Bell’s Inequality sits at the heart of one of the most profound debates in modern physics: whether the world is fundamentally local and deterministic, or whether Quantum Mechanics really does allow nature to behave in ways that defy classical intuition?
Proposed by physicist John Stewart Bell in 1964, the inequality provides a precise mathematical test to distinguish between “hidden‑variable” explanations (where particles carry pre‑existing properties) and the genuinely nonlocal correlations predicted by quantum theory.
Experiments have repeatedly shown that Quantum Mechanics violates Bell’s Inequality, forcing us to confront a universe where entanglement links distant particles in ways that no classical picture can fully explain.
Bell’s inequality provides a mathematical physical constraint that shows the limitations of Local Hidden-Variable theories in explaining the correlations observed in Quantum Mechanics – a mathematical tool to test whether the strange predictions of Quantum Mechanics – such as entanglement and non-locality – can be explained by more traditional, “classical” ideas.
Physicists tested entangled particles (like photons) and measured their properties, like polarization at different angles.
The result violates Bell’s inequality.
And it would appear Nature does not follow classical rules.
Quantum entanglement (Quantenverschränkung) is REAL. And lo and behold, “Spooky action at a distance” (unheimliche Fernwirkung) actually does happen.
In Classical Physics, it is assumed that particles have definite properties (like spin or position) even when we are not observing them, and that no information can travel faster than light.
These classical assumptions lead to certain statistical limits on how correlated the measurements of distant particles can be.
Bell’s inequality captures those limits.
If the world obeys classical rules with hidden variables, then the correlations between the measurements should never exceed a specific threshold.
However, experiments with entangled particles, whereby two particles are linked in such a way that measuring one instantly affects the other, consistently violate Bell’s inequality. This indicates that the correlations are stronger than what Classical Physics allows.
The violation suggests that either particles do not have definite properties until measured, or that some kind of “spooky action at a distance” is happening as Einstein famously put it.
In short, Bell’s inequality shows that the quantum world defies classical intuition and that reality might be fundamentally interconnected in ways we are only beginning to understand.