Relativity Comes Down to Earth Over a century ago, Einstein presented the theory of relativity—where space and time are no longer fixed concepts, but are relative to an observer and their frame of reference. Tests of relativity have usually been confined to space-based measurements and/or with objects with velocities approaching the speed of light. Time-dilation and length-contraction have been confirmed and are used in satellite communication and global positioning systems. Using the precision of state-of-the-art optical clocks, Chou et al. (p. 1630 ) have confirmed that relativistic effects can now be measured at speeds attained by 100 meters sprinters (10 m/s) and gravitational effects due to just one meter height difference.