For more than a century, Einstein's description of gravity as the curvature of spacetime has endured every experimental challenge placed before it — yet physicists have always known the theory cannot be the final word. Now, researchers have created a muonium beam of unprecedented intensity, fashioning a delicate instrument from one of nature's most fleeting particles, and pointed it toward the oldest of questions: does gravity truly behave as Einstein said it does? The answer, still ahead in a series of careful experiments, could either deepen our confidence in a century-old framework or crack