Page 8 - trends in automation

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.
How do you measure time?
Ullrich: Using a pendulum, for example. The shorter the pendu-
lum, 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 posi-
tioned detector. In our two most accurate clocks, the CSF1 and
CSF2 (see image on right), we have a different layout and fire
the caesium atoms upwards like a fountain through the micro-
wave 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.
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 rela-
tively 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 funda-
mental 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 far­
reaching 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
The keeper
of time
Interview