Physicists have proven Einstein’s theory of relativity by using two super-accurate optical clocks, one held a few centimeters above the other.
Einstein's theory of relativity says that time passes slower closer to the ground because of the Earth's gravitational field, but this is the first time that researchers used clocks to prove it.
It is well known and widely accepted that time passes at different rates, depending on the circumstances, and that a clock at lower altitude, thus lower gravitational energy, should run by about 3 microseconds per year per kilometer of elevation slower than one at higher altitude,.
This has already been tested by comparing super precise atomic clocks on the ground with those in high-flying jets, and satellite-based GPS take this small difference into consideration.
The researchers that managed to find 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 an optical clock, are Chin-wen Chou, Till Rosenband, and colleagues at the National Institute of Standard and Technology (NIST) in Boulder, Colorado.
An atomic clock is based on the fact that the electrons in an atom occupy "states" with distinct energies and are able to hop between two states by emitting or absorbing electromagnetic waves of a certain frequency.
These waves are shone on the atoms and a feedback loop keeps their frequency tuned so that the atoms keep jumping back and fourth between the two states.
These waves are then assimilated to the movements of a pendulum, only much evenly and faster, Science NOW reports.
The international time standard is now set by atomic clocks that use microwaves with a frequency of 9.2 billion cycles per second, but the NIST researchers' clock used laser light with a frequency of 1,120,000 billion cycles per second.
This way the energy jump has higher and it became an optical transition in a single aluminum ion held in an elaborate trap.
The accuracy of the cesium standard is of 3 parts in 10 million billion, and the new aluminum clock is almost 40 times more precise, so it can prove the effects of relativity at a more human scale.
The scientists built two aluminum clocks and to test the speed effect, they set the ion in one to move back and forth in its trap with a speed as low as 4 meters per second, and found the 2-parts-in-10-million-billion slowing caused by motion in the clock with the moving ion.
For the test of the gravity effect, they started with one clock 17 centimeters below the other and then raised the first clock by 33 centimeters; they found a 4-parts-in-100-million-billion shift in the frequency of the raised clock.
Daniel Kleppner, a physicist at the Massachusetts Institute of Technology in Cambridge, who was not involved in the work, said that “the demonstration of the gravitational shift by elevating a clock about one foot is quite stunning, [even though it] does not change anyone's view on relativity.”
Nan Yu, a quantum physicist at NASA Jet Propulsion Laboratory in Pasadena, California, said that “what amazes me is the advancement of the optical clocks—10 or 20 years ago they were only a dream.”
they are now a reality thanks to recent advances in laser techniques, Yu adds and he predicts that within a few years the cesium microwave standard could be replaced by some type of optical clock.
“These clocks are making rapid progress, but they do not run continuously” by themselves, noted Christophe Salomon, a physicist at the École Normale Supérieure in Paris, and added that the optical clocks aren't quite mature technologically.
This experiment was reported in the 24 September issue of Science.