A Science cikk:
http://news.sciencemag.org/sciencenow/2010/09/superaccurate-clocks-confirm-you.html
"Now Chin-wen Chou, Till Rosenband, and colleagues at the National Institute of Standard and Technology (NIST) in Boulder, Colorado, have detected changes in the passage of time caused by speeds of less than 10 meters per second and height changes of less than a meter, using a new type of atomic clock called an optical clock.
An atomic clock exploits the fact that the electrons in at atom occupy "states" with distinct energies and can hop between two states by emitting or absorbing electromagnetic waves of a set frequency. Researchers shine such waves on the atoms, and a feedback loop keeps their frequency tuned so that the atoms continually jump back and fourth between the two states. The oscillating waves then mark time just as a pendulum does, only very much faster and more evenly. The atomic clocks that now set the international time standard use microwaves with a frequency of 9.2 billion cycles per second to make cesium atoms flip between two states of nearly the same energy.
In contrast, the NIST researchers' clock uses laser light with a frequency of 1,120,000 billion cycles per second to drive a higher-energy jump called an optical transition in a single aluminum ion held in an elaborate trap. The cesium standard is accurate to three parts in 10 million billion; the new aluminum clock has an accuracy nearly 40 times better. That extra accuracy makes it possible to demonstrate the effect of relativity on a more human scale. The researchers built two aluminum clocks, and to test the velocity effect they set the ion in one jiggling back and forth in its trap with a speed as low as 4 meters per second. They were able to resolve the 2-parts-in-10-million-billion slowing that motion caused in the clock with the moving ion. To test the gravity effect, the physicists started with one clock 17 centimeters below the other and then raised the first clock by 33 centimeters. This time they detected a 4-parts-in-100-million-billion shift in the frequency of the raised clock, as predicted by the theory of general relativity"
