# The spacetime is symmetrical

## Atomic Clock experiment confirms a key message of Einstein's relativity theory

Read out**Einstein is right: even the smallest elementary particles react independently of their space-time orientation - when all other conditions are the same.** **This key statement of the special relativity theory of Albert Einstein has now confirmed an experiment with two optical atomic clocks.** **Although the clocks were perpendicular to each other, their frequency developed to a measurement accuracy of three trillions no deviations, as the researchers report in the journal "Nature".**

Albert Einstein published his theory of relativity more than 100 years ago. But so far no test has been able to disprove its validity - whether in the curvature of space-time, time-stretching, the equivalence principle or the local position invariance. Also a key statement of the special relativity theory, the so-called Lorentz transformation, has withstood all tests so far. According to this, not only is the speed of light in all directions the same, but also in other processes the space-time is symmetrical, provided that the remaining conditions are the same.

### Are there any deviations?

But now there are quantum physical models that predict deviations from the Lorentz transformation, especially at high energies. If this is true, then some elementary particles would have to show differences in their behavior depending on the direction. "Modern tests of Einstein's theory of relativity therefore attempt to measure hitherto undiscovered violations of the Lorentzian symmetry, " explain Christian Sanner of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig and his colleagues.

So far, however, no test has been able to detect deviations within its respective measuring limits - neither with atomic nucleus components such as neutrons and protons, nor with photons or electrons, as the researchers report. However, modern atomic clocks now make it possible to check the fundamental symmetry properties of space-time more accurately than before. That's why Sanner and his team have once again put Einstein's Theory of Relativity to the test.

### Ytterbium ions in common mode

For their experiment, the physicists compared the "ticking" of two optical ytterbium atomic clocks for six months. At the center of these clocks is an ytterbium ion whose electrons in the excited state vibrate more strongly in one direction. The researchers now aligned the clocks using magnetic fields in such a way that the wave functions of the electrons were perpendicular to each other in both ytterbium ions. display

Swinging ytterbium ions remain in sync, even though the wave functions of their electrons (yellow) are perpendicular to each other. PTBThe trick here: If space-time acts symmetrically on these clock-ions, their vibration frequencies must remain the same regardless of their spatial orientation. "If there are any objections to the Lorentzian symmetry, then they must lead to periodic frequency deviations between the two clocks while the earth revolves and revolves around the sun, " explain the physicists. For six months, they therefore compared the frequencies of both atomic clocks every 2.36 seconds a total of approximately 1.7 million times.

### No evidence of a violation

The result: Although the optical atomic clocks register each change to within trillionth of a second, there were no deviations. "We find no convincing evidence that any of the comparison parameters deviate from zero, " report Sanner and his colleagues. Over the entire measuring time, both atomic clocks worked precisely in a common mode the measuring limit was below three trillionths, according to the physicists.

With this, the researchers have now checked the Lorentz transformation for electrons a hundred times more accurately than all previous measurements. "From the absence of modulations down to the level of 10 ^{-19} we conclude that the Lorentz symmetry is confined to strict limits of orders of 10 ^{-21} ", state Sanner and his team. This precision can help to put theories of quantum gravity to the test in the future.

It seems clear: Einstein's theory was right again and passed this test with flying flags. (Nature, 2019; doi: 10.1038 / s41586-019-0972-2)

Source: Physikalisch-Technische Bundesanstalt

- Nadja Podbregar