Ever since Albert Einstein published his theory of general relativity back in 1915, a principle known as the gravitational redshift has played a major role in stellar physics and space engineering. In a nutshell, this refers to the way that time speeds up and slows down depending on the strength of gravity, meaning that clocks tick faster in space than they do on Earth.

In other words, time moves slower as you get closer to a massive object like a planet, thanks to the increasing gravitational potential. This is nothing new, and without an understanding of this phenomenon, we would never have been able to develop satellite-based navigation systems like GPS.

However, while the difference in time between a person on the ground and a satellite in orbit has been demonstrated many times, researchers have for the first time measured the strength of the gravitational redshift from the observation deck of a skyscraper.

Publishing their work in the journal Nature Photonics, the researchers reveal that time moves around four nanoseconds per day faster on the 450-meter-high (1,476-foot) observation deck of the Tokyo Skytree than it does on the ground. That time is marginally accelerated at such an altitude comes as no major surprise, but what is significant here is that it has actually been measured. Performing such a calculation can only be achieved using an optical lattice clock, which is a highly expensive and bulky piece of equipment, often occupying an entire laboratory.

Yet study author Hidetoshi Katori and his colleagues were able to produce a much smaller device, roughly the size of a regular household fridge, which can take measurements of time that are “comparable to space experiments” in their accuracy.

In their write-up, the study authors declare that their small, portable optical lattice clock is now “ready for field applications,” and hope to see this exquisitely sensitive piece of equipment put to a number of uses other than telling time. For example, the researchers explain that these clocks could also be used for “monitoring spatiotemporal changes of geopotentials caused by active volcanoes or crustal deformation,” thereby helping to predict earthquakes and other natural disasters.