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"After two hours, I looked at my watch," a reviewer of Wagnerian opera is said to have written. "I found that 17 minutes had gone by."
In 1905, Albert Einstein wrote his own treatise on the relativity of time, famously theorising that time speeds up or slows down according to how fast an object is moving in relation to another object.
Thus, according to his hypothesis, a clock which is in motion ticks more slowly than an identical clock which is at rest -- a phenomenon that Einstein called time dilation.
In a study published on Sunday, the most accurate experiment yet into time dilation has proven the great German physicist to be bang on target.
An international team of researchers used a particle accelerator to whizz two beams of atoms around a doughnut-shaped course to represent Einstein's faster-moving clocks.
They then timed the beams using high-precision laser spectroscopy and found that, compared with the outside world, time for these atomic travellers did indeed slow down.
"We were able to determine the effect more precisely than ever before," said lead researcher Gerald Gwinner of the University of Manitoba in Winnipeg, Canada.
"We found the observed effect to be in complete agreement."
The experiments, said Gwinner, confirm the technology aboard US military satellites that provide the signals for the Global Positioning System (GPS) -- the "satnav" network that is used as a navigational aid around the world.
GPS satellites have precise atomic clocks on board in order to send out synchronised signals that are then transcribed by trigonometry to give one's position.
"GPS uses satellites to measure the position of objects on the ground, but it needs to take into account the fact that the satellites themselves are in motion at high speeds as they orbit the Earth," said Gwinner.
"Our test validates the theory used by the devices to compensate for the satellites' motion."
The experiments took place at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and include researchers from that organisation, the Max Planck Institute for Quantum Optics in Garching, and Mainz University.
The findings were published online Sunday by the journal Nature Physics.
The first measurement of Einstein's time dilation took place in 1938, when US scientists used the Doppler effect -- the change in pitch when a sound and the person hearing it are moving apart or closer together -- as the measuring tool.
Einstein's theory of relativity has become the basis for innumerable science fiction tales, for it opens up the prospect of bending and distorting time.
If one of two identical twins were launched into space at very high speed, when he returned home, he would be younger than his earthbound twin.
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