A group of physicists has stared at 12 atomic clocks located in the United Kingdom, France, the United States, Germany, and Italy from 1999 to 2014.
Eight of these clocks are one of the most precise cesium fountain atomic clocks operated by the National Institute of Standards and Technology. While other clocks would tick according to the swing of a pendulum or counts of crystals, these clocks tick following the beat of an atom. The atoms produce light waves that move back and forth at a steady rate of several billion times per second. The cycles of the light being emitted is extremely consistent that the clock will not miss even one second in tens of millions of years.
By watching the clocks' each and every tick from November 1999 until October 2014, the physicists found that the movement of the clocks did not change as the Earth orbits the Sun.
Hence, the experiment becomes the most accurate confirmation yet of Albert Einstein's theory of general relativity.
Einstein's Theory of General Relativity
The theory of general relativity goes by the idea that all objects inside an elevator would accelerate at the same rate even with the presence or absence of a steady gravitational field. The theory also follows the principle called "local position invariance" or LPI. This principle states that the properties of objects inside a free-falling elevator will remain constantly relative to one another.
In their experiment, the physicists from NIST assumed that the Earth is the elevator falling through the Sun's gravitational field. The atomic clocks, meanwhile, represent all the objects inside that elevator.
Earth As An Elevator
The NIST team, headed by Bijunath Patla, a researcher from NIST, compared the ticks of the eight cesium fountain atomic clocks with the four other hydrogen masers. Their observation revealed that all the clocks located across the world remained in sync with one another over the span of 14 years despite different amount and directions of gravitational pull as the Earth orbits around the sun.
The team ran several measurements to compare and derive their conclusions. One measurement involved comparing the frequencies of electromagnetic radiation from the atomic clocks. The team yielded an extremely small value for a quantity that Einstein predicted to be at zero value.
"The researchers constrained the violation of LPI to a value of 0.00000022 plus or minus 0.00000025 — the most miniscule number yet, consistent with general relativity's predicted result of zero, and corresponding to no violation," the team explained in a paper published in the journal Nature Physics.
Essentially, this means that the ratio of the hydrogen-to-cesium frequencies inside the clock remained the same even if the clocks moved in synchrony to the "falling elevator" or the Earth's orbit.
Importance Of Atomic Clocks
Patla explained that while the main thesis of the experiment is to test Einstein's theory of general relativity, the main reason they conducted the experiment is to emphasize the importance of the atomic clocks.
The experiment highlighted that atomic clocks can, indeed, prove or disprove fundamental physics or, in this case, the foundations of general relativity. Hence, it will be important to constantly develop highly advanced atomic clocks in the future.
Based on their present experiment, the researchers predicted that hydrogen and cesium clocks may not be able to obtain further limits on LPI in the future. This is, however, possible with experimental next-generation clocks such as atomic clocks with ytterbium and strontium.