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Optical clocks on Tokyo Skytree help prove time passes faster at higher altitude

Professor Hidetoshi Katori is seen with an optical clock at the Faculty of Engineering at the University of Tokyo, in the capital's Bunkyo Ward, on April 3, 2020. (Mainichi/Suzuko Araki)

TOKYO -- Tokyo Skytree, the world's highest broadcasting tower, has become host to an experiment by researchers at the University of Tokyo and others to prove that time passes more quickly at higher altitude where gravity is weaker, as hypothesized in Einstein's general theory of relativity.

The experiment was made possible at Japan's tallest structure, in Tokyo's Sumida Ward, by using a portable optical clock capable of highly accurate time measurements, which can measure a period of time spanning less than a second by up to 18 decimal figures.

Placing one of the clocks 450 meters up from ground level on the tower's observation deck, the team was able to confirm that each second at the top of Tokyo Skytree passes approximately five in a hundred-trillionths of a second faster.

The theory has been proven by University of Tokyo professor Hidetoshi Katori, who specializes in quantum electronics, and others. The optical clock uses quantum particles to record time with such accuracy that it would take it 30 billion years to go off by a second. The technology is thought to be a candidate for the next generation of time-measuring devices.

Professor Katori announced the concept of the research in 2001, and some have called for his work to be recognized with the Nobel Prize in physics. Until now, the clocks have required the majority of the space in a 20-square-meter laboratory to be set up.

But the miniaturized versions used for the experiment can be constructed from three boxes, each measuring 44 centimeters wide, 64 cm deep and 30 cm high. Two optical clocks were installed at Tokyo Skytree; one on the observation deck, and another in a meeting room on the structure's first floor. By placing them this far apart, the team was able to observe how time passes differently depending on the gravitational force.

Their results showed that time was observed to pass on the observation deck around 4.3 nanoseconds faster per day. A single nanosecond constitutes one billionth of a second. Converted into a per year measurement, the time at the higher-placed optical clock would pass 1.6 microseconds faster; 1 microsecond is a millionth of a second.

Based on calculations from the general theory of relativity, a difference of 450 meters in height represented a change of 4.3 nanoseconds per day, a result that the team says proves Einstein's hypothesis.

The researchers went on to say that the findings could have practical applications including by using optical clocks placed on mountains and on the ocean floor to take detailed measurements of changes in the Earth's crust. It's hoped that the optical clocks could help bring about a new system of measurement.

The results were published in the electronic edition of the U.K. science journal Nature Photonics on April 6.

(Japanese original by Suzuko Araki, Science & Environment News Department)

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