RISE keeps track of time

Our perception of time is tied to the movement of the sun across the sky. The rising and setting of the sun, night and day, the changes of the seasons. For a long time, the sun was the only reference needed. It didn't matter much if time differed in different parts of the country. But as transportation and communication developed, new demands on common time arose. When the railway made its entry, it simply did not do to have different times in Stockholm and in Gothenburg. Nowadays, time is an international matter that is measured by very accurate atomic clocks around the world.

The UTC(SP) time scale

At the National Laboratory of Time and Frequency at RISE, Kenneth Jaldehag and his colleagues are responsible of maintaining and providing Swedish standard time, through the UTC(SP) time scale.

"Many systems depend on accurate time, such as communication systems or financial transactions. We maintain the Swedish time scale and distribute correct time to the companies and organizations that need very accurate time. We also help setting up our customers own time systems and help them with monitoring", says Kenneth Jaldehag.

Measuring the second

Although we all measure time through our watches or phones, we cannot see or touch it. What we are measuring is actually an interval between two events. The time scales are in turn an accumulation of these time intervals. The SI unit for the time interval is the second, which is measured with very high accuracy at the National Laboratory.

"We have five atomic clocks for measuring the second at the National Laboratory in Borås which we use to produce the time scale UTC(SP). The measurements are constantly compared with each other so that we get as good an accuracy as possible and to detect any errors", says Kenneth Jaldehag.

Leap seconds

UTC(SP) in turn follows the international time scale UTC which is coordinated and calculated by the International Bureau of Weights and Measures (BIPM) in Paris.

"UTC is a time scale that is based on comparisons between atomic clocks and is calculated, somewhat simplified, by taking the average value from approximately 500 atomic clocks worldwide, including our atomic clocks", says Kenneth Jaldehag.

Back to the sun and our perception of time. The atomic clocks generate the time based on the definition of the second in the SI system. The problem is that the length of the day varies slightly due to changes in the Earth's rotation. The tendency in the long term has been for the earth's rotation speed to slowly decrease; thus, the duration of a solar day slowly increases. For UTC to match the rotation of the earth and the length of the solar day, leap seconds are added from time to time. In recent years, however, the rotation speed has increased, which causes problems:

“In the future we may need to add negative leap seconds, i.e., remove seconds, which can cause major problems for technical solutions”, says Kenneth Jaldehag.

At the General Conference on Weights and Measures in Paris in the fall of 2022, the countries of the world therefore decided to investigate the conditions for a different handling of leap seconds in the future. Read more about the decision (in Swedish).

Predicting time

The process of calculating UTC means that it is not available in real time. For example, the UTC for September is not available until October. The time scales maintained by various time laboratories are, however, available in real time.

"Our task is to match UTC with great accuracy, but there is no clock that has UTC in real time. This means we need to compare our time scale UTC(SP) in relation to UTC in retrospect, but also that we need to predict what we think UTC will be right now and correct our time scale in relation to our prediction. We have a very good grasp of the properties of our atomic clocks and can make these predictions very well", says Kenneth Jaldehag.

Given that you have worked for 27 years at the national laboratory. What is time?

“That is usually the first question I get, and my simple answer is that time is the fourth dimension. In the same way that we are in a position in space, we are also at a point in time and moving towards the next point in time. Our job is about measuring that movement very carefully. But I must admit, considering the work I do, that for me time is mostly a lot of numbers”, says Kenneth Jaldehag.