Highly precise timekeepers
To study this intersection, the researchers used optical lattice clocks, the most accurate timekeepers ever created. They trapped thousands of strontium atoms vertically in grids of laser light.
Atomic clocks rely on the steady frequency at which electrons in atoms transition between energy levels. Keeping a count of these oscillations allows atomic clocks to measure time with extremely high precision.
The effects of Einstein’s mass-energy equivalence come into play when performing high-precision measurements through two effects.
First is the gravitational redshift which causes atoms at different heights to tick at different rates, as gravity is slightly weaker for atoms higher up. Second, as atoms move, the second-order Doppler shift generates tiny changes in frequency.