Principle of locality

The principle of locality is a fundamental concept in physics. It states that an object is influenced only by its immediate surroundings. This means that for one object to affect another, there must be a mediator. A mediator is something that travels through the space between two points. This could be a particle or a wave that carries the influence. A theory that follows this rule is called a local theory. This concept is the opposite of "action at a distance." Action at a distance suggests that an effect happens instantly across space without a middle step.

To understand how locality works, we must look at the speed of signals. The special theory of relativity sets a maximum speed for causal influence. This speed is the speed of light in a vacuum. Because of this limit, an event at one point cannot cause a simultaneous result elsewhere. The time it takes for an influence to travel depends on the distance. If the distance is $d$ and the speed of light is $c$, the time required is $d/c$. This ensures that the universe operates in a step-by-step sequence. No signal can bypass the space between two locations to cause an instant change.

Physics has moved through different models of how these influences travel. In the 17th century, Isaac Newton described gravity as an instant action at a distance. This idea violated the principle of locality, even though Newton found it absurd. Later, in 1880, James Clerk Maxwell developed field equations for electromagnetism. These equations obey locality and show that electromagnetic forces travel at the speed of light. In 1905, Albert Einstein proposed the special theory of relativity. He sought to reformulate physics to obey locality by stating that no energy can travel faster than light. He later succeeded with general relativity, which also obeys the principle of locality.

Quantum mechanics introduced a new and difficult challenge to locality. In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published the EPR paradox. They used a thought experiment to suggest that quantum mechanics might be incomplete. They described two systems that interact and then separate. In modern terms, these systems are called entangled. The researchers argued that without "hidden variables," quantum mechanics would predict illogical results. They believed that local measurements at one site should not cause changes at a distant site. They thought the wavefunction could not be a complete description of reality if it required non-local action.

In 1964, physicist John Stewart Bell developed a way to test these ideas. He formulated Bell's theorem, which provides a mathematical inequality. This inequality creates a limit on the correlations between measurements of separated particles if local causality holds. Bell used a concept called local causality to describe his model. This model assumes that the results for two people, often named Alice and Bob, are only linked by shared past events. He used a mathematical "screen" to show which events could affect the measurements.

Experimental tests have since challenged the idea of local hidden variables. Many experiments have shown that quantum mechanics violates Bell's inequalities. This means that the correlations between entangled particles are stronger than what local rules allow. Some interpretations suggest that certain quantum effects actually violate the principle of locality. These experiments have been successful even when measurement locations are more than one kilometer apart. The results imply that the submicroscopic world does not follow the same local rules as large objects like planets.

The significance of this discovery was recognized with the 2022 Nobel Prize in Physics. The prize was awarded to Alain Aspect, John Clauser, and Anton Zeilinger. They were honored for their experiments with entangled photons. Their work established the violation of Bell's inequalities and confirmed the reality of quantum entanglement. This research connects the study of individual particles to the broader structure of spacetime. It forces scientists to debate the true nature of reality and how information is shared across the universe.