A key principle underlying Albert Einstein's famous theory of general relativity, which describes how gravity relates to space and time, has just been given its most accurate test ever.
The test, performed by physicists at the National Institute of Standards and Technology (NIST), was made possible by continual improvements in the world's most accurate atomic clocks. It yielded a record-low, exceedingly small value for a quantity that Einstein predicted to be zero.
Einstein had suggested a thought experiment imagining Earth as a free-falling elevator. He theorized that all objects located in such an elevator would accelerate at the same rate, as if they were in a uniform gravitational field — the same as if they were in no gravity at all. He also predicted that, during the fall, these objects' properties relative to each other would remain constant. Einstein’s principle of local position invariance (LPI) holds that measures of nongravitational effects are independent of time and place.
One of those measures compares the frequencies of electromagnetic radiation from atomic clocks at different locations. The NIST researchers compared recorded data on the "ticks" of two different types of atomic clocks located around the world — four hydrogen maser and eight cesium fountain clocks, for a total of 12 clocks in all — to show they remained in sync over 14 years. That was the case despite variations on the gravitational pull on the “elevator,” which result from Earth's slightly off-kilter orbit.
The team was able to constrain the violation of LPI to its most miniscule number to date: a value of 0.00000022 plus or minus 0.00000025 — five times more sensitive than NIST’s best previous measurement of LPI violation, which itself was 20 times more sensitive than previous tests. The advance was credited to more accurate cesium fountain atomic clocks; better time transfer processes that enable devices at different locations to compare their time signals; and the latest data for computing the position and velocity of Earth in space, according to researcher Bijunath Patla.
Because so many scientific theories intertwine, researchers used their new value for the LPI violation to calculate variations in several fundamental "constants" of nature — physical quantities thought to be universal and widely used in physics. In the future, experimental clocks based on optical frequencies — which are much higher than the frequencies of hydrogen and cesium clocks — could offer much more sensitive results.
