We deal with time every day. In our daily life, it's enough to measure time accurately to the minute. In competitive sports, such as sprint races, it's often necessary to measure time accurately to 0.01 seconds, and for high-speed sports like Formula 1 racing, it requires accuracy down to 0.001 seconds (1 millisecond). However, in some scientific research fields, the requirements for time accuracy are much higher, often requiring accuracy down to one hundredth of a second or even a millionth of a second. What kind of timing tools can meet such requirements?
At present, the most accurate timing instrument is the cesium atomic clock. Scientists have developed cesium atomic clocks that can run continuously for 6 million years with an accumulated error of less than 1 second. Why can cesium be used to make such high-precision timing instruments? We know that the Earth rotates once every 24 hours, and each hour is divided into 3,600 parts, which is 1 second. Therefore, one 86,400th of the time required for the Earth to rotate once is 1 second. However, the Earth's rotation speed is not stable, sometimes faster and sometimes slower, so the defined second does not meet the high accuracy requirements of some scientific experiments. Cesium can be selected as an atomic clock because it is the most active member of the metal family. That is, its outermost electrons have a lower transition energy, and the electromagnetic wave signals it emits are in the microwave frequency range of radio, which coincidentally is similar to the transmission frequency of broadcasting satellites. At present, human technology for manufacturing instruments in this frequency range is very mature. Therefore, scientists use the electromagnetic waves emitted by cesium atoms jumping between two energy levels as a standard to control and calibrate electronic oscillators, and then control the running of the clock. Thus, people use the characteristics of cesium atoms to create a new type of clock - the cesium atomic clock, and stipulate that 1 second is the time required for cesium atoms to jump from one energy level to another 919,263,177 times. This is the most accurate definition of a second so far.
In addition to cesium atomic clocks, people have also invented hydrogen atomic clocks, rubidium atomic clocks, etc. With atomic clocks, it has become possible to engage in more precise scientific research such as atomic and hydrogen bomb explosions, rocket and missile launches, space navigation, and artificial Earth satellites, and to achieve high-precision control and operation.