At University College London, a long-standing tension at the heart of physics — the incompatibility of Einstein's gravity and quantum mechanics — is being brought from the blackboard into the laboratory. Funded by an £8 million gift honoring the late physicist and entrepreneur David Potter, the new institute will use quantum sensors and interferometers to test whether spacetime itself obeys quantum rules, or stands apart as something irreducibly classical. It is a rare moment when a question that has resisted resolution for generations becomes, at last, experimentally approachable.