Principle of locality

Things need to touch to move.

A push travels from one spot to another. It cannot jump across space. It must move through the air. It must travel through the space between.

This move takes time. It cannot happen all at once. Light is the fastest thing. Nothing can move faster than light. This rule keeps things in order. Scientists study how things work this way.

In physics, there is a rule called the principle of locality. This rule says that an object is only changed by its immediate surroundings. For one thing to affect another, something must travel between them. This could be a wave or a tiny particle.

This movement takes time. It cannot happen instantly. Albert Einstein said nothing can travel faster than light. This sets a speed limit on how fast things can happen.

Scientists found that tiny particles do not always follow this rule. In 1935, Albert Einstein and his team studied this. They looked at two systems that were entangled. Entangled means the two parts are linked in a special way. Even when they are far apart, they stay connected.

In 1964, John Stewart Bell made a math rule called Bell's theorem. He wanted to see if local rules could explain these links. Experiments later showed that quantum mechanics breaks these local rules. This means some tiny effects happen in ways that defy locality. This work helped win the Nobel Prize in 2022.

The principle of locality is a big idea in physics. It says that an object is only changed by things right next to it. For one thing to affect another, something must travel through the space between them. This could be a wave or a tiny particle.

This idea is different from "action at a distance." Action at a distance means things change instantly without anything moving between them. Locality suggests that everything has a middle step to carry the influence. This keeps the world feeling organized and predictable.

How does this work in real life? Imagine you want to push a ball. You must touch the ball to make it move. In physics, the same rule applies to forces. For a force to work, a signal must travel from one point to another. This signal cannot go faster than the speed of light. Because light has a speed limit, nothing can happen truly at the same time in two different places. If an event happens far away, it takes time to reach you. This time depends on how far away the event is. This makes the universe follow a steady, step-by-step flow.

Scientists have studied these rules for a long time. In the 1600s, Isaac Newton described gravity as an instant action at a distance. He actually thought this idea was quite strange. Later, in 1880, James Clerk Maxwell showed how light and electricity work using field equations. These equations follow the rules of locality. In 1905, Albert Einstein created the special theory of relativity. This theory said no energy can travel faster than light. Einstein wanted physics to obey locality, so he later created general relativity.

In 1935, Einstein, Boris Podolsky, and Nathan Rosen looked at a new problem. They used a thought experiment called the EPR paradox. They studied two systems that were "entangled." This means the two parts are linked even when they are far apart. They thought quantum mechanics might be missing some hidden information. In 1964, John Stewart Bell created Bell's theorem to test this. He made a math rule to see if local rules could explain these links. Experiments later showed that quantum mechanics actually breaks these local rules.

Today, we know that the tiny world of atoms is very strange. Many experiments show that quantum effects do not follow the principle of locality. These tests have been done even when the two parts are over a kilometer apart. This work is very important for our understanding of reality. In 2022, the Nobel Prize in Physics was given to Alain Aspect, John Clauser, and Anton Zeilinger. They won for their experiments with entangled photons. Their work proved that these strange quantum links are real. This shows us that the smallest parts of our world work in surprising ways.

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.