The theory of general relativity describes gravity and gravitational phenomena, usually having to do with really massive objects, like stars, black holes, whole galaxies. Physicists consider it a particularly elegant and significant theory. However, the conditions of our daily life are too “small-scale” for it to get involved.
Now, scientists set out to find if general-relativistic effects can be measured in everyday conditions. Not only did they succeed, they also made their point in a cool way. Even better? They made it possible to use general relativity for practical purposes here on Earth.
To begin with, the team formed by five Japanese research groups needed very precise clocks. Relativity says that time passes more slowly in stronger gravity, so the plan was to detect small changes in the ticking speed at different heights. The go-to choice for such precision is optical lattice clocks, where two lasers pointing against each other form “nodes” of light that trap atoms, vibrating in well-defined ways. The team added improvements to the quality of the atomic signal and fitted the system in a portable 60cm-tall box, as the whole point was to take gravity measurements out of the lab.
Selecting a real-world setting for the experiment mustn’t have been hard; the team picked Tokyo Skytree, an iconic television tower with a convenient public observation floor at 453 meters. After installing the clocks at the observatory and on the ground floor they spent five days measuring in October 2018.
The result was a tiny but clear shift in the frequency of the two identical clocks, with the one at the top of Skytree “ticking” faster. This shift allowed them to calculate the difference in the strength of Earth’s gravity between the two points, and from it the difference in height.
Moreover, the experiment’s accuracy permits the detection of a difference of even a few centimeters. This could turn gravity into our aid for detecting small position changes in places difficult to access — no wonder that one of the first applications could be in monitoring the motion of Earth’s crust.