surrounding the
transition to general relativity are of a nature similar to hydrodynamics and
mesoscopic physics.Comment: Latex 18 pages. Expanded version of an invited talk given at the
Second Sakharov International Symposium, Lebedev Physical Institute, Moscow,
May 20-24, 199

### Hawking-Unruh Thermal Radiance as Relativistic Exponential Scaling of Quantum Noise

The Hawking-Unruh effect of thermal radiance from a black hole or observed by
an accelerated detector is usually viewed as a geometric effect related to the
existence of an event horizon. Here we propose a new viewpoint, that the
detection of thermal radiance in these systems is a local, kinematic effect
arising from the vacuum being subjected to a relativistic exponential scale
transformation. This kinematic effect alters the relative weight of quantum
versus thermal fluctuations (noise) between the two vacua. This approach can
treat conditions which the geometric approach cannot, such as systems which do
not even have an event horizon. An example is the case of an observer whose
acceleration is nonuniform or only asymptotically uniform. Since this approach
is based on concepts and techniques of non-equilibrium statistical mechanics,
it is more adept to dynamical problems, such as the dissipation, fluctuation,
and entropy aspects of particle creation and phase transitions in black hole
collapse and in the early universe.Comment: Latex 13 pages; Invited talk at the Fourth International Workshop on
Thermal Field Theory and Applications, Dalian, China, August, 1995.
Proceedings edited by Y. X. Gui and F. C. Khanna (World Scientific,
Singapore, 1996

### Decoherence of Two-Level Systems Can Be Very Different From Brownian Particles

In quantum computation, it is of paramount importance to locate the parameter
space where maximal coherence can be preserved in the qubit system. In recent
years environment-induced decoherence based the quantum Brownian motion (QBM)
models have been applied to two level systems (2LS) interacting with an
electromagnetic field, leading to the general belief that 2LS are easily
decohered. In a recent paper C. Anastopoulos and B. L. Hu [Phys. Rev. A62,
(2000) 033821] derived a new exact non-Markovian master equation at zero
temperature, from which they showed that this belief is actually misplaced. For
a two-level atom (2LA)- electromagnetic field (EMF) system the decoherence time
is rather long, comparable to the relaxation time. Theoretically this is
because the dominant interaction is the $\hat \sigma_{\pm}$ type of coupling
between the two levels (what constitutes the qubit) and the field, not the
$\hat \sigma_z$ type, which shows the QBM behavior. Depending on the coupling
the field can act as a resonator (in an atom cavity) or as a bath (in QBM) and
produce very different decoherent behavior in the system. This is not new to
Cavity QED experimentalists: the 2LA-EMF system maintaining its coherence in
sufficiently long duration is the reason why they can manipulate them so well
to show interesting quantum coherence effects.Comment: Latex 15 pages Invited talk given at the Workshop on Mechanisms of
Decoherence, University of Texas, Austin. Oct. 2001 Summary of PRA (2000)
paper with Charis Anastopoulo

### Fractal Spacetimes in Stochastic Gravity? -- Views from Anomalous Diffusion and the Correlation Hierarchy

We explore in stochastic gravity theory whether non-Gaussian noises from the
higher order correlation functions of the stress tensor for quantum matter
fields when back-reacting on the spacetime may reveal hints of multi-scale
structures. Anomalous diffusion may depict how a test particle experiences in a
fractal spacetime. The hierarchy of correlations in quantum matter field
induces the hierarchy of correlations in geometric objects via the set of
Einstein-Langevin equations for each correlation order. This correlation
hierarchy kinetic theory conceptual framework, aided by the characteristics of
stochastic processes, may serve as a conduit for connecting the low energy
`Bottom-Up' approach with the `Top-Down' theories of quantum gravity which
predict the appearance of fractal spacetimes at the Planck scale.Comment: 16 pages. Invited talk at DICE2016 conference: "Spacetime - Matter -
Quantum Mechanics", September 201

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