Physics
Quantum Entanglement and Nonlocality in Particle Physics
Quick fact
In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for experiments that proved quantum entanglement is real and that the universe does not obey local realism—Einstein's 'spooky action at a distance' is actually a fact of nature.
Why this is interesting
Imagine two coins that always land on opposite sides, no matter how far apart they are tossed. But these coins don't decide until you look at one—and then the other instantly 'knows'. How can that be?
Read the full explanation
Understanding Quantum Entanglement and Nonlocality in Particle Physics
In the quantum world, particles like electrons can exist in a superposition of states—for example, 'spin up' and 'spin down' at the same time. When two particles interact, they can become entangled, meaning their states are linked. The combined system is described by a single wavefunction that connects both particles, even if they are separated by kilometres. Instead of each particle having its own independent properties, the pair shares one quantum state. When you measure one particle, the superposition collapses, and the outcome for the other particle is instantly determined. This happens without any signal travelling between them, breaking our classical notion of locality—the idea that an object is only influenced by its immediate surroundings.
A deeper explanation
The mechanism behind entanglement lies in the mathematics of quantum mechanics. The wavefunction of an entangled pair is non‑separable: it cannot be written as a product of independent states for each particle. For example, if particle A is measured to have spin up, particle B must have spin down, and this correlation is perfect. Albert Einstein, Boris Podolsky, and Nathan Rosen (EPR) argued in 1935 that this would require 'hidden variables'—unknown properties that predetermine outcomes. However, John Bell in 1964 derived an inequality that any local hidden‑variable theory must satisfy. Experiments measuring the polarisation of entangled photons have violated Bell's inequality, confirming that nature is nonlocal: the correlations cannot be explained by any local theory. This means that measurement on one particle affects the other instantaneously across space, although it cannot be used to send information faster than light because the outcomes are random. Entanglement is a fundamental resource for quantum technologies: quantum computing uses entangled qubits to perform parallel computations, and quantum cryptography uses entanglement to detect eavesdropping, enabling secure communication.