Portable clock provides new ruler for measuring the Earth

Enlarge / Researchers measured how tall this building is by throwing a portable atomic clock in its elevator. (credit: Philip Fong/Getty Images)

When it comes to measuring our position on the Earth, we tend to turn to one of the global positioning satellite constellations. For standard commercial devices, the absolute position accuracy is a few meters, though it can be reduced to a few centimeters with more advanced techniques. But there are applications where satellite-based measurements aren't right for the job. Luckily, the Earth supplies its own measuring stick: gravity.

For instance, if you want to measure subsidence and swelling around a volcano, then an accuracy of a few centimeters would be highly desirable, and standard GPS isn't accurate enough without some specialized hardware. To get around this problem, researchers in Japan have proposed and demonstrated possibly the most expensive altitude sensor in the history of humanity. Fortunately, it comes with a bonus feature: it will continuously test general relativity.

Shining a light on time

Einstein's general theory of relativity tells us that clocks tick a little slower in a gravitational field. So if one clock is a centimeter higher than another clock, it will speed up. How much? Not a lot: a clock needs to be accurate to within a few attoseconds (10^-18s) to be able to detect centimeter differences in height. As it happens, optical lattice clocks can achieve this accuracy.

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