Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Time travel is possible

J. B. Kennedy

Published Jul 30, 2002• 0 citations
Publisher

Abstract

Causal order The iron chains of causality link events together into a definite order: a cause always precedes its effect. If, however, there are distant events that do not influence each other, what decides their order? As noted above, time dilation implies that different sets of rulers and clocks moving relative to each other will assign different orders to distant events. Consider, for example, three events: event A, which causes event B, and a distant event, X. Since A and B are connected by some causal process, their order is fixed. But if event X is distant enough from both, then different clocks may register any of the three orders A, B, X or A, X, B or X, A, B That is, the distant event may follow both A and B, happen between them or precede both. According to the mainstream interpretation of special relativity, durations and other temporal intervals are not invariant, and are therefore not real. According to this view, there is no fact of the matter about which of the three orders above is real and physical. The events all occur and are all real, but there is no physical fact that makes X later or earlier than the others. Just as there are no unicorns or pink elephants, there is no order between distant events that do not influence each other. Consider another illustration. Suppose that there are two long queues leading into two doors at a club or music concert.

DOI
10.1017/upo9781844653447.007

Related papers

Causal and causally separable processes

Ognyan Oreshkov, Christina Giarmatzi

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.

arXiv:1506.05449•New Journal of Physics•Sep 7, 2016•140 citations

EPR, time, irreversibility and causality

Umberto Lucia, Giulia Grisolia

Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.

•Aug 2, 2021•0 citations

EPR, time, irreversibility and causality

Umberto Lucia, Giulia Grisolia

Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.

•Aug 18, 2021•0 citations

EPR, time, irreversibility and causality

Umberto Lucia, Giulia Grisolia

Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.

•Aug 3, 2021•0 citations

EPR, time, irreversibility and causality

Umberto Lucia, Giulia Grisolia

Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.

•Aug 18, 2021•0 citations

EPR, time, irreversibility and causality

Umberto Lucia, Giulia Grisolia

Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.

•Mar 21, 2022•4 citations
Causal order The iron chains of causality link events together into a definite order: a cause always precedes its effect. If, however, there are distant events that do not influence each other, what decides their order? As noted above, time dilation implies that different sets of rulers and clocks moving relative to each other will assign different orders to distant events. Consider, for example, three events: event A, which causes event B, and a distant event, X. Since A and B are connected by some causal process, their order is fixed. But if event X is distant enough from both, then different clocks may register any of the three orders A, B, X or A, X, B or X, A, B That is, the distant event may follow both A and B, happen between them or precede both. According to the mainstream interpretation of special relativity, durations and other temporal intervals are not invariant, and are therefore not real. According to this view, there is no fact of the matter about which of the three orders above is real and physical. The events all occur and are all real, but there is no physical fact that makes X later or earlier than the others. Just as there are no unicorns or pink elephants, there is no order between distant events that do not influence each other. Consider another illustration. Suppose that there are two long queues leading into two doors at a club or music concert.
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.
Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.
Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.
Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.
Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.
Causality is the relationship between causes and effects. Following Relativity, any cause of an event must always be in the past light cone of the event itself, but causes and effect must always be related to some interactions. In this paper, causality is developed as a consequence of the analysis of the Einstein, Podolsky and Rosen paradox. Causality is interpreted as the result of the time generation due to irreversible interactions of real systems among them. Time results as a consequence of irreversibility, so any state function of a system in its space cone, when affected by an interaction with an observer, moves into a light cone or within it, with the consequence that any cause must precede its effect in a common light cone.