Einstein said that time goes a little faster where gravity is weaker. Gravity is a little weaker in high places. Japanese scientists tested this at Tokyo Skytree. They put one very exact clock on the ground and another about 450 meters up.

The clock at the top went faster by about four billionths of a second each day. People cannot feel this, but it is real.

According to Einstein's general theory of relativity, time runs slightly faster where gravity is weaker, such as high above the ground. The difference is tiny, so it requires extremely precise clocks. Hidetoshi Katori of the University of Tokyo invented the optical lattice clock, which counts the vibrations of atoms trapped by lasers. It is so precise that it would lose only about one second in 30 billion years.

In 2020, his team placed two portable clocks at Tokyo Skytree, one on the ground floor and one about 450 meters up. The upper clock gained about four nanoseconds per day, matching Einstein's prediction. The same effect must be corrected in GPS satellites.

Einstein's general theory of relativity predicts that clocks tick faster where gravity is weaker, so a clock high above the ground should slowly pull ahead of one below. Measuring this over everyday heights demands extraordinary precision. The optical lattice clock, proposed in 2001 by Hidetoshi Katori, traps many atoms in a grid of laser light and counts the oscillations of the light they absorb. Because many atoms are measured simultaneously, it reaches 18-digit precision quickly, losing roughly one second in 30 billion years, over a hundred times better than the cesium clocks that define the second.

In 2020, Katori's team installed two transportable clocks at Tokyo Skytree, about 450 meters apart in height, and found that the upper clock gained about four nanoseconds per day, consistent with relativity and with surveyed elevation data. Earlier precise tests had required satellites or rockets, and a 2010 laboratory experiment detected a 33-centimeter height difference, but this was a pioneering measurement with portable clocks in an ordinary building. Since 18-digit precision corresponds to about one centimeter of height, such clocks could one day detect ground uplift associated with volcanic activity.