Ultraprecise atomic clocks put Einstein’s special relativity to the test

The ticktock of two ultraprecise clocks has proven Einstein right, once again.

A pair of atomic clocks made of single ions of ytterbium kept pace with one another over six months, scientists report March 13 in Nature. The timepieces’ reliability supports a principle known as Lorentz symmetry. That principle was the foundation for Einstein’s special theory of relativity, which describes the physics of voyagers dashing along at nearly the speed of light.

Lorentz symmetry states that the rules of physics should remain the same whether you’re standing still or moving at a breakneck speed, and no matter what direction you’re facing (SN: 7/8/17, p. 14). The clocks kept up with one another as the Earth rotated, confirming that idea.

The two ytterbium ions — positively charged atoms — absorbed and emitted light at a particular frequency, functioning like the ticking of a clock hand. The ions, which were oriented in different directions, rotated as the Earth turned, making a full cycle each day. If the atomic clocks’ ticks varied based on their orientation in space, the experiment would reveal a daily variation in the relative frequencies from the two clocks — a violation of Lorentz symmetry. But the atomic clocks agreed within about a tenth of a quadrillionth of a percent, confirming with about 100 times the precision of previous tests that Lorentz symmetry held.

Although Lorentz symmetry has been confirmed repeatedly, some scientists predict that it won’t hold up to increasingly precise tests. Some theories of quantum gravity, which aim to unite scientists’ understanding of gravity with the theory of very small objects (SN: 10/17/15, p. 28), suggest that Lorentz symmetry’s days are numbered. But so far, there’s no hint of its demise.