Why are atomic clocks so important

Atomic clocks: second errors after billions of years

Atomic clocks that use the vibrations of cesium atoms are extremely accurate. But physicists all over the world are not satisfied with that. Their goal: the nuclear clock based on changes in energy in the thorium atomic nucleus.

Photo: Physikalisch-Technische Bundesanstalt

All clocks work on the same principle. Something is kicked periodically so that it vibrates. Then the vibrations are counted. A certain number corresponds to one second. With the first clocks it was the pendulum. Later the balance. In modern clocks it is quartz crystals that are electrically excited and in the most common atomic clocks it is cesium atoms, more precisely the cesium isotope 133, which has been shown to be the easiest to care for. It is evaporated in an oven. According to this, the isotopes have two different energy levels. Those who are unsuitable for timing will be flipped away magnetically. What remains are isotopes that are all at the same energy level.

Now they are excited by the microwave. They absorb energy in order to emit it again. This happens with a certain frequency, namely 9,192,631,770 Hertz, which is recorded by a counter. That corresponds to a single second. In Germany, the atomic clock with the designation CS2, operated by the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, sets the pace. For example, it provides the time signal that the long-wave transmitter in Mainflingen in Offenbach, Hesse, broadcasts. It is received by radio clocks throughout Central Europe. And she also reveals her knowledge on the Internet, at uhr.ptb.de the CS2 time can be found on the Internet.

Next leap second on June 30, 2019?

In order to compensate for the tiny differences between the globally uniform atomic clock time and the world time based on the rotation of the earth, leap seconds have to be added every now and then, which bring the two times back into line. The last time there was such a correction was on December 31, 2016. The date for the next one has not yet been set, but is imminent. Presumably it will happen on June 30, 2019.

Atomic clocks are even more accurate with strontium

CS2 goes wrong six billionths of a second in a year. In other words: in six billion years, the Braunschweig atomic clock will go one second behind - or ahead. For some physicists this is not enough. You want to measure the time even more precisely. Scientists at the University of Colorado and the National Institute of Standards and Technology succeeded in this two years ago. They used an optical grid made of laser beams that looks like a tiny cage. They locked a few thousand atoms of the alkaline earth metal strontium in it and hit them with another laser beam with a wavelength of exactly 698 nanometers. Its energy causes the strontium atoms to oscillate between two energy states, 430 trillion times per second. In 15 billion years, the clock would only lose one second.

More about the atomic clock, which is accurate to 1.5 trillionths of a second

The core clock becomes the precision product of time measurement

A core clock would be even more accurate. In contrast to normal atomic clocks, it is not the oscillations of electrons that circle around an atomic nucleus that serve as a clock, but rather an excitation state in the atomic nucleus itself. Thorium is currently the most promising candidate for the construction of a nuclear clock. Some may still know it as breeding material from the time of the pebble bed reactors - it turned into fissile uranium while the reactor was in operation. Now it should bring Kernuhren to life. It doesn't exist yet, but researchers like Ekkehard Peik from PTB already know how to do it. And above all, why such a clock is far more accurate than all other atomic clocks.

He called the decisive advantage of this type of clock “world of physics”. Huge forces prevail in the core - a lot of energy is released when it is split in reactors. Unlike whole atoms, the nuclear forces are immune to external influences such as vibrations and temperature changes. That is their advantage. The disadvantage is that large forces are also required to achieve other energy levels. Usually you can only do something with radioactive gamma radiation. And this is where thorium comes in because it is more frugal. Ultraviolet radiation, which can easily be produced with lasers, is sufficient here.

The thorium isotope 229 is still not an optimal candidate. It is radioactive and decays with a half-life of 8,000 years. This means that after this period of time only half of the material is left. Peik thinks that it is "long-lasting enough to be able to operate a core clock with it".

Precise time is important for navigation systems and autonomous cars

"Of course, you don't need such a complicated clock for everyday use and normal scientific purposes," says the physicist. It would only bring progress in basic research. For example, satellite navigation is part of everyday life for atomic clocks. This is based on time-of-flight measurements of signals transmitted by the American GPS, the Russian Glonass, the Chinese Beidou and the European Galileo satellites, for example. There is an atomic clock in each satellite. The better it corresponds to the world atomic time, the more precisely the system works. The position of the receiver in a car, for example, can be determined from the transit times of the signals from several satellites.

A brief history of the atomic clock

The American physicist Isidor Isaac Rabi at Columbia University in New York laid the foundations for the atomic clock. In 1944 he received the Nobel Prize in Physics for this. Harold Lyons, microwave expert at the US National Bureau of Standards, built the first atomic clock in 1949. It was based on the vibrations of ammonia molecules. However, it did not run as precisely as it should. That is why it was converted three years later. Now cesium atoms set the pace. This principle still applies today. Worldwide there are more than 260 atomic clocks in 60 time centers, which are regularly compared with each other. The result is the International Atomic Time, which is binding for the whole world.


More articles about the atomic clock:

Braunschweig researchers are working on the successor to the atomic clock, the optical super timer with a significantly higher clock frequency

We have also reported earlier about the strontium atomic clock from the USA:

  1. 2016: This atomic clock is accurate to 1.5 trillionths of a second
  2. 2015: The most accurate watch in the world comes from the USA
  3. 2014: New strontium atomic clock: Accurate to the second for five billion years

A contribution from:

  • Wolfgang Kempkens

    Wolfgang Kempkens studied electrical engineering at RWTH Aachen University and graduated with a diploma. He worked for a daily newspaper and magazine before establishing himself as a freelance journalist. He mainly deals with environmental, energy and technology issues.