Editing Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? (section)

=== Entangled Particles Reveal the Paradox ===

They imagined two interacting particles. When the interaction ends, the particles stay "entangled". 

Measuring one particle affects the probabilities for the other. This lets you predict two contradictory things about the other particle with certainty, violating the idea they can't both be real.

So quantum theory must be incomplete, because its wave function can't describe the full reality. There must be more underlying variables it does not capture.