Time doesn't always go by at the same speed everywhere.

This is one of the strange consequences of the theory of relativity.

Not only moving clocks tick slower than stationary ones.

Gravity also influences the passage of time: clocks run slower in the immediate vicinity of a massive body than at a certain distance.

Physicists have been able to experimentally confirm this prediction of the general theory of relativity several times over the past few decades by bringing precision clocks to great speeds or heights with airplanes or rockets, or by locating their experiments on mountains or towers.

Now, researchers at the National Institute of Standards and Technology in Boulder, Colorado, have subjected the effect of gravitational time stretching, or time dilation, to the most precise test yet.

Manfred Lindinger

Editor in the department "Nature and Science".

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The General Theory of Relativity, published by Albert Einstein in 1915, describes gravitation as a result of the curvature of space and time.

Massive objects would then deform the four-dimensional space-time structure - similar to how a heavy ball deforms a rubber membrane.

The consequence: not only the paths of all bodies and also those of light rays are subject to the curvature of space-time, time itself is also subject to the force of gravity.

These relativistic effects must be taken into account for precise location determinations using satellite navigation.

Optical clocks beat every second

If you want to measure tiny time differences, you need a chronometer that is as accurate as possible.

However, even the best Swiss clocks do not tick precisely enough to demonstrate time dilation in gravitational fields.

Such experiments only became possible with the development of atomic clocks in the 1960s, which are set by the microwave oscillations of a cesium atom.

In 1976, an atomic clock on board a rocket (Gravity Probe A experiment) was shot to an altitude of around 10,000 kilometers.

The time difference compared to an identical atomic clock on earth was one second in 73 years.

Time measurements at significantly lower altitudes also confirmed Einstein's predictions.

Two years ago, Japanese researchers subjected gravitational time dilation to the most precise test to date on the Tokyo TV tower "Skytree" at a height of 450 meters.

On average, the atomic clock ran four nanoseconds faster per day than an identical atomic clock on the ground.

But what about even smaller height differences - in the range of centimeters or millimeters?

In order to be able to measure an effect of time dilation and a possible deviation here, clocks must be extremely accurate - that of cesium clocks is no longer sufficient.

One therefore resorts to optical atomic clocks, in which light oscillates at a much higher frequency, which affects the rate accuracy.

In fact, an optical chronometer would only go wrong by one second in billions of years.

For comparison: A cesium clock “only” ticks exactly one second for about 30 million years.

Relativity meets quantum physics

The researchers in Boulder used the currently best optical atomic clock for their measurements.

It is more accurate than a cesium clock by a factor of fifty.

Around one hundred thousand deep-frozen strontium atoms are held in suspension in an optical lattice structure by intersecting laser beams and excited with an extremely stable red laser beam.

If the atoms and the external laser field oscillate in sync, the frequency of the light emitted by the atoms and thus the clock rate of the strontium atomic clock is measured.

Since there are many thousands of synchronously beating clocks, a significantly higher measurement accuracy is achieved than if one were to work with a single ion.

The researchers led by Jun Ye lifted some of the atoms trapped in the light lattice by one millimeter, creating a kind of second strontium atomic clock.

The researchers were able to prove that the slightly higher-lying atoms actually beat the clock slightly faster than the lower ones.

The frequency difference was noticeable in the nineteenth decimal place, as Jun Ye and his colleagues write in "Nature".

This small effect has no effect on the everyday world.

However, the researchers see possible applications in geodesy.

The precise chronometers could be used to create precise height profiles of mountains or to precisely gauge the depth of the oceans.

The precise strontium atomic clocks can also be used to predict earthquakes.

With their measurements, the researchers working with Ye have also come a good deal closer to a vision: it would be possible to test the predictions of the general theory of relativity and quantum physics simultaneously with unprecedented precision.

In this way one could check how the quantum mechanical matter waves of the atoms behaved in a curved space.

To do this, the scientists would have to determine the influence of gravity on time more precisely by a factor of ten.