>The specific way they measured the shift — comparing two parts of the same cloud — allowed them to cancel out a lot of noise that was common to both parts. It’

>The specific way they measured the shift — comparing two parts of the same cloud — allowed them to cancel out a lot of noise that was common to both parts. It’s like measuring a sailboat in rough seas. Even as it lurches up and down unpredictably, the distance between the keel and mast will always stay constant. While a clock made of a cloud of atoms can drift due to any number of things — electric fields, magnetic fields, the laser light itself, heat from the environment — the difference in frequencies between the top and bottom of the cloud remains the same. Measuring that difference revealed the effect of gravity. “That’s not trivial to do,” said Andrew Ludlow, an atomic clock expert at the National Institute of Standards and Technology, who was not involved with the research.