5.0 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Action at a distance | 4/4 | https://en.wikipedia.org/wiki/Action_at_a_distance | reference | science, encyclopedia | 2026-05-05T13:41:21.251432+00:00 | kb-cron |
Though Albert Einstein played a pivotal role in the development of quantum mechanics, he himself never fully accepted the theory. While he recognized that it made correct predictions, he believed a more fundamental description of nature must be possible. Over the years he presented multiple arguments to this effect, but the one he preferred most dated to a debate with Bohr in 1930. Einstein suggested a thought experiment in which two objects are allowed to interact and then moved apart a great distance from each other. The quantum-mechanical description of the two objects is a mathematical entity known as a wavefunction. If the wavefunction that describes the two objects before their interaction is given, then the Schrödinger equation provides the wavefunction that describes them after their interaction. But because of what would later be called quantum entanglement, measuring one object would lead to an instantaneous change of the wavefunction describing the other object, no matter how far away it is. Moreover, the choice of which measurement to perform upon the first object would affect what wavefunction could result for the second object. Einstein reasoned that no influence could propagate from the first object to the second instantaneously fast. Indeed, he argued, physics depends on being able to tell one thing apart from another, and such instantaneous influences would call that into question. Because the true "physical condition" of the second object could not be immediately altered by an action done to the first, Einstein concluded, the wavefunction could not be that true physical condition, only an incomplete description of it. In 1947, Einstein expressed his dissatisfaction with quantum theory in a letter to Max Born. "I cannot seriously believe in" quantum mechanics, he wrote, "because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance." In 1964, John Stewart Bell carried the analysis of quantum entanglement much further by proving the first version of Bell's theorem. In the context of Bell's theorem, "local" refers to the principle of locality, the idea that a particle can only be influenced by its immediate surroundings, and that interactions mediated by physical fields cannot propagate faster than the speed of light. "Hidden variables" are supposed properties of quantum particles that are not included in quantum theory but nevertheless affect the outcome of experiments. In the words of Bell, "If [a hidden-variable theory] is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local." In his original paper, Bell deduced that if measurements are performed independently on the two separated particles of an entangled pair, then the assumption that the outcomes depend upon hidden variables within each half implies a mathematical constraint on how the outcomes on the two measurements are correlated. Such a constraint would later be named a Bell inequality. Bell then showed that quantum physics predicts correlations that violate this inequality. Multiple variations on Bell's theorem were put forward in the years following his original paper, using different assumptions and obtaining different Bell (or "Bell-type") inequalities. The phrase "spooky action at a distance" has been adopted to describe the violation of Bell inequalities. Whether these phenomena involve real action at a distance, or in other words whether the need for nonlocality in hidden-variable models implies true nonlocality in nature, is a subject of debate.
== Force in quantum field theory ==
Quantum field theory does not need action at a distance. At the most fundamental level, only four forces are needed. Each force is described as resulting from the exchange of specific bosons. Two are short range: the strong interaction mediated by mesons and the weak interaction mediated by the weak boson; two are long range: electromagnetism mediated by the photon and gravity hypothesized to be mediated by the graviton. However, the entire concept of force is of secondary concern in advanced modern particle physics. Energy forms the basis of physical models and the word action has shifted away from implying a force to a specific technical meaning, an integral over the difference between potential energy and kinetic energy.
== See also == Central force – Mechanical force towards or away from a point Principle of locality – Physical principle that only immediate surroundings can influence an object Quantum nonlocality – Deviations from local realism
== References ==
== External links == This article incorporates text from a free content work. Licensed under CC-BY-SA. Text taken from Newton’s action at a distance – Different views, Nicolae Sfetcu.