· Physicist Portrait
What Is the Born Rule, and Why Is Probability |ψ|²?
Quantum mechanics describes a particle with a wavefunction ψ, but experiments record definite clicks. The Born rule is the bridge: the probability of finding the particle is the squared magnitude of the wavefunction, |ψ|². Here is what the rule says, where Max Born got it, how well it has been tested, and which parts are still argued over.