Causality (physics)
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Physical causality is a physical relationship between
Macroscopic vs microscopic causality
Causality can be defined macroscopically, at the level of human observers, or microscopically, for fundamental events at the atomic level. The strong causality principle forbids information transfer faster than the speed of light; the weak causality principle operates at the microscopic level and need not lead to information transfer. Physical models can obey the weak principle without obeying the strong version.[3][4]
Macroscopic causality
In classical physics, an effect cannot occur before its cause which is why solutions such as the advanced time solutions of the
Another requirement of causality is that cause and effect be mediated across space and time (requirement of contiguity). This requirement has been very influential in the past, in the first place as a result of direct observation of causal processes (like pushing a cart), in the second place as a problematic aspect of Newton's theory of gravitation (attraction of the earth by the sun by means of
Simultaneity
In
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 those two theories, causality is closely related to the principle of locality.
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 (or at least outline)
Determinism (or, what causality is not)
The word causality in this context means that all effects must have specific physical causes due to fundamental interactions.
The empiricists' aversion to metaphysical explanations (like Descartes' vortex theory) meant that scholastic arguments about what caused phenomena were either rejected for being untestable or were just ignored. The complaint that physics does not explain the cause of phenomena has accordingly been dismissed as a problem that is philosophical or metaphysical rather than empirical (e.g., Newton's "Hypotheses non fingo"). According to Ernst Mach[7] the notion of force in Newton's second law was pleonastic, tautological and superfluous and, as indicated above, is not considered a consequence of any principle of causality. Indeed, it is possible to consider the Newtonian equations of motion of the gravitational interaction of two bodies,
as two coupled equations describing the positions and of the two bodies, without interpreting the right hand sides of these equations as forces; the equations just describe a process of interaction, without any necessity to interpret one body as the cause of the motion of the other, and allow one to predict the states of the system at later (as well as earlier) times.
The ordinary situations in which humans singled out some factors in a physical interaction as being prior and therefore supplying the "because" of the interaction were often ones in which humans decided to bring about some state of affairs and directed their energies to producing that state of affairs—a process that took time to establish and left a new state of affairs that persisted beyond the time of activity of the actor. It would be difficult and pointless, however, to explain the motions of binary stars with respect to each other in that way which, indeed, are time-reversible and agnostic to the arrow of time, but with such a direction of time established, the entire evolution system could then be completely determined.
The possibility of such a time-independent view is at the basis of the
Confusion between causality and determinism is particularly acute in
Distributed causality
Theories in
"Small variations of the initial condition of a nonlinear dynamical system may produce large variations in the long term behavior of the system."
This opens up the opportunity to understand a distributed causality.
A related way to interpret the butterfly effect is to see it as highlighting the difference between the application of the notion of causality in physics and a
Causal sets
In causal set theory, causality takes an even more prominent place. The basis for this approach to quantum gravity is in a theorem by
Interaction, force and the conservation of momentum
By physical causation is meant an effect that was caused by physical interference propagated by force from object A to object B. Momentum is propagated by force according to the
See also
- Causality – How one process influences another (general)
- Causal contact – Sharing an event that affects entities in a causal way
- Causal system – System where the output depends only on past and current inputs
- Particle horizon – Distance measurement used in cosmology
- Philosophy of physics – Truths and principles of the study of matter, space, time and energy
- Retrocausality – Concept in which the future affects the past
- Synchronicity – Jungian concept of the meaningfulness of acausal coincidences
- Wheeler–Feynman time-symmetric theory for electrodynamics – Interpretation of electrodynamics
References
- ^ 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.
- ^ Bunge, Mario (1959). Causality: the place of the causal principle in modern science. Cambridge: Harvard University Press.
- ISSN 0556-2821.
- ISBN 978-0-19-511798-1.
- ^ A. Einstein, "Zur Elektrodynamik bewegter Koerper", Annalen der Physik 17, 891–921 (1905).
- ^ "Causality." Cambridge English Dictionary. Accessed November 18, 2018. https://dictionary.cambridge.org/us/dictionary/english/causality
- ^ Ernst Mach, Die Mechanik in ihrer Entwicklung, Historisch-kritisch dargestellt, Akademie-Verlag, Berlin, 1988, section 2.7.
Further reading
- Bohm, David. (2005). Causality and Chance in Modern Physics. London: Taylor and Francis.
- Espinoza, Miguel (2006). Théorie du déterminisme causal. Paris: L'Harmattan. ISBN 2-296-01198-5.
External links
- Causal Processes, Stanford Encyclopedia of Philosophy
- Caltech Tutorial on Relativity — A nice discussion of how observers moving relatively to each other see different slices of time.
- 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.