To test Einstein’s theory of relativity, we need extremely accurate clocks. Two research groups have now developed atomic clocks that set new precision records. These test the theory of relativity on the millimeter scale.
According to Einstein’s theory of general relativity, the gravity of objects such as planets or stars distorts spacetime. This makes time slow down as you get closer to the object. The further a clock is above the earth’s surface, the faster it ticks.
Ultra Accurate Atomic Clock
Because gravity affects the ticking of a clock, you can use ultra-accurate clocks to map the Earth’s gravitational field. And you can use it to detect gravitational waves, for example.
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For such detailed measurements, the atomic clocks must be very precise. It is also useful for some applications if the devices are compact. Now these kinds of clocks often fill a lab.
Two independent American research groups have taken important steps in this direction. They published their results in the same issue of the scientific journal Nature† One publication is about the most accurate atomic clock ever. The second publication is about a clock that is slightly less precise, but which demonstrates how the technology can be made more compact.
Laser measurements
The most accurate clocks are optical atomic clocks. They use the regularity with which atoms ‘tick’ by jumping back and forth between two energy levels. The ticking goes with the frequency of the light that is received or emitted during these energy transitions. What frequency that is depends on the type of atoms. For example, cesium clocks use a cloud of cesium atoms. Both new atomic clocks work with strontium.
The frequency of the atoms is measured by shining a laser of that frequency on a cloud of atoms. Why not immediately use the laser frequency as a clock? It is not accurate enough, because laser frequencies are easily distorted. The clocks use the cloud of atoms to continuously check whether the laser’s frequency is still correct and to adjust it if necessary.
Millimeter scale
The difference between the two new optical atomic clocks is that one focuses the laser on a single cloud of strontium atoms about a millimeter in size. The other experiment uses one laser to look at two clouds of strontium atoms, one of which is one centimeter higher.
By dividing the measurements over two clouds, the researchers were able to measure stably for longer and they were less dependent on the quality of the laser. This technique could therefore also be used in a more compact setup with a simpler laser. Their setup can measure the difference between two clocks that are one second apart after ticking 300 billion years.
The other researchers had a better quality laser. They managed to measure even more accurately. In addition, they were able to measure the time difference between the atoms at the top and bottom of the millimeter-sized cloud. Never before has the theory of relativity been tested and confirmed on such a scale. As the clock ticks at home, it doesn’t tick a millimeter higher.