Is gravity secretly changing the color of light all around you? For 50 years, following Einstein’s publication of General Relativity, astronomers looked to the stars, hoping to confirm the prediction that the frequency of light changes when it travels up or down in a gravitational field. Einstein expressed the importance of this effect, “should [gravitational redshift] be absent, the whole theory should be abandoned.” The ingenious experiment that finally proved the effect was real didn’t involve looking to the sky, but instead looking just six stories upward through holes smashed through the floors of Harvard’s Jefferson Laboratory. This video brings us inside the building and the minds of the experimenters.