Causality (physics)
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'''Causality'''<ref>Green, Celia (2003). ''The Lost Cause: Causation and the Mind-Body Problem''. Oxford: Oxford Forum. ISBN 0-9536772-1-4. Includes three chapters on causality at the microlevel in physics.</ref> describes the relationship between [[cause]]s and [[effect]]s, is fundamental to all natural [[science]], especially [[physics]], and has a basis in [[logic]]. It is also studied from the perspectives of [[philosophy]], [[computer science]], and [[statistics]].
In [[classical physics]], it was assumed that all events are caused by earlier ones according to the known laws of nature, culminating in [[Pierre-Simon Laplace]]'s claim that if the current state of the world were known with precision, it could be computed for any time in the future or the past. (see [[Laplace's demon]])
According to classical physics, the cause simply had to precede its effect, or at most be simultaneous with it (like force and acceleration in [[Newton's laws of motion|Newton's second law]]). In [[modern physics]], the notion of causality had to be clarified.
The insights of the theory of [[special relativity]] confirmed the assumption of causality, but they made the meaning of the word "simultaneous" observer-dependent<ref>A. Einstein, "Zur Elektrodynamik bewegter Koerper", ''Annalen der Physik'' '''17''', 891-921 (1905).</ref>. Consequently, the relativistic principle of causality says that the cause must precede its effect according to all [[inertial]] observers. This is equivalent to the statement that the cause and its effect are separated by a [[timelike]] interval, and the effect belongs to the future [[light cone]] of its cause. Equivalently, special relativity has shown that it is not only impossible to influence the past, it is also impossible to influence distant objects with signals that travel faster than the [[speed of light]].
In the theory of [[general relativity]], the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the [[spacetime]] is curved. New subtleties must be taken into account when we investigate causality in [[quantum mechanics]] and relativistic [[quantum field theory]] in particular. In quantum field theory, causality is closely related to the [[principle of locality]]. A careful analysis of the phenomena is needed, and the outcome slightly depends on the chosen [[interpretation of quantum mechanics]]: this is especially the case of the experiments involving [[quantum entanglement]] that require [[Bell's Theorem]] for their implications to be fully understood.
Despite these subtleties, causality remains an important and valid concept in physical theories. For example, the notion that events can be ordered into causes and effects is necessary to prevent [[causality paradox]]es such as the [[grandfather paradox]], which asks what happens if a time-traveller kills his own grandfather before he ever meets the time-traveller's grandmother. See also [[Chronology protection conjecture]].
'''Distributed causality'''
Theories in [[physics]] like the [[Butterfly effect]] from [[chaos theory]] open up the possibility of a type of [[Distributed parameter systems]] in causality. The butterfly effect theory proposes:
<blockquote>"Small variations of the initial condition of a nonlinear dynamical system may produce large variations in the long term behavior of the system."</blockquote> This opens up the opportunity to understand a distributed causality.
==References==
<references/>
== See also ==
* [[Causality]]
* [[Retrocausality]]
* [[Causal Structure]]
* [[Causal Sets]]
* [[Particle horizon]]
* [[Philosophy of physics]]
* [[Causal contact]]
==External links==
* [http://plato.stanford.edu/entries/causation-process/ Causal Processes, Stanford Encyclopedia of Philosophy]
* [http://www.black-holes.org/relativity3.html Caltech Tutorial on Relativity] — A nice discussion of how observers moving relatively to each other see different slices of time.
* [http://arxiv.org/abs/gr-qc/0107091 Faster-than-c signals, special relativity, and causality]. This article explains that faster than light signals do not necessarily lead to a violation of causality.
* by John G. Cramer:
**[http://www.analogsf.com/0612/altview.shtml EPR Communication: Signals from the Future?] "In this column I want to tell you about this causality-violating communications scheme and its possible consequences."
**[http://mist.npl.washington.edu/npl/int_rep/tiqm/TI_toc.html The Transactional Interpretation of Quantum Mechanics] "3.10 The Arrow of Time in the Transactional Interpretation - The formalism of quantum mechanics, at least in its relativistically invariant formulation, is completely even handed in dealing with the "arrow" of time, the distinction between future and past time directions."
[[Category:Causality]]
[[Category:Fundamental physics concepts]]
[[Category:Philosophy of physics]]
[[Category:Physics]]
[[Category:Time travel]]
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