The changing definitions of the second
We all know that a day has 24 hours and an hour has 60 minutes and that every minute includes 60 seconds; In other words, the day includes 86,400 seconds. But how is the exact length of the second determined? In general, in order for us to measure time effectively, we first and foremost need to observe a phenomenon that is repeated regularly enough. The phenomenon that traditionally defined the second and the rest of the time units was the rotation of the earth on its axis; And the historical definition of the second was simply the 1/86,400th part of the day. But from the middle of the 20th century, the understanding deepened that relying on the day for the second definition is very problematic, since the length of the day is not fixed. Therefore, it was decided to change the basis of the second definition of the day, in the tropical year – that is, the period of time that passes between two consecutive transitions of the sun across the same point on its horizontal orbit in the sky – which changes much more slowly than day. Accordingly, in 1960 the second in the system of international units (SI) was defined as part of the 1⁄31,556,925.9747 of the tropical year of 1900. But only seven years later a completely new formal definition of the second was adopted, relying on a phenomenon at the atomic rather than the astronomical level. In 1955, the first precision atomic clock was built, based on the passage of 133 cesium atoms between two specific energy levels: the atoms aroused by microwaves to a higher energy level, and faded back to the ground level, emitting photons with very precise frequency – and this was the new regular phenomenon. Measuring time with unprecedented accuracy.
What led scientists to change the second definition in 1960? And seven years later, change it again?
Accordingly, in 1967 the standard second definition was replaced by the following definition: the duration of the 9,192,631,770 radiation cycles emitted in the transition of cesium-133 between the two energy levels; The number of cycles was chosen so that this duration would match the length of the second as defined in 1960. Moreover, the new second definition affected the redefinition of additional basic units of measure. In 1983 the meter was set based on the second, and in 2018 the kilogram was redefined on the basis of a number of natural constants, including the same frequency of radiation emanating from the cesium-133 transition between energy levels.
What motivated this fascinating process that the latter went through, from a unit without a standard formal definition (before 1960) to a unit that is not only defined (as of 1967) in an unprecedentedly precise way, but is now used as a basis for defining other units of measurement? Prof. Shaul Katzir, head of the Cohen Institute for the History and Philosophy of Sciences and Ideas at Tel Aviv University, is researching this issue, along with many other accompanying questions. How and why do scientists determine that a size that has so far been defined as fixed is not actually fixed? How was the conclusion reached that the speed of the earth’s rotation – the original index per second – is decreasing? Why did the scientists think that the second definition should be changed, and why did they choose a rather strange definition in the first place that provoked criticism (based on the tropical year, and of 1900 in particular), and only seven years later replaced it with a completely different definition? This study, which received a grant from the National Science Foundation, raises many issues related to the needs of astronomy, physics and technology at the time; As well as methodological and technical questions concerning requirements from basic units of measure and ways in which time can be measured accurately and determine how constant they are. In the background is also the reform of the basic units of measure from 2018, which raised on the agenda the fundamental question of how the basic units of measure should be defined and when there is a need and the possibility of replacing them.
The new second definition affected the redefinition of additional basic units of measure. In 1983 the meter was set based on the second, and in 2018 the kilogram was redefined.
The study includes an examination of the factors and moves that led astronomers between 1925 and 1939 to conclude that the speed of the Earth’s rotation on an axis is indeed gradually decreasing, in order to determine the exact date they were convinced of it and the reasons why. The study suggests that despite the existence of new external circumstances (more accurate time-measuring instruments such as quartz clocks), the main factors that led to this conclusion were internal considerations that touched on the theory used to predict planetary motions and relied on comparing it to actual observational data. The suspicion of slowing down the Earth’s rotation arose following observations from the mid-19th century that showed that the moon was moving faster than the theory predicted. Following more careful calculations and observations, it became clear that the motions of planets were also accelerated relative to what the theory predicts, and at a rate similar to the acceleration of the moon. Finally, using complex calculations – examined in depth in this study – astronomers, including Willem de Sitter, showed that the assumption that the Earth’s self-rotation rate decreases explains all the anomalies measured between theory and observations.
As for the changes in the second definition, it seems that the main motive for them was the physical and technological need to measure frequency (and therefore also time) and the ability to produce increasingly accurate time and frequency meters – first quartz clocks, then atomic clocks. The initiative for the first change came from astronomers, who initially believed that only they had the ability to calculate time absolutely and accurately based on the well-known laws of celestial mechanics, so the first exact measure chosen (instead of orbiting the Earth) was orbiting the sun. But then it became clear that the latest atomic physics could provide an equally reliable and accurate mechanism that was much more convenient to use for measuring time – using the 133 cesium atom – and the base of the second definition was replaced accordingly.
Life itself:
Prof. Katzir is a “heavy” fan of classical, ancient and contemporary music alike – and tends to listen to it at all hours of his notes, including while working on clocks.