138,702 research outputs found
On causality and superluminal behavior in classical field theories. Applications to k-essence theories and MOND-like theories of gravity
Field theories whose full action is Lorentz invariant (or diffeomorphism
invariant) can exhibit superluminal behaviors through the breaking of local
Lorentz invariance. Quantum induced superluminal velocities are well-known
examples of this effect. The issue of the causal behavior of such propagations
is somewhat controversial in the literature and we intend to clarify it. We
provide a careful analysis of the meaning of causality in classical
relativistic field theories, and we stress the role played by the Cauchy
problem and the notions of chronology and time arrow. We show that superluminal
behavior threaten causality only if a prior chronology on spacetime is chosen.
In the case where superluminal propagations occur, however, there is at least
two non conformally related metrics on spacetime and thus two available notions
of chronology. These two chronologies are on equal footing and it would thus be
misleading to choose \textit{ab initio} one of them to define causality.
Rather, we provide a formulation of causality in which no prior chronology is
assumed. We argue this is the only way to deal with the issue of causality in
the case where some degrees of freedom propagate faster than others. We
actually show that superluminal propagations do not threaten causality. As an
illustration of these conceptual issues, we consider two field theories, namely
k-essences scalar fields and bimetric theories of gravity, and we derive the
conditions imposed by causality. We discuss various applications such as the
dark energy problem, MOND-like theories of gravity and varying speed of light
theories.Comment: 15 pages, 2 figures; minor changes, references added, submitted to
Phys.Rev.
On the duality in CPT-even Lorentz-breaking theories
In this paper, we generalize the duality between self-dual and
Maxwell-Chern-Simons theories for the case of a CPT-even Lorentz-breaking
extension of these theories. The duality is demonstrated with use of the gauge
embedding procedure, both in free and coupled cases, and with the master action
approach. The physical spectra of both Lorentz-breaking theories are studied.
The massive poles are shown to coincide and to respect the requirements for
unitarity and causality at tree level. The extra massless poles which are
present in the dualized model are shown to be nondynamical.Comment: 17 pages, version accepted to EPJ
Causality and CPT violation from an Abelian Chern-Simons-like term
We study a class of generalized Abelian gauge field theories where CPT
symmetry is violated by a Chern-Simons-like term which selects a preferred
direction in spacetime. Such Chern-Simons-like terms may either emerge as part
of the low-energy effective action of a more fundamental theory or be produced
by chiral anomalies over a nonsimply connected spacetime manifold.
Specifically, we investigate the issues of unitarity and causality. We find
that the behaviour of these gauge field theories depends on whether the
preferred direction is spacelike or timelike. For a purely spacelike preferred
direction, a well-behaved Feynman propagator exists and microcausality holds,
which indicates the possibility of a consistent quantization of the theory. For
timelike preferred directions, unitarity or causality is violated and a
consistent quantization does not seem to be possible.Comment: LaTeX, 27 pages, v4: to appear in NP
An action-related theory of causality
The paper begins with a discussion of Russell?s view that the notion of cause is unnecessary for science, and can therefore be eliminated. It is argued that this is true for theoretical physics, but untrue for medicine where the notion of cause plays a central role. Medical theories are closely connected with practical action (attempts to cure and prevent disease), whereas theoretical physics is more remote from applications. This suggests the view that causal laws are appropriate in a context where there is a close connection to action. This leads to a development of an action-related theory of causality which is similar to the agency theory of Menzies and Price, but differs from it in a number of respects, one of which is the following. Menzies and Price connect ?A causes B? with an action to produce B by instantiating A, but, particularly in the case of medicine, the law can also be linked to the action of trying to avoid B by ensuring that A is not instantiated. The action-related theory has in common with agency theory of Menzies and Price, the ability to explain causal asymmetry in a simple fashion, but the introduction of avoidance actions together with some ideas taken form Russell enable some of the objections to agency accounts of causality to be met. The paper begins with a discussion of Russell?s view that the notion of cause is unnecessary for science, and can therefore be eliminated. It is argued that this is true for theoretical physics, but untrue for medicine where the notion of cause plays a central role. Medical theories are closely connected with practical action (attempts to cure and prevent disease), whereas theoretical physics is more remote from applications. This suggests the view that causal laws are appropriate in a context where there is a close connection to action. This leads to a development of an action-related theory of causality which is similar to the agency theory of Menzies and Price, but differs from it in a number of respects, one of which is the following. Menzies and Price connect ?A causes B? with an action to produce B by instantiating A, but, particularly in the case of medicine, the law can also be linked to the action of trying to avoid B by ensuring that A is not instantiated. The action-related theory has in common with agency theory of Menzies and Price, the ability to explain causal asymmetry in a simple fashion, but the introduction of avoidance actions together with some ideas taken form Russell enable some of the objections to agency accounts of causality to be met
Inferring Narrative Causality between Event Pairs in Films
To understand narrative, humans draw inferences about the underlying
relations between narrative events. Cognitive theories of narrative
understanding define these inferences as four different types of causality,
that include pairs of events A, B where A physically causes B (X drop, X
break), to pairs of events where A causes emotional state B (Y saw X, Y felt
fear). Previous work on learning narrative relations from text has either
focused on "strict" physical causality, or has been vague about what relation
is being learned. This paper learns pairs of causal events from a corpus of
film scene descriptions which are action rich and tend to be told in
chronological order. We show that event pairs induced using our methods are of
high quality and are judged to have a stronger causal relation than event pairs
from Rel-grams
Relativistic hydrodynamic fluctuations from an effective action: causality, stability, and the information current
Causality is necessary for retarded Green's functions to remain retarded in
all inertial frames in relativity, which ensures that dissipation of
fluctuations is a Lorentz invariant concept. For first-order BDNK theories with
stochastic fluctuations, introduced via the Schwinger-Keldysh formalism, we
show that imposing causality and stability leads to correlation functions of
hydrodynamic fluctuations that only display the expected physical properties at
small frequencies and wavenumber, i.e., within the expected regime of validity
of the first-order approach. For second-order theories of Israel and Stewart
type, constructed using the information current such that entropy production is
always non-negative, a stochastic formulation is presented using the
Martin-Siggia-Rose approach where imposing causality and stability leads to
correlators with the desired properties. We also show how Green's functions can
be determined from such an action. We identify a symmetry,
analogous to the Kubo-Martin-Schwinger symmetry, under which this
Martin-Siggia-Rose action is invariant. This modified Kubo-Martin-Schwinger
symmetry provides a new guide for the effective action formulation of
hydrodynamic systems with dynamics not solely governed by conservation laws.
Furthermore, this symmetry ensures that the principle of detailed balance is
valid in a covariant manner. We employ the new symmetry to further clarify the
connection between the Schwinger-Keldysh and Martin-Siggia-Rose approaches,
establishing a precise link between these descriptions in second-order theories
of relativistic hydrodynamics. Finally, the modified Kubo-Martin-Schwinger
symmetry is used to determine the corresponding action describing diffusion in
Israel-Stewart theories in a general hydrodynamic frame.Comment: 28 page
Weyl corrections to holographic conductivity
For conformal field theories which admit a dual gravitational description in
anti-de Sitter space, electrical transport properties, such as conductivity and
charge diffusion, are determined by the dynamics of a U(1) gauge field in the
bulk and thus obey universality relations at the classical level due to the
uniqueness of the Maxwell action. We analyze corrections to these transport
parameters due to higher-dimension operators in the bulk action, beyond the
leading Maxwell term, of which the most significant involves a coupling to the
bulk Weyl tensor. We show that the ensuing corrections to conductivity and the
diffusion constant break the universal relation with the U(1) central charge
observed at leading order, but are nonetheless subject to interesting bounds
associated with causality in the boundary CFT.Comment: 15 pages, v2: references adde
Causality, Human Action and Experimentation: Von Wright's Approach to Causation in Contemporary Perspective
This paper discusses von Wright's theory of causation from Explanation and Understanding and Causality and Determinism in contemporary context. I argue that there are two important common points that von Wright's view shares with the version of manipulability currently supported by Woodward: the analysis of causal relations in a system modelled on controlled experiments, and the explanation of manipulability through counterfactuals - with focus on the counterfactual account of unmanipulable causes. These points also mark von Wright's departure from previous action-based theories of causation. Owing to these two features, I argue that, upon classifying different versions of manipulability theories, von Wright's view should be placed closer to the interventionist approach than to the agency theory, where it currently stands. Furthermore, given its relevance in contemporary context, which this paper aims to establish, I claim that von Wright's theory can be employed to solve present problems connected to manipulability approaches to causation
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