Locality

Abstract

Locality: The locality assumption is sometimes called “local causes”. It is the requirement that each physical event or change has a physical cause, and that this cause can be localized in the immediate space-time neighborhood of its effects. A collision of two billiard balls or the mechanical connections between the parts of a steam engine are clear examples. A more subtle example is the feature of classical electromagnetism that any change in the velocity of a moving charged particle can be regarded as being caused by the action upon this particle of the electric and magnetic fields existing in the immediate space-time neighborhood of that particle at the moment at which the change in velocity occurs, and that any change in the electric and magnetic fields are likewise caused by physically describable properties that are located very close to where that change occurs. This idea that all physical effects are consequences of essentially “contact” interactions was part of the intellectual milieu, stemming from the ideas of Rene Descartes, in which Isaac Newton worked while creating the foundations of modern physics. However, his universal law of gravitational attraction was stated as a law of instantaneous action over astronomical distances, a clear violation of the idea that all physical effects have local causes.