Machine LearningAI 'Lie Detectors' Have a Mathematical Blind Spot — and a 90-Year-Old Theorem Explains WhySafety researchers train 'truth probes' to catch language models lying. A new analysis uses Tarski's undefinability theorem — the 1930s cousin of Gödel's incompleteness — to show why no universal AI lie detector can exist.Jul 29, 2026