Lorentz Invariance and Time Dilation – Einstein is still Right on Time

Since the formulation of Special Relativity by Albert Einstein more than 100 years, its main ingredient, the space-time symmetry of local Lorentz invariance (LI), forms one of the corner stones of all currently accepted theories describing nature on a fundamental level. Already this fundamental role alone demands thorough experimental affirmation of this symmetry. Further motivation for incessant experimental tests with ever increasing scrutiny comes from theoretical attempts to solve some of the unsettled problems in contemporary physics, such as the reconciliation of quantum theory and general relativity, which allow LI to be violated.   

Within the wealth of LI tests, ‘Ives-Stilwell’ experiments stand out for their large Lorentz boost, which neither depend on sidereal variations nor on a special reference frame. These experiments, which are based on the optical Doppler effect, directly determine the relativistic time dilation effect, one of the most fascinating and at the same time most disconcerting aspects of the space-time symmetry of Special Relativity as it abolishes the notion of absolute time. I will report on our modern versions of this experiment, which involves metastable ⁷Li⁺ ions moving with velocities of up to 1/3 of the velocity of light as atomic clocks. The experiments combine ion storage and ion cooling in heavy ion storage rings together with laser induced saturation and optical-optical double resonance spectroscopy to read out the clock frequencies.  Comparing these frequencies with those measured at rest allowed us to verify the relativistic time dilation with unprecedented precision.