Causality (physics)

Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics. Causality means that an effect can not occur from a cause which is not in the back (past) light cone of that event. Similarly, a cause can not have an effect outside its front (future) light cone.

In classical physics, an effect can not occur before its cause. In relativity theory, causality means that an effect can not occur from a cause which is not in the back (past) light cone of that event. Similarly, a cause can not have an effect outside its front (future) light cone. These restrictions are consistent with the grounded belief (or assumption) that causal influences cannot travel faster than the speed of light and/or backwards in time. In quantum field theory, observables of events with a spacelike relationship, "elsewhere", have to commute, so the order of observations or measurements of such observables do not impact each other.

Causality should not be confused with Newton's second law, which is related to the conservation of momentum, and is a consequence of the spatial homogeneity of physical laws. The name causality suggests that all effects must have specific causes, which is a concept unrelated to the common use of causality in physics, and is violated in some mainstream interpretations of quantum mechanics.

Another requirement, at least valid at the level of human experience, 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 action at a distance) replacing mechanistic proposals like Descartes' vortex theory; in the third place as an incentive to develop dynamic field theories (e.g., Maxwell's electrodynamics and Einstein's general theory of relativity) restoring contiguity in the transmission of influences in a more successful way than did Descartes' theory.

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