3,359 research outputs found
Quantum-Gravity Phenomenology: Status and Prospects
Over the last few years part of the quantum-gravity community has adopted a
more optimistic attitude toward the possibility of finding experimental
contexts providing insight on non-classical properties of spacetime. I review
those quantum-gravity phenomenology proposals which were instrumental in
bringing about this change of attitude, and I discuss the prospects for the
short-term future of quantum-gravity phenomenology.Comment: 28 pages, LaTex, invited Brief Review to appear in a a special issue
of Modern Physics Letters A devoted to the First IUCAA Meeting on the
Interface of Gravitational and Quantum Realm
Observers and Measurements in Noncommutative Spacetimes
We propose a "Copenhagen interpretation" for spacetime noncommutativity. The
goal is to be able to predict results of simple experiments involving signal
propagation directly from commutation relations. A model predicting an energy
dependence of the speed of photons of the order E/E_Planck is discussed in
detail. Such effects can be detectable by the GLAST telescope, to be launched
in 2006.Comment: 10 pp; v2: equivalence of observers explicitely stated; v3: minor
changes, references and remarks added, burst spreading with energy emphasized
as a signature rather than nois
Particle Creation from Vacuum by Lorentz Violation
It is shown that the vacuum state in presence of Lorentz violation can be
followed by a particle-full universe that represents the current status of the
universe. In this model the modification in dispersion relation (Lorentz
violation) is picked up representing the regime of quantum gravity. The result
can be interpreted such that the existence of the particles is an evidence for
quantum effects of gravity in the past. It is concluded that only the vacuum
state is sufficient to appear the matter fields spontaneously after the process
of semi-classical analysis.Comment: 9 pages, 2 figure
Lorentz invariance with an invariant energy scale
We propose a modification of special relativity in which a physical energy,
which may be the Planck energy, joins the speed of light as an invariant, in
spite of a complete relativity of inertial frames and agreement with Einstein's
theory at low energies. This is accomplished by a non-linear modification of
the action of the Lorentz group on momentum space, generated by adding a
dilatation to each boost in such a way that the Planck energy remains
invariant. The associated algebra has unmodified structure constants, and we
highlight the similarities between the group action found and a transformation
previously proposed by Fock. We also discuss the resulting modifications of
field theory and suggest a modification of the equivalence principle which
determines how the new theory is embedded in general relativity
Relativistic Planck-scale polymer
Polymer quantum mechanics has been studied as a simplified picture that
reflects some of the key properties of Loop Quantum Gravity; however, while the
fate of relativistic symmetries in Loop Quantum Gravity is still not
established, it is usually assumed that the discrete polymer structure should
lead to a breakdown of relativistic symmetries. We here focus for simplicity on
a one-spatial-dimension polymer model and show that relativistic symmetries are
deformed, rather than being broken. The specific type of deformed relativistic
symmetries which we uncover appears to be closely related to analogous
descriptions of relativistic symmetries in some noncommutative spacetimes. This
also contributes to an ongoing effort attempting to establish whether the
"quantum-Minkowski limit" of Loop Quantum Gravity is a noncommutative
spacetime.Comment: 5 pages, no figures. v2: minor changes in Section I
Time Uncertainty in Quantum Gravitational Systems
It is generally argued that the combined effect of Heisenberg principle and
general relativity leads to a minimum time uncertainty. Most of the analyses
supporting this conclusion are based on a perturbative approach to
quantization. We consider a simple family of gravitational models, including
the Einstein-Rosen waves, in which the (non-linearized) inclusion of gravity
changes the normalization of time translations by a monotonic energy-dependent
factor. In these circumstances, it is shown that a maximum time resolution
emerges non-perturbatively only if the total energy is bounded. Perturbatively,
however, there always exists a minimum uncertainty in the physical time.Comment: (4 pages, no figures) Accepted for publication in Physical Review
Loop Quantum Gravity Modification of the Compton Effect
Modified dispersion relations(MDRs) as a manifestation of Lorentz invariance
violation, have been appeared in alternative approaches to quantum gravity
problem. Loop quantum gravity is one of these approaches which evidently
requires modification of dispersion relations. These MDRs will affect the usual
formulation of the Compton effect. The purpose of this paper is to incorporate
the effects of loop quantum gravity MDRs on the formulation of Compton
scattering. Using limitations imposed on MDRs parameters from Ultra High Energy
Cosmic Rays(UHECR), we estimate the quantum gravity-induced wavelength shift of
scattered photons in a typical Compton process. Possible experimental detection
of this wavelength shift will provide strong support for underlying quantum
gravity proposal.Comment: 12 pages, 2 eps figures, revised versio
Quantum Corrections to Lorentz Invariance Violating Theories: Fine-Tuning Problem
It is of general agreement that a quantum gravity theory will most probably
mean a breakdown of the standard structure of space-time at the Planck scale.
This has motivated the study of Planck-scale Lorentz Invariance Violating (LIV)
theories and the search for its observational signals. Yet, it has been
recently shown that, in a simple scalar-spinor Yukawa theory, radiative
corrections to tree-level Planck-scale LIV theories can induce large Lorentz
violations at low energies, in strong contradiction with experiment, unless an
unnatural fine-tuning mechanism is present. In this letter, we show the
calculation of the electron self-energy in the framework given by the
Myers-Pospelov model for a Lorentz Invariance Violating QED. We find a
contribution that depends on the prefered's frame four-velocity which is not
Planck-scale suppressed, showing that this model suffers from the same disease.
Comparison with Hughes-Drever experiments requires a fine-tuning of 21 orders
of magnitude for this model not to disagree with experiment.Comment: 10 pages, no figures. Version acceptd in Physics Letters
On the true nature of renormalizability in Horava-Lifshitz gravity
We argue that the true nature of the renormalizability of Horava-Lifshitz
gravity lies in the presence of higher order spatial derivatives and not in the
anisotropic Lifshitz scaling of space and time. We discuss the possibility of
constructing a higher order spatial derivatives model that has the same
renormalization properties of Horava-Lifshitz gravity but that does not make
use of the Lifshitz scaling. In addition, the state-of-the-art of the Lorentz
symmetry restoration in Horava-Lifshitz-type theories of gravitation is
reviewed.Comment: Latex file in Revtex style, 5 pages, no figures. v2: references
added, version accepted for publication in Foundations of Physic
Generalized Uncertainty Principle and Quantum Electrodynamics
In the present work the role that a generalized uncertainty principle could
play in the quantization of the electromagnetic field is analyzed. It will be
shown that we may speak of a Fock space, a result that implies that the concept
of photon is properly defined. Nevertheless, in this new context the creation
and annihilation operators become a function of the new term that modifies the
Heisenberg algebra, and hence the Hamiltonian is not anymore diagonal in the
occupation number representation. Additionally, we show the changes that the
energy expectation value suffers as result of the presence of an extra term in
the uncertainty principle. The existence of a deformed dispersion relation is
also proved.Comment: Acceptted in General Relativity and Gravitatio
- …