# The comparison of 2 atomic clocks has confirmed their excellent accuracy as well as a fundamental hypothesis of the theory of relativity — ScienceDaily

According to Einstein, the speed of light is always the same. However, according to the theoretical model of quantum gravity, this spatiotemporal uniformity does not apply to particles. Physicists are now trying to detect changes in spatiotemporal uniformity using two optical cuckoo clocks. Their results are published in the latest issue of Nature.

In his special theory of relativity, Einstein proposed the hypothesis that the speed of light is always the same regardless of the conditions. However, according to the theoretical model of quantum gravity, this spatiotemporal uniformity may not be applicable to particles. Physicists now test this hypothesis through the first long-term comparison of two optical cuckoo clocks of Physikalisch-Technische Bundesanstalt (PTB). Using these clocks, the error is only one second in 10 billion years, and it should be possible to measure the small deviation of the electron motion in the sputum. But when the clocks were in different directions in space, the scientists did not notice any changes. Due to this result, the current limit of the space-time symmetry has been greatly improved by 100 times. In addition, the minimum system measurement uncertainty of the optical chirp clock is less than 4 × 10E-18 has been confirmed. A team of physicists from PTB and the University of Delaware published the results in the latest issue of * Nature *.

It is one of the most famous physics experiments in history: as early as 1887, Michelson and Morley proved what Einstein later expressed in theory. By means of a rotating interferometer, they compared the speed of light along two perpendicularly intersecting optical axes. The result of this experiment became one of the basic statements of Einstein's special theory of relativity: the speed of light in all directions is the same. Now someone can ask: Is the symmetry of space (named after Hendrick Anton Lorenz) also applicable to the movement of matter particles? Or is there any direction that moves faster or slower despite the constant energy? Especially for the high energy of particles, the theoretical model prediction of quantum gravity violates Lorentz symmetry.

Experiments have now been carried out with two atomic clocks in order to study this problem with high precision. The frequencies of these atomic clocks are each controlled by the resonant frequency of a single Yb+ ion stored in the trap. Although the electrons of Yb + ions have a spherically symmetric distribution in the ground state, they exhibit a significantly elongated wave function in the excited state, and thus mainly move in a spatial direction. The direction of the wave function is determined by the magnetic field applied within the clock. The field orientation is chosen to be approximately at right angles to the two clocks. The clock is securely mounted in the laboratory and rotates with the Earth once a day relative to a fixed star (more precisely: once within 23.9345 hours). If the velocity of the electron depends on the direction of the space, this will result in a frequency difference between the two atomic clocks that occur periodically, as well as the rotation of the Earth. In order to be able to distinguish this effect from any possible technical effects, the frequency of the Yb + clock is compared over 1000 hours. During the experiment, no change between the two clocks was observed over the acceptable period duration from a few minutes to 80 hours. For theoretical interpretation and calculation of the atomic structure of Yb + ions, the PTB team worked with the theorists of the University of Delaware (USA). The results now have improved the limits set by researchers at the University of California at Berkeley in 2015, and Ca+ ions have increased dramatically by a factor of 100.

On average, the relative frequency deviation of the two clocks is less than 3 × 10E-18. This confirms the combined uncertainty of the clock previously estimated to be 4 x 10E-18. In addition, it is an important step in characterizing optical atomic clocks at this level of precision. These clocks will deviate from each other for a second only after about 10 billion years.

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Materials are provided by ** Physikalisch-Technische Bundesanstalt (PTB) ** . * Note: Content can be edited for style and length. *

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