9,721 research outputs found

### Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction

A Planck-scale minimal observable length appears in many approaches to
quantum gravity. It is sometimes argued that this minimal length might conflict
with Lorentz invariance, because a boosted observer could see the minimal
length further Lorentz contracted. We show that this is not the case within
loop quantum gravity. In loop quantum gravity the minimal length (more
precisely, minimal area) does not appear as a fixed property of geometry, but
rather as the minimal (nonzero) eigenvalue of a quantum observable. The boosted
observer can see the same observable spectrum, with the same minimal area. What
changes continuously in the boost transformation is not the value of the
minimal length: it is the probability distribution of seeing one or the other
of the discrete eigenvalues of the area. We discuss several difficulties
associated with boosts and area measurement in quantum gravity. We compute the
transformation of the area operator under a local boost, propose an explicit
expression for the generator of local boosts and give the conditions under
which its action is unitary.Comment: 12 pages, 3 figure

### The century of the incomplete revolution: searching for general relativistic quantum field theory

In fundamental physics, this has been the century of quantum mechanics and
general relativity. It has also been the century of the long search for a
conceptual framework capable of embracing the astonishing features of the world
that have been revealed by these two ``first pieces of a conceptual
revolution''. I discuss the general requirements on the mathematics and some
specific developments towards the construction of such a framework. Examples of
covariant constructions of (simple) generally relativistic quantum field
theories have been obtained as topological quantum field theories, in
nonperturbative zero-dimensional string theory and its higher dimensional
generalizations, and as spin foam models. A canonical construction of a general
relativistic quantum field theory is provided by loop quantum gravity.
Remarkably, all these diverse approaches have turn out to be related,
suggesting an intriguing general picture of general relativistic quantum
physics.Comment: To appear in the Journal of Mathematical Physics 2000 Special Issu

### A simple background-independent hamiltonian quantum model

We study formulation and probabilistic interpretation of a simple
general-relativistic hamiltonian quantum system. The system has no unitary
evolution in background time. The quantum theory yields transition
probabilities between measurable quantities (partial observables). These
converge to the classical predictions in the $\hbar\to 0$ limit. Our main tool
is the kernel of the projector on the solutions of Wheeler-deWitt equation,
which we analyze in detail. It is a real quantity, which can be seen as a
propagator that propagates "forward" as well as "backward" in a local parameter
time. Individual quantum states, on the other hand, may contain only "forward
propagating" components. The analysis sheds some light on the interpretation of
background independent transition amplitudes in quantum gravity

### Graviton propagator from background-independent quantum gravity

We study the graviton propagator in euclidean loop quantum gravity, using the
spinfoam formalism. We use boundary-amplitude and group-field-theory
techniques, and compute one component of the propagator to first order, under a
number of approximations, obtaining the correct spacetime dependence. In the
large distance limit, the only term of the vertex amplitude that contributes is
the exponential of the Regge action: the other terms, that have raised doubts
on the physical viability of the model, are suppressed by the phase of the
vacuum state, which is determined by the extrinsic geometry of the boundary.Comment: 6 pages. Substantially revised second version. Improved boundary
state ansat

### Relational evolution of the degrees of freedom of generally covariant quantum theories

We study the classical and quantum dynamics of generally covariant theories
with vanishing a Hamiltonian and with a finite number of degrees of freedom. In
particular, the geometric meaning of the full solution of the relational
evolution of the degrees of freedom is displayed, which means the determination
of the total number of evolving constants of motion required. Also a method to
find evolving constants is proposed. The generalized Heinsenberg picture needs
M time variables, as opposed to the Heisenberg picture of standard quantum
mechanics where one time variable t is enough. As an application, we study the
parameterized harmonic oscillator and the SL(2,R) model with one physical
degree of freedom that mimics the constraint structure of general relativity
where a Schrodinger equation emerges in its quantum dynamics.Comment: 25 pages, no figures, Latex file. Revised versio

### The complete LQG propagator: II. Asymptotic behavior of the vertex

In a previous article we have show that there are difficulties in obtaining
the correct graviton propagator from the loop-quantum-gravity dynamics defined
by the Barrett-Crane vertex amplitude. Here we show that a vertex amplitude
that depends nontrivially on the intertwiners can yield the correct propagator.
We give an explicit example of asymptotic behavior of a vertex amplitude that
gives the correct full graviton propagator in the large distance limit.Comment: 16 page

### Multiple-event probability in general-relativistic quantum mechanics

We discuss the definition of quantum probability in the context of "timeless"
general--relativistic quantum mechanics. In particular, we study the
probability of sequences of events, or multi-event probability. In conventional
quantum mechanics this can be obtained by means of the ``wave function
collapse" algorithm. We first point out certain difficulties of some natural
definitions of multi-event probability, including the conditional probability
widely considered in the literature. We then observe that multi-event
probability can be reduced to single-event probability, by taking into account
the quantum nature of the measuring apparatus. In fact, by exploiting the
von-Neumann freedom of moving the quantum classical boundary, one can always
trade a sequence of non-commuting quantum measurements at different times, with
an ensemble of simultaneous commuting measurements on the joint
system+apparatus system. This observation permits a formulation of quantum
theory based only on single-event probability, where the results of the "wave
function collapse" algorithm can nevertheless be recovered. The discussion
bears also on the nature of the quantum collapse

### Spacetime as a quantum many-body system

Quantum gravity has become a fertile interface between gravitational physics
and quantum many-body physics, with its double goal of identifying the
microscopic constituents of the universe and their fundamental dynamics, and of
understanding their collective properties and how spacetime and geometry
themselves emerge from them at macroscopic scales. In this brief contribution,
we outline the problem of quantum gravity from this emergent spacetime
perspective, and discuss some examples in which ideas and methods from quantum
many-body systems have found a central role in quantum gravity research.Comment: 15 pages; invited contribution to "Many-body approaches at different
scales: A tribute to Norman H. March on the occasion of his 90th birthday",
edited by G. G. N. Angilella and C. Amovilli (New York, Springer, 2017 - to
appear

### Quantum Loop Representation for Fermions coupled to Einstein-Maxwell field

Quantization of the system comprising gravitational, fermionic and
electromagnetic fields is developed in the loop representation. As a result we
obtain a natural unified quantum theory. Gravitational field is treated in the
framework of Ashtekar formalism; fermions are described by two Grassmann-valued
fields. We define a $C^{*}$-algebra of configurational variables whose
generators are associated with oriented loops and curves; ``open'' states --
curves -- are necessary to embrace the fermionic degrees of freedom. Quantum
representation space is constructed as a space of cylindrical functionals on
the spectrum of this $C^{*}$-algebra. Choosing the basis of ``loop'' states we
describe the representation space as the space of oriented loops and curves;
then configurational and momentum loop variables become in this basis the
operators of creation and annihilation of loops and curves. The important
difference of the representation constructed from the loop representation of
pure gravity is that the momentum loop operators act in our case simply by
joining loops in the only compatible with their orientaiton way, while in the
case of pure gravity this action is more complicated.Comment: 28 pages, REVTeX 3.0, 15 uuencoded ps-figures. The construction of
the representation has been changed so that the representation space became
irreducible. One part is removed because it developed into a separate paper;
some corrections adde

### The physical hamiltonian in nonperturbative quantum gravity

A quantum hamiltonian which evolves the gravitational field according to time
as measured by constant surfaces of a scalar field is defined through a
regularization procedure based on the loop representation, and is shown to be
finite and diffeomorphism invariant. The problem of constructing this
hamiltonian is reduced to a combinatorial and algebraic problem which involves
the rearrangements of lines through the vertices of arbitrary graphs. This
procedure also provides a construction of the hamiltonian constraint as a
finite operator on the space of diffeomorphism invariant states as well as a
construction of the operator corresponding to the spatial volume of the
universe.Comment: Latex, 11 pages, no figures, CGPG/93/

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