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Testing the symmetry of space-time by means of atomic clocks


A long-term comparison of two optical ytterbium clocks has confirmed the fundamental hypothesis of Einstein's Theory of Relativity according to which the speed of light is always the same, no matter what the conditions are.

The experimental setup at PTB: A tunable laser excited an extremely narrow-band resonance in an Yb+ ion of an atomic clock. Two ions with wave functions (yellow in the figure) that are oriented at right angles were interrogated by means of laser light with an adjustable frequency shift Δf to measure a possible frequency difference. The whole experimental setup rotated together with the Earth once a day relative to the fixed stars.

According to theoretical models of quantum gravitation, this uniformity of space-time may not apply to particles. With the atomic clocks used, whose error amounts to only one second in ten billion years, it would have been possible to measure even extremely small deviations of the movement of the electrons in ytterbium. But the scientists did not detect any change when the clocks were oriented differently in space. Due to this result, the current limit for testing the space-time symmetry by means of experiments has been drastically improved by a factor of 100. In addition to this, the extremely small systematic measurement uncertainty of the optical ytterbium clocks of less than 4 · 10−18 has been confirmed. The team consisting of physicists from PTB and from the University of Delaware published its results in the scientific journal “Nature”.