Atomic clocks achieved an unheard accuracy, the FOCS1 activated in 2004 in Switzerland has an accuracy of 1 second in 30 million years. It would seem that any improvement in accuracy would not only be difficult but also useless!
Well, it is not so. NIST physicists have managed to create a clock that has an accuracy better than 1 second every 13.8 billion years (that is the estimated age of our Universe). The clock is based on ytterbium atoms and on the measurement of their frequency.
This is an amazing accuracy but what would be the usefulness of such accuracy? It turns out that with such accuracy it becomes possible to measure the effect of gravity on time. Einstein pointed out that time and space are the fabric of the universe, connected one to the other. He also showed that gravity warps the space time continuum, hence gravity affects time as well as space.
Since Earth gravity is slightly different in different places, depending on altitude (the higher up you go the farther from Earth gravitational center and therefore the lesser the gravity), one should notice that one of this clock based at an higher altitude should tick faster than one located at a lower altitude.
This might provide a further confirmation to Einstein theory of relativity (both special and general) but as Einstein would have said, we do not need further prove of his theory. What scientists are planning to do is to use these clocks to search for dark matter.
This dark matter should be somewhere (plenty of it actually) but being “dark” we cannot see it. The idea is that we should be able to see the effect that the gravity has on time (spacetime warp) and possibly detect ripple in the space time.
We’ll see in the next few years if this clock will let scientists detect the dark matter. Whether or not that will happen the amazing feat of measuring time with this accuracy remains as a tremendous achievement.