The keeper of time


Joachim Ullrich is President of the PTB – Physikalisch-Technische Bundesanstalt in Braunschweig, Germany’s national metrology institute. The clocks that tick here are among the most accurate in the world. In this interview, the physicist explains how they work, what this has to do with Coordinated Universal Time and why it’s a good idea to define a second to sixteen decimal places.

trends in automation: Professor Ullrich, the PTB is known for its accurate clocks and is thus regarded as an authority in matters of time. But what exactly is time?

Prof. Dr. Joachim H. Ullrich: This is a highly complex question. We physicists make it easy on ourselves and define time using predictable, recurring processes, such as the earth’s rotation or a pendulum, for example. Nobel prize winner and curator of the Physikalisch-Technische Bundesanstalt Albert Einstein gave us a very pragmatic definition of time: “Time is what you read on a clock.” Since Einstein, however, we also know that time is relative. For example, it passes more slowly when we are in motion or in a gravitational field. Human perception of time is also relative. Here’s another quote often attributed to Einstein: “When you sit on a hot stove for two minutes, it feels like two hours. But, when you sit with a nice girl for two hours, it feels like two minutes. That’s relativity!”
There are also many unanswered questions, such as whether time has a beginning or an end and how long the present lasts in human perception. The body’s biological clock and cultural differences in how we deal with time are also highly interesting topics in science at present.

trends in automation: How do you measure time?

Ullrich: Using a pendulum, for example. The shorter the pendulum, the faster it swings and the more accurately it measures time. Even more accurate are quartz clocks in which an electrical charge causes a crystal to oscillate over 30,000 times per second. The most accurate clocks we have at present are atomic clocks, even though the atoms themselves don’t oscillate. Instead, we use electromagnetic radiation or, to be more precise, microwaves. They oscillate much faster than a quartz crystal, in the region of nine billion times per second. We use this microwave radiation to excite the electrons of caesium atoms. And because this only works when the radiation has a very specific oscillation frequency, we can use this to define and precisely determine the value of a second.
Of course, we must constantly check that we have set the correct clock pulse and that the electrons are actually excited. To do this, we begin by sending the caesium atoms in a horizontal beam through magnetic and microwave fields and then count only the atoms with excited electrons using a carefully positioned detector. In our two most accurate clocks, the CSF1 and CSF2, we have a different layout and fire the caesium atoms upwards like a fountain through the microwave field. They pass through the field a second time on the way back down. With these atomic clocks we can accurately define a second to 16 decimal places.

trends in automation: Wristwatches or station clocks don’t need such precision. So what do we need it for?

Ullrich: Inaccuracy has a cumulative effect and it happens relatively quickly. In order to guarantee a high degree of accuracy over the long term, we need very precise clocks. Exact timing is particularly important in the world of science. One of our key tasks at the P is investigating the question of whether fundamental physical constants such as the fine-structure constant, which includes the speed of light and Planck’s constant, really are constant. There is evidence to suggest that this is not the case. Should this theory be substantiated, it would have farreaching consequences, as many laws and models are based on fundamental physical constants.
Time researchers previously experienced supposedly secure assumptions being torpedoed by precise measurements back in the 1930s, when the second was still defined as a fraction of the earth’s rotation. The quartz clocks commissioned here at our institute were the most accurate clocks available at the time. Researchers discovered that the earth’s rotation is slowing down and highly variable and doesn’t always move at the same speed, as had been the assumed for the existing definition of time.

trends in automation: Are there also practical applications for atomic clocks?

Ullrich: Atomic clocks are used by the positioning satellites for the American GPS system or the Russian GLONASS, for example, as well as for the first satellites of the European Galileo system. These systems identify locations using signal running times between the satellite and the earth and therefore need very precise time specifications. There are also plans to use clocks for measurement in space in the near future. This will allow highprecision measurement of the relative position of two satellites and their change can be used to fully plot the earth’s gravitational field. With similar measurements on the earth and even more accurate clocks, it may even be possible to detect different mass distributions and thus trace mineral resources in the future. These are the kinds of topics that we are currently working on with other researchers in the QUEST excellence cluster at the Leibniz University of Hanover.

trends in automation: Do the clocks on the satellites have the same complex structures as the atomic clocks of the PTB?

Ullrich: They work according to the same principle, but are slightly more compact and don’t have to be quite as accurate. The previous signal transmission results in minor deviations in any case. Atomic clocks can be easily acquired nowadays for a wide variety of uses. They cost between a few hundred and around 100,000 euros for earth-based applications. For satellite applications they are significantly more expensive – and thanks to sophisticated technology can generally run for many years without any maintenance.

trends in automation: You say that the technology is sophisticated. However, can an atomic clock such as the one here at the Physikalisch-Technische Bundesanstalt fail?

Ullrich: Of course that is a possibility, but we have back-ups. In our institute alone we have four primary atomic clocks ticking away as our contribution to Coordinated Universal Time. To provide the time for radio clocks, for example, which is transmitted from Mainflingen, near Frankfurt, using a longwave transmitter, there are a further three atomic clocks on site, which are regularly synchronized with our clocks.

trends in automation: How do you ensure that the clocks all over the world are correct?

Ullrich: As I mentioned earlier, we have what we call Coordinated Universal Time, which is valid for 24 time zones and is determined by around 400 atomic clocks worldwide. The clocks are compared with one another and a mean value is created. Less accurate clocks have a lower weighting than more accurate clocks. This value is then checked to establish if it matches the best clocks in the world, including our atomic clocks at the PTB. The values determined in this process are released as Coordinated Universal Time by the International Bureau of Weights and Measures (BIPM), which has been based in Sèvres, near Paris, since 1875. This happens once a month. It is also important that all atomic clocks take altitude into account because, according to Einstein, time is influenced by the gravitational field.

trends in automation: How long is the current definition of time likely to be valid?

Ullrich: Certainly for a few more years, though the next generation of clocks is already in sight. These so-called optical clocks are likely to be at least several hundred times more accurate than the best atomic clocks available today. They operate according to a similar principle. However, the radiation that we use here to excite the electrons has an oscillation frequency 100,000 times higher and is in the visible range. Instead of
microwave radiation, optical clocks run with light from highprecision lasers.
At the PTB we already have two different optical clocks, both of which are around ten times more accurate than our atomic clocks. Over the coming years, however, we will have to compare and observe different optical clocks around the world to determine whether they all tick the same and with what kind of inaccuracy. This will take at least as long as it takes for the definition of a second to be adapted to the new technical possibilities.

trends in automation: Which role does collaboration with international partners play in such new developments?

Ullrich: Since the signing of the Metre Convention in 1875, we metrologists work together very closely and constructively, which I think is excellent. Of course, there is also competition. At the end of the day, everyone wants to have the best clock. In this regard, we have been very successful. Our fountain atomic clocks are among the most accurate in the world. In the area of optical clocks, we are currently involved in a friendly head-to-head race with our partner, the National Institute of Standard and Technology (NIST) in the USA.

trends in automation: Professionally, you work very intensively on this topic. Does your job influence your own personal relationship with time?

Ullrich: I think time is an extremely valuable asset. I therefore try to use it wisely. For example, I complete various tasks that require intensive concentration in blocks if possible. When that is the case I don’t like to be disturbed, because you work extremely inefficiently if you have to keep starting over again. I generally tend to work like that early in the morning or at weekends, and tend to use my mobile phone and the Internet very little.
The most difficult thing is getting the balance right between time spent working and time with my family. This is partly due to the fact that I love my job and often don’t even see it as work. I sometimes forget the time.

Prof. Dr. Joachim Ullrich

Joachim Ullrich has been President of the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, based in Braunschweig since 2012. Prior to that, he was was Director of the Max Planck Institute for Nuclear Physics in Heidelberg and headed up the Experimental Few-Particle Quantum Dynamics division. He is internationally renowned not only as President of the National Metrology Institute, but also as an expert in quantum physics and experiments with free-electron lasers carried out at DESY in Hamburg and the SLAC National Accelerator Laboratory in Stanford, USA. He has received numerous awards for his work, including the Gottfried Wilhelm Leibniz Award of the German Research Foundation (DFG) and the Philip Morris Research Prize.

  1. This article was published in trends in automation 2.2013, the customer magazine of Festo
  2. Photos: Fotodesign Bierwagen

September 2013