According to my computer, it was 12:51:23 pm at the time I wrote the last digit. What is the precision of that measurement? Probably plus or minus 1 second, limited by my typing speed (appalling) and by my internet connection. Having checked the “Set date and time automatically” box, my computer’s time is updated via the internet to a server somewhere, which itself syncs with UTC time. UTC time, or Coordinated Universal Time, is the successor to GMT (Greenwich Mean Time). The precision of UTC time itself, limited by the precision of the atomic clocks that keep it, is about 0.1 femtoseconds. This means that the human race, assuming no global electricity blackouts affecting all atomic clocks simultaneously, will be able to reference this exact moment 100 million years from now, to the accuracy of a second. (Since there are about 10^16 seconds in 100 million years).
International Atomic Time (IAT) is the time actually kept by a network of hundreds of atomic clocks around the world. UTC is not quite the same as IAT, because we humans like our days to be 24 hours from noon to noon. Though this is true on average, it is not true for any given day if you measure it with fixed length seconds. This is due to a variety of factors such as the earth’s elliptical orbit and its axial tilt. Corrections are made to International Atomic Time by adding one second here and subtracting some there, to give us UTC time, which is followed by everything from the Stock Market to cellphones and home computers.
So how do atomic clocks keep such accurate time? Atomic clocks are really frequency calibration sources. Their input is a range of frequencies, and their output is one specific frequency. Time intervals can then be precisely measured since the time intervals are just the inverse of frequencies. There are several kinds of atomic clocks, some of which work at microwave frequencies (GHz), and some of which work at optical frequencies (hundreds of THz). The International Standard (SI) unit for time is the second, and it is defined to be 9192631770 periods of a certain frequency naturally present in caesium (Cs) atoms. By definition, this frequency is exactly 9.192631770 GHz.
The atomic clocks used to define this standard and to keep IAT and UTC are called caesium fountain clocks. They start by cooling and trapping a small amount of caesium-133 atoms to a few microKelvin in a vacuum chamber using laser beams. This technique uses the fact that photons (particles of light), have momentum. By hitting a cloud of atoms with light from all six directions, the atoms can be compressed into a small ball, slowed down (and therefore cooled), and trapped. Another laser beam is then used to give a push to this bunch of atoms, sending them up into a resonant cavity. The resonant cavity is a chamber with an adjustable length that is filled with microwaves. For every length, only a certain frequency of the microwave can exist, so by changing the length, the frequency of the microwaves can be swept over. If the microwave frequency is resonant with a frequency in the caesium atoms, then the caesium atoms absorb the microwaves and change their state. In this changed state, they can interact with a laser and then fluoresce. Only when the input frequency is resonant with the caesium atoms will they fluoresce and by detecting the fluorescence, the resonant frequency of 9.192631770 GHz is found. There is nothing special about this number, it was chosen to match the previous definition of the second. The important part is that it will be the same for cold dilute samples of caesium atoms regardless of the specific setup, time of year, or location on Earth so it can be measured and agreed upon by anyone with a caesium fountain clock.
There are other kinds of atomic clocks. One kind is an optical lattice clock. Instead of using a microwave resonant frequency of caesium atoms, an optical resonant frequency of strontium atoms is used. Also, instead of kicking the atoms up in a fountain, the atoms are trapped in an optical lattice. An optical lattice is a two dimensional pattern of standing waves, created by a laser beam interfering with itself. The standing waves create periodic traps for the atoms, which look kind of like a chessboard of hills and valleys. In this way, the atoms are held in place during the measurement. The higher frequency of the resonant light and longer measurement times make this type of atomic clock more precise than the caesium fountain clocks. Recently, a journal article in Nature Communications from a research group in Paris showed that this is the case by comparing several optical lattice clocks to each other and also to caesium atom clocks. There is talk that this will lead to a redefinition of the SI second to a more precise number. This would improve the accuracy of devices such as GPS, which depend on a precise knowledge of what time it is.
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