Causal Ordering Of Events Research Articles

A no-go theorem for superpositions of causal orders

The causal order of events need not be fixed: whether a bus arrives before or after another at a certain stop can depend on other variables – like traffic. Coherent quantum control of causal order is possible too and is a useful resource for several tasks. However, quantum control implies that a controlling system carries the which-order information – if the control is traced out, the order of events remains in a probabilistic mixture. Can the order of two events be in a pure superposition, uncorrelated with any other system? Here we show that this is not possible for a broad class of processes: a pure superposition of any pair of Markovian, unitary processes with equal local dimensions and different causal orders is not a valid process, namely it results in non-normalised probabilities when probed with certain operations. The result imposes constraints on novel resources for quantum information processing and on possible processes in a theory of quantum gravity.

Functorial Evolution of Quantum Fields

We present a compositional algebraic framework to describe the evolution of quantum fields in discretised spacetimes. We show how familiar notions from Relativity and quantum causality can be recovered in a purely order-theoretic way from the causal order of events in spacetime, with no direct mention of analysis or topology. We formulate theory-independent notions of fields over causal orders in a compositional, functorial way. We draw a strong connection to Algebraic Quantum Field Theory (AQFT), using a sheaf-theoretical approach in our definition of spaces of states over regions of spacetime. We introduce notions of symmetry and cellular automata, which we show to subsume existing definitions of Quantum Cellular Automata (QCA) from previous literature. Given the extreme flexibility of our constructions, we propose that our framework be used as the starting point for new developments in AQFT, QCA and more generally Quantum Field Theory.

Quantum clocks and the temporal localisability of events in the presence of gravitating quantum systems

The standard formulation of quantum theory relies on a fixed space-time metric determining the localisation and causal order of events. In general relativity, the metric is influenced by matter, and is expected to become indefinite when matter behaves quantum mechanically. Here, we develop a framework to operationally define events and their localisation with respect to a quantum clock reference frame, also in the presence of gravitating quantum systems. We find that, when clocks interact gravitationally, the time localisability of events becomes relative, depending on the reference frame. This relativity is a signature of an indefinite metric, where events can occur in an indefinite causal order. Even if the metric is indefinite, for any event we can find a reference frame where local quantum operations take their standard unitary dilation form. This form is preserved when changing clock reference frames, yielding physics covariant with respect to quantum reference frame transformations.

Causal and causally separable processes

The idea that events are equipped with a partial causal order is central to our understanding of physics in the tested regimes: given two pointlike events A and B, either A is in the causal past of B, B is in the causal past of A, or A and B are space-like separated. Operationally, the meaning of these order relations corresponds to constraints on the possible correlations between experiments performed in the vicinities of the respective events: if A is in the causal past of B, an experimenter at A could signal to an experimenter at B but not the other way around, while if A and B are space-like separated, no signaling is possible in either direction. In the context of a concrete physical theory, the correlations compatible with a given causal configuration may obey further constraints. For instance, space-like correlations in quantum mechanics arise from local measurements on joint quantum states, while time-like correlations are established via quantum channels. Similarly to other variables, however, the causal order of a set of events could be random, and little is understood about the constraints that causality implies in this case. A main difficulty concerns the fact that the order of events can now generally depend on the operations performed at the locations of these events, since, for instance, an operation at A could influence the order in which B and C occur in A’s future. So far, no formal theory of causality compatible with such dynamical causal order has been developed. Apart from being of fundamental interest in the context of inferring causal relations, such a theory is imperative for understanding recent suggestions that the causal order of events in quantum mechanics can be indefinite. Here, we develop such a theory in the general multipartite case. Starting from a background-independent definition of causality, we derive an iteratively formulated canonical decomposition of multipartite causal correlations. For a fixed number of settings and outcomes for each party, these correlations form a polytope whose facets define causal inequalities. The case of quantum correlations in this paradigm is captured by the process matrix formalism. We investigate the link between causality and the closely related notion of causal separability of quantum processes, which we here define rigorously in analogy with the link between Bell locality and separability of quantum states. We show that causality and causal separability are not equivalent in general by giving an example of a physically admissible tripartite quantum process that is causal but not causally separable. We also show that there are causally separable quantum processes that become non-causal if extended by supplying the parties with entangled ancillas. This motivates the concepts of extensibly causal and extensibly causally separable (ECS) processes, for which the respective property remains invariant under extension. We characterize the class of ECS quantum processes in the tripartite case via simple conditions on the form of the process matrix. We show that the processes realizable by classically controlled quantum circuits are ECS and conjecture that the reverse also holds.

A set of 265 pictures standardized for studies of the cognitive processing of temporal and causal order information

Planning, predicting, reasoning, and acting often depend crucially on the correct encoding and application of knowledge concerning the temporal and causal ordering of events. Yet no pictorial stimulus set is optimized for investigating the processing of temporal and causal order information. We introduce a novel stimulus set of 265 black-and-white line drawings depicting a diverse array of recognizable events. Most of the images in the stimulus set (N = 222) share a thematic or conceptual association with one other image in the set, and the stimuli were created and extensively normed such that the image pairs vary in the degrees to which they share a causal, ordered relation with one another. The stimuli were standardized in a series of normative tasks, including concept/noun/verb agreement, perceived frequency, visual similarity, and indexes of three features of causal associations between events (i.e., temporal proximity, exclusivity, and priority). Both younger adults (ages 18-30 years) and older adults (ages 60-80 years) contributed normative data, allowing for broad applications of the stimuli to the study of normal and age-related changes in the encoding, retention, and retrieval of information regarding temporal and causal order. Complete normative data sets are available in the online supplemental materials, and the full stimulus set is available by contacting the first author.

Environmental Factors and Puberty Timing: Expert Panel Research Needs

Serono Symposia International convened an expert panel to review the impact of environmental influences on the regulation of pubertal onset and progression while identifying critical data gaps and future research priorities. An expert panel reviewed the literature on endocrine-disrupting chemicals, body size, and puberty. The panel concluded that available experimental animal and human data support a possible role of endocrine-disrupting chemicals and body size in relation to alterations in pubertal onset and progression in boys and girls. Critical data gaps prioritized for future research initiatives include (1) etiologic research that focus on environmentally relevant levels of endocrine-disrupting chemicals and body size in relation to normal puberty as well as its variants, (2) exposure assessment of relevant endocrine-disrupting chemicals during critical windows of human development, and (3) basic research to identify the primary signal(s) for the onset of gonadotropin-releasing hormone-dependent/central puberty and gonadotropin-releasing hormone-independent/peripheral puberty. Prospective studies of couples who are planning pregnancies or pregnant women are needed to capture the continuum of exposures at critical windows while assessing a spectrum of pubertal markers as outcomes. Coupled with comparative species studies, such research may provide insight regarding the causal ordering of events that underlie pubertal onset and progression and their role in the pathway of adult-onset disease.

A circular and dynamic model of the process of job design

We present a model of the process of job design that attempts to develop theory in five ways. First, critical variables in the model include performance, perceived competence, trust, knowledge and self‐efficacy. Second, job design is proposed to be a dynamic and circular process, with the variables acting as both predictors and outcomes. Third, the process is seen, at least in part, as a social one, involving job‐holders, supervisors and peers. Fourth, we identify some of the contingencies affecting the model. Finally, we specify the causal ordering of events and speculate on the nature of the time lags involved in the process. We make a number of model‐based predictions and consider the implications of the model for theory, research and practice.

Constructing reliable distributed communication systems with CORBA

Communication software and distributed services for next-generation applications must be reliable, efficient, flexible, and reusable. These requirements motivate the use of the Common Object Request Broker Architecture (CORBA). However, building highly available applications with CORBA is hard. Neither the CORBA standard nor conventional implementations of CORBA directly address complex problems related to distributed computing, such as real-time or high-speed quality of service, partial failures, group communication, and causal ordering of events. This article describes a CORBA-based framework that uses the virtual synchrony model to support reliable data- and process-oriented distributed systems that communicate through synchronous methods and asynchronous messaging.