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Scientists from the US National Institute of Standards and Technology (NIST) conducted the most thorough test of the basic assumption of Einstein's general theory of relativity, which describes the relationship between gravity and time and space. The test was carried out by comparing different types of atomic clocks and was made possible by the fact that these devices are constantly being improved. During the test, a record low score was achieved for the famous Einstein thought experiment with a falling lift. Einstein theorized that all objects in such an elevator would accelerate equally, as if they were in a uniform gravity field, or as if they were influenced by gravity. In other words, the relative properties of such objects in relation to each other will remain the same during the free fall of the lift.
NIST's scientists used the solar system, and their lift was the Earth falling in the gravitational field of the Sun. To investigate the relative properties of the objects, they compared data from two types of atomic clocks over their 14 years of operation to see if they were synchronized with each other, even in the face of changes in the solar gravitational impact on the Earth. The data came from the years 1999-2014 from 12 clocks. Four of them are hydrogen masks owned by NIST and eight are the world's most accurate caesium atomic clocks, located in laboratories in the USA, the UK, France, Germany and Italy.
During one of such measurements the frequencies of electromagnetic radiation of clocks were compared. The measurements showed a breach of the Einstein principle of 0.00000022 (±0.00000025). This is the smallest result achieved so far, which means no infringement. This means that the ratio of hydrogen to caesium remained identical in a freely falling elevator. NIST has thus broken its own 2007 measurement record. The uncertainty of the current measurement is 5 times lower than then.
As Bijunath Patla from NIST said, this was possible thanks to increasingly accurate caesium clocks, improving data transfer between clocks so that they can compare their signals and more accurately calculate the position and velocity of the Earth.
Scientists believe that it is unlikely that Einstein's assumptions will be tested further with hydrogen and caesium clocks. This will be possible with future generations of optical clocks. NIST already has such clocks based on yttrium and strontium atoms.