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. 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.Īnother requirement, at least valid at the level of human experience, is that cause and effect be mediated across space and time (requirement of contiguity). In relativity theory this requirement is strengthened so as to limit causes to the back (past) light cone of the event to be explained (the "effect") nor can an event be a cause of any event outside the former event's front (future) light cone. In classical physics, a cause should always precede its effect. So Newton's second law can be used to predict the force necessary to realize a desired acceleration. For instance, in Newtonian mechanics, an observed acceleration can be explained by reference to an applied force. So what constitutes a "cause" and what constitutes an "effect" depends on the total system of explanation in which the putative causal sequence is embedded.Ī formulation of physical laws in terms of cause and effect is useful for the purposes of explanation and prediction. Also, the meaning of "uncaused motion" is dependent on the theory being employed: for Aristotle it is (absolute) rest, for Newton it is inertial motion (constant velocity with respect to an inertial frame of reference), in the general theory of relativity it is geodesic motion (to be compared with frictionless motion on the surface of a sphere at constant tangential velocity along a great circle). In the general theory of relativity, too, acceleration is not an effect (since it is not a generally relativistic vector) the general relativistic effects comparable to those of Newtonian mechanics are the deviations from geodesic motion in curved spacetime. For instance, in Aristotelian physics the effect is not said to be acceleration but to be velocity (one must push a cart twice as hard in order to have its velocity doubled). For different physical theories the notions of cause and effect may be different. Thus, in classical (Newtonian) mechanics a cause may be represented by a force acting on a body, and an effect by the acceleration which follows as quantitatively explained by Newton's second law. In physics it is useful to interpret certain terms of a physical theory as causes and other terms as effects.
Various Concepts of Cause and Effect in Physics
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