the alternating definitions of the second
We all know that there are 24 hours in a day and 60 minutes in an hour 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 need an observable phenomenon that repeats itself regularly enough. The phenomenon that traditionally defined the second and the other units of time 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 starting in the middle of the 20th century, the understanding deepened that relying on the day to define the second is very problematic, since the length of the day is not fixed. Therefore, it was decided to replace the basis of the definition of the second from the day, with the tropical year - that is, the period of time that passes between two successive passes of the sun over the same point on its horizontal path in the sky - which changes much more slowly than the day. Accordingly, in 1960 the second was defined in the International System of Units (SI) as the 1⁄31,556,925.9747 part of the tropical year of 1900. But only seven years later, a completely new formal definition of the second was adopted, based on a phenomenon at the atomic level rather than the astronomical one. In 1955, the first precise atomic clock was built, which was based on the transition of cesium-133 atoms between two certain energy levels: the atoms were excited by microwaves to a higher energy level, and when they decayed back to the fundamental level, they emitted photons at a very precise frequency - and this was the new regular phenomenon that made it possible Time measurement with unprecedented precision.
What led the scientists to change the definition of the second in 1960? And seven years later, to change it again?
Accordingly, in 1967 the standard second definition was replaced by the following definition: the duration of 9,192,631,770 cycles of radiation emitted in the transition of cesium-133 between the two energy levels; The number of cycles is chosen so that this duration corresponds to the length of the second as defined in 1960. What's more: the definition of the new second affected the redefinition of additional basic measurement units. In 1983, the meter was defined based on the second, and in 2018, the kilogram was redefined based on several natural constants, among them the same frequency of radiation emanating from the transition of cesium-133 between energy levels.
What motivated this fascinating process that the second went through, from a unit without a formal standard definition (before 1960) to a unit that is not only defined (starting in 1967) in an unprecedentedly precise way, but is currently used as a basis for defining other measurement units? Prof. Shaul Katzir, head of the Cohen Institute for the History and Philosophy of Science and Ideas at Tel Aviv University, investigates this issue, along with many other questions that accompany it. How and why do the scientists determine that a size that has been defined so far as constant is actually not constant? How was the conclusion reached that the speed of the earth's rotation - the original measure per second - is getting smaller? Why did the scientists think that the second definition should be changed, and why did they initially choose a rather strange definition that provoked criticism (based on the tropical year, and 1900 to be exact), and only seven years later replaced it with a completely different definition? In this research, which received a grant from the National Science Foundation, many issues arise related to the needs of astronomy, physics and technology at that time; As well as methodological and technical questions concerning the requirements for basic measurement units and the ways in which time can be measured accurately and determine how constant the frequencies are. In the background is also the reform of the basic units of measure from 2018, which put on the agenda the fundamental question of how the basic units of measure should be defined and when the need exists and there is the possibility of replacing them.
The new second definition affected the redefinition of additional basic measurement units. In 1983 the meter was defined based on the second, and in 2018 the kilogram was redefined.
The study includes an examination of the factors and moves that led the astronomers between 1925 and 1939 to the conclusion that the speed of the earth's rotation on its axis is indeed gradually decreasing, with the aim of determining the exact time when they became convinced of this and the reasons why they concluded this. The research shows that despite the existence of new external circumstances (more accurate time-measuring devices such as quartz watches), the main factors that led to this conclusion were internal considerations that touched on the theory used to predict the movements of the planets and were based on a comparison between the prediction and the actual observational data. The suspicion that the Earth's rotation is slowing arose following observations from the mid-19th century that showed that the moon was moving faster than the theory predicts. Following more rigorous calculations and observations, it became clear that the movements of planets are also accelerated relative to what the theory predicts, and at a rate similar to the acceleration of the moon. In the end, through complex calculations - which were examined in depth in this study - astronomers, including Willem de Sitter, showed that the assumption that the rate of self-rotation of the earth is getting smaller explains all the deviations measured between theory and observations.
As for the changes in the second definition, it seems that the main motivation for them was the physical and technological need to measure frequency (and therefore also time) and the ability to produce more and more accurate time and frequency meters - first the quartz clocks, and then the atomic clocks. The initiative for the first change came from the astronomers, who first believed that only they had the ability to calculate time in an absolute and precise manner based on the well-known laws of celestial mechanics, and therefore the first precise measure chosen (instead of the rotation of the earth) was the time of the revolution around the sun. But then it became clear that precisely modern atomic physics can provide an equally reliable and accurate mechanism that is much more convenient for use in measuring time - using the cesium-133 atom - and the basis of the definition of the second was changed accordingly.
Life itself:
Prof. Katzir is a "heavy" fan of classical, ancient and contemporary music alike - and listens to it in all his waking hours, including while working on watches.
4 תגובות
The refutation of Asbar's claim will be done using Permafrost's Lemma:
----------------
Let us consider a group of circles in the Euclidean plane with a common center point and a common central angle whose lines, the rays, intersect all these circles.
If the circles are "scattered" in a plane and do not have a common center - we will first copy them to an arbitrary common center point using a compass and ruler(*).
The rays of the common angle cut a sector or "slice" from each circle belonging to the group and an arc from each circle belonging to the group.
All these "slices" are similar, but differ in a single parameter - the radius.
The difference between them is only scaling.
Due to the similarity, it immediately follows that the ratio between the different arcs on which the common central angle rests and the radii of the circles from which these arcs were respectively cut is constant.
This ratio is, according to its definition, the central angle, no matter how big it is, and it is common and the same for everyone.
Expanding the common angle and including it to the ratio of the full angle that extends from the center point means that the ratio between the radii of the circles and the corresponding circumferences is constant, and it does not matter at all what the ratio of this constant is.
(*) The ruler - for drawing straight lines only. Not to measure. There are no standard rulers in geometry.
The discussion does not dwell on the concept of leap second.
The self-rotating speed of KDA decreases by about 1.4-1.7 milliseconds per day every hundred years, due to tectonic plate shifts (a matter that affects the rotating speed due to conserved angular momentum), but also due to tides.
After all, Earth is not a solid body, but has a liquid content, magma, and a liquid cover, seas and oceans.
The angular momentum of the Earth slowly "flows" mainly to the moon and the moon moves away from us as a result by about 3.5 cm per year.
Due to the slowing down of the self-rotation speed of KDA, starting in 1972, the addition of a passing second was introduced.
When the deceleration accumulates to about 0.9 second (every 500 days) - a leap second is added at the end of June or at the end of December.
In fact, the matter was felt already in 1957 when the use of atomic clocks began, but it only began to be addressed in 1972 when the slowdown had already accumulated to ten seconds, and since then another 27 seconds have been added for a total of 37 seconds.
Many computer systems require extremely accurate timekeeping, for the purpose of scheduling events between them, especially when they are geographically distributed, and are not all placed together in one room on a single site.
They subscribe to a time synchronization service from standards institutes or observatories, based on an atomic clock, and subtract 37 seconds (currently) from the value sent to them to get the correct national time.
Stupidity is what drives most of the shitty scientists in the world crazy!
The clear sign of physical reality is that there is nothing permanent about it.
There is no constant speed in nature.
There is also no constant acceleration in nature, and free fall has a variable acceleration.
There is no permanent change in nature, and change is always changing.
There is no phenomenon in nature that repeats itself in equal periods of time.
There is a change of forms in nature, and uniformity in forms does not exist.
There is no movement in nature in a straight line that is uniform in form.
There is no movement in nature in a circular path that is uniform in shape.
There is no straight geometric line in nature, and there is no circular geometric line in nature.
There is no point in striving for perfect standards of length of time and energy, and enough is enough
that the standards should be suitable for practical use.
There is no way to know if a phenomenon that is seen as a regular cycle, is really regular.
The longing for something permanent has always existed, but one has to accept the statement that in physical reality there is no permanent "thing".
The ratio between the circumference of the circle and its diameter is also not constant
Mathematicians believe (for thousands of years) that a single number whose value is approximately 3.14 allows the transition from the diameter of any circle to its circumference.
Each circle means, from the smallest whose diameter approaches zero mm, to the largest whose diameter approaches infinity mm.
Whereas Asbar believed that each size of a circle should have a unique transition number, and all these transition numbers are in a narrow range between 3.14 and 3.16
According to the belief of mathematicians, it is accepted that if two random circles are chosen, the ratio of their circumferences (equal) to the ratio of their diameters.
According to Esbar belief, if we choose two random circles, the ratio of their circumferences (is not equal) to the ratio of their diameters,
This is a very small inequality, given the narrow range between 3.14 and 3.16
These two beliefs can be tested by means of an accurate practical experiment with two metal cylinders, whose diameter ratio is, for example, 7
The experiment begins when the small cylinder is pressed in its circumference to the circumference of the large cylinder, and when the small cylinder is rotated, the large cylinder rotates.
First result of the experiment:
If the small cylinder rotates 7 times, and the large cylinder completes exactly one revolution, then the belief of the mathematicians is correct….. the ratio of the diameters (equal) to the ratio of the circumferences.
Second result of the experiment: if the small cylinder rotates 7 times and the large cylinder rotates a single rotation (plus or minus) 1 degree, then Nesbar's belief is correct. (The ratio of the diameters (is not equal) to the ratio of the circumferences, when it is a tiny inequality.
Asbar conducted this experiment in 2017, and he received his result.
Mathematicians are reluctant to conduct this experiment, for fear that their thousand-year-old belief will turn out to be a terrible mistake, handed down from generation to generation as a solid mathematical truth.
A. Asbar