The problem is that even though you can get exactly 9,,, Hz dialed in, it will "drift" over time, so the clocks need to be calibrated constantly. The calibration is done with the alkali metal cesium. A cesium atom has one electron orbiting its nucleus in the highest energy level, all by itself.
This lone outer electron is either "spin up" or "spin down," which refers to a quantum measurement of the electron's angular momentum. This spin produces a magnetic field, and the magnetic field is either aligned with the magnetic field of the atom's nucleus, or it's not. A maser that is exactly 9,,, Hz will force that outer electron to transition from spin up to spin down, or vice versa. There are lasers on in here, dangerous ones. So here's what's going on.
A gaseous ball of about 10 million cesium atoms is released into the bottom of a cylindrical vacuum chamber, which is the "fountain. Lasers are used to slow and cool the atoms to near absolute zero, and then more lasers are used to elevate the ball of cesium up the chamber. As a result, all of the atoms' outer electrons are aligned according to their nuclei. The ball of cesium passes through the maser at the top of the chamber. If calibrated to exactly 9,,, Hz, the maser will force every single one of the atoms' outer electrons to transition.
The ball of atoms then settles back down to the bottom of the chamber and additional lasers are used to measure them, checking to see how many transitioned. The problem is that if you measure 10 million cesium atoms and all but three transition, you know the maser frequency is ever-so-slightly off, but you don't know if it is too high or too low. So Jefferts intentionally calibrates the maser at a frequency that's too high and one that's too low.
When he has the two measurements equal, he can calculate the average frequency to land on 9,,, hertz. That maser, running through a cable, is then preserved in a high-tech temperature and pressure vessel—a converted egg incubation chamber. The measurement is then used to evaluate a suite of commercial atomic clocks, also stored in egg incubation chambers, and give them a weighted grade. The time of the commercial clocks is averaged, according to their weighted grade, and that sets the official civilian time for the United States.
Simple right? So had a leap second. But unlike leap years, leap seconds are not predictable in advance. Climate change, which will send ice at the poles to the equator as water, will make the inconsistencies in Earth's rotation even more pronounced.
Continuing to correct for these changes with leap seconds is a controversial issue. The leap second is expensive for computing and financial institutions, which must account for it in calculations and modifications, so many people think we should simply let leap seconds accumulate until we have a full leap minute , then make the adjustment.
Jefferts, however, is in agreement with most astronomers and thinks we should keep the leap second. I'm a sailor, and many years ago I taught celestial navigation, and so I sort of have this very personal linkage idea that, dammit, the sun should be overhead on the 21st of March or the 20th of March [the spring equinox] at noon at Greenwich We should not be accumulating leap seconds so that that is no longer true.
Leap seconds are far from the only ongoing uncertainty about time. The current definition of a second, 9,,, periods of a maser that will cause cesium to transition, isn't perfect. For one thing, the duration of a second, as currently defined, is slightly different at altitude compared to sea level due to general relativity, so corrections need to be made. The amount of time it takes for the Earth to turn once about its axis, or for it to rotate once about the sun, is fairly stable, and for much of human history, it sufficed as a way of marking the passage of time.
Today, however, when computers perform operations at the rate of 4 billion cycles per second, we need a better measure. The rotation of Earth, and its orbit, change slightly over time. So measuring a second based on rotation would mean that a second would get slowly longer over time.
So, to pin down a truly timeless measure of a second, scientists in the s devised a better clock, one based not on astronomical processes but on the movement of fundamental bits of matter — atoms — whose subtle vibrations are, for all intents and purposes, locked in for eternity.
That number seems random because each and every definition of a second has by necessity been based on the one that came before. By isolating and cooling cesium atoms to near absolute zero, researchers can measure each and every flip by the pulse of electromagnetic radiation it gives off. It was the exact same length of time, but now, it would remain permanently fixed. In , German researchers proposed an even better atomic clock, one based on the element strontium, and which uses optical light, rather than microwave emissions for calibration.
Greek astronomers were the first to establish the modern hour, by dividing the day into six parts and then dividing those parts into four smaller parts. They also had an early version of the minute, which was based on how long it took for the sun to travel one degree along the sky about four minutes.
In , a Persian scholar named al-Biruni first termed the word second when he defined the period of time between two new moons as a figure of days, hours, minutes, seconds, thirds, and fourths. The minute was the first subdivision of the hour by 60, then the second, and so on. In the late s, true standard seconds came into existence with the advent of mechanical clocks , so that the time could be measured objectively from mean-time instead of deriving it from the apparent-time.
The first clock with a hand indicating seconds was built between and in we had the first clock with second markings. But, in , William Clement tinkered with the physics and built a clock precise enough that the second was now an established unit of time.
By it was established that the second would be the base unit of time for all scientific research , along with the millimeter and milligram.
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