# TL;DR Science: The Atomic Clock

By Thomas P.
September 23, 2022 · 2 minute read

Physics

Chemistry

Mathematics

Engineering

Perhaps you have a clock set to the Atomic Clock. However, have you ever considered the complexities of calculating the exact time, and the coordination that goes into correctly matching it with the length of the Earth’s rotation?

### The Atomic Clock

Basically, the atomic clock is an instrument that uses microwaves to cause cesium-133 to oscillate between two hyperfine states (one being a “ground” state and the other being an “excited” state).

To go into specific detail, the machine sends cesium-133 atoms into a resonating chamber from one side of the machine called the “oven” where they are hit by microwaves that are traveling at 9,192,631,770 Hz (cycles/oscillations per second [frequency]). During the process the cesium atoms reach a hyperfine excited state, where the electrons receive a jolt of energy. The time that it takes for this to occur is one second. The excited atoms are then removed from the resonating chamber by a magnet and sent to a detector. The detector ensures that all of the atoms have reached their excited state. Should a portion of the cesium-133 atoms not be in the excited state, it is likely that the machine does not have the microwaves exactly set to the frequency of 9,192,631,770 Hz; in which case, the detector will communicate with the microwave radiator in order to refine the frequency. Should the clock be working correctly, it will only lose 30 picoseconds (3 10-11 sec) per day, which is amazingly precise.

### Leap seconds

The atomic clock is an instrument of extreme precision, almost to a fault. Scientists have had to add “leap seconds” to the world clock (Universal Time Coordinated [UTC]) in order to correctly match the correct time with length of day.

Looking at the time above, we can see that it is 16:03:00 or 4:03:00 PM UTC-4* (or Eastern Daylight Time)**. These clocks are exactly thirty-six seconds behind the International Atomic Time (TAI). This is as a result of leap seconds, which are subtracted from the International Atomic Time, which there are 36 of. You will also see that the world Global Positioning System master clock has also remained unperturbed since being set to the atomic clock in 1980, and is as a result 18 seconds ahead of UTC.

*4 hours behind UTC (the world clock).

*When Daylight Savings Time is not in effect (after “fall back”), Eastern Standard Time is used. Eastern Standard Time is UTC-5, or 5 hours behind UTC.

### Sources

https://www.nasa.gov/feature/jpl/what-is-an-atomic-clock