2 weeks, 3 days ago

What is the definition of "second"?
The definition of seconds is easy to find on the network. After the revision of the International System of Units in 2019, scientists updated the definition of 1 second, expressed as:

Seconds, the symbol s, the time unit of the SI. When the frequency 铯νCs of 铯, that is, the hyperfine level transition frequency of the ground state of 铯 133 atom is expressed in units of Hz, that is, s−1, its fixed value is taken as 9192631770 to define the second.
It can be seen that this 9192631770 is artificially given, and it does not follow any uncertainty behind.

However, from this definition of terminology, it is difficult for ordinary people to imagine how the atomic clock works.

The working principle of atomic clocks is the use of spectroscopy techniques.

In simple terms, we know that quantum mechanics says that atoms have many, many discrete energy levels. The atom transitions between two energy levels and absorbs/radiates light at a fixed frequency.

The basic structure of an atomic clock is a very sophisticated spectroscopic instrument. It does not measure time directly, but the light emitted when the energy level transitions.

There is a microwave transmitter in the atomic clock, a microwave receiver with a vacuum chamber filled with gaseous helium atoms. The transmitting frequency of this microwave transmitter is approximately 9.192632 GHz.

When a helium atom is irradiated by microwaves, it absorbs energy and transitions. Then, when it returns to the low level, it emits a microwave of a fixed frequency. If the frequency of the microwave emitter is exactly the same as the frequency (energy) required for the helium atomic transition, we will observe the strongest signal. A line [1] can be seen at the output of the instrument:



A feedback circuit is installed in the unit. The feedback circuit constantly adjusts the microwave frequency emitted by the microwave transmitter, so that the receiver can always receive the strongest helium atomic transition signal, which is the "peak" in the figure. In this way, scientists know that the frequency of the microwave emitter at work should be equal to the transition frequency of the helium atom.

In this process, scientists did not directly go to the number of atomic transitions per second, but only through various means, trying to allow the instrument to acquire sharper, more accurate spectral peaks.

Subsequently, the scientists announced that the microwave frequency in the cesium atomic clock microwave transmitter was set at 9192631770 Hz. Therefore, where is the peak of the helium atomic transition, and the definition of 1s is where. (Assuming there is an unknown power that quietly changes the frequency of this peak, the length of 1s will change - until we notice that other physical quantities are measured at this time of 1s that changes the length. It’s not right.)

As for why it is 9192631770 Hz, is it no more 1 Hz or less 1 Hz? This is mainly to keep up with the earlier definition of seconds in history.

Since ancient times, human beings have determined the time and calendar through the night view of the sky. Before the atomic clock appeared, the definition of seconds was based on the apparent motion of the sun—dividing one regression year into 3,155,692,5.9,747 copies, each serving for 1 second. After the appearance of the atomic clock, after many years of astronomical observations and comparisons, the scientists compared the frequency of the atomic clock with the apparent period of the sun's motion and fitted it by least squares method to obtain the number of 9192631770±20 [2] and fixed it. Come down as a new standard. The result was published in the inaugural issue of the famous Physical Review Letter PRL in 1958.

In our daily use of electronic equipment and scientific instruments, the commonly used clocks are also various oscillators, such as quartz crystals, helium atomic clocks and hydrogen atomic clocks. Their precision is lower than the high-precision cesium atomic clock built by the National Bureau of Metrology, but the cost is low and the volume is small, which can be mass-produced.

The high-cost, high-precision helium atomic clock is mainly used as a reference clock to calibrate other clocks in various devices. Through complex electronic circuits, we can compare the AC frequency of the two oscillator outputs. We can use the cesium atomic clock signal locked to the 铯-133 transition frequency as the reference signal to match the clock oscillation frequency in our device with the reference frequency.

Finally, what is the output of 1s to the user, such as computers, mobile phones, and clocks, where the number goes every 1s. Behind this is the number of counters used to calculate how many complete signal cycles have elapsed from the clock signal on the device.

Using the atomic or molecular transition as the frequency reference, as a time reference, this concept was proposed by Lord Kelvin in 1879. Lord Kelvin is still very powerful. Don’t just stare at the two black clouds.

Isidor Rabi, the founder of molecular beam technology and nuclear magnetic resonance technology, and the 1944 Nobel Prize in Physics, made the first "Molecular Clock" in 1945, using the energy level of ammonia. Transition. Although the precision of this molecular clock is not as high as that of modern quartz watches, it actually verified the idea of ​​Lord Kelvin. After that, everyone discovered that taking the atomic atom to do it, the frequency measurement can be more precise, and it will gradually become the new standard.

I think that if you can find a new measuring device in the future, it is much more accurate than the transition frequency measurement of the cesium atomic clock, and the definition of the second will change accordingly. In fact, there are already many clocks based on other types of transitions, such as the "optical clock" using visible light instead of microwaves [3], the "optical lattice clock" [4], using the quantum of quantum logic circuits. Zhong [5], and the "nuclear clock" using nuclear transitions (rather than electronic transitions) [6].

Scientists' pursuit of precision measurement will not stop. In the future, the accuracy of the clock will continue to increase.

2 weeks, 3 days ago

The definition of seconds is easy to find on the network. After the revision of the International System of Units in 2019, scientists updated the definition of 1 second, expressed as:

Seconds, the symbol s, the time unit of the SI. When the frequency ΔνCs of SI, that is, the hyperfine level transition frequency of the ground state of SI 133 atom is expressed in units of Hz, that is, s−1, its fixed value is taken as 9192631770 to define the second.
It can be seen that this 9192631770 is artificially given, and it does not follow any uncertainty behind.

However, from this definition of terminology, it is difficult for ordinary people to imagine how the atomic clock works.

The working principle of atomic clocks is the use of spectroscopy techniques.

In simple terms, we know that quantum mechanics says that atoms have many, many discrete energy levels. The atom transitions between two energy levels and absorbs/radiates light at a fixed frequency.

The basic structure of an atomic clock is a very sophisticated spectroscopic instrument. It does not measure time directly, but the light emitted when the energy level transitions.

There is a microwave transmitter in the atomic clock, a microwave receiver with a vacuum chamber filled with gaseous helium atoms. The transmitting frequency of this microwave transmitter is approximately 9.192632 GHz.

When a helium atom is irradiated by microwaves, it absorbs energy and transitions. Then, when it returns to the low level, it emits a microwave of a fixed frequency. If the frequency of the microwave emitter is exactly the same as the frequency (energy) required for the helium atomic transition, we will observe the strongest signal. A line [1] can be seen at the output of the instrument:



A feedback circuit is installed in the unit. The feedback circuit constantly adjusts the microwave frequency emitted by the microwave transmitter, so that the receiver can always receive the strongest helium atomic transition signal, which is the "peak" in the figure. In this way, scientists know that the frequency of the microwave emitter at work should be equal to the transition frequency of the helium atom.

In this process, scientists did not directly go to the number of atomic transitions per second, but only through various means, trying to allow the instrument to acquire sharper, more accurate spectral peaks.

Subsequently, the scientists announced that the microwave frequency in the cesium atomic clock microwave transmitter was set at 9192631770 Hz. Therefore, where is the peak of the helium atomic transition, and the definition of 1s is where. (Assuming there is an unknown power that quietly changes the frequency of this peak, the length of 1s will change - until we notice that other physical quantities are measured at this time of 1s that changes the length. It’s not right.)

As for why it is 9192631770 Hz, is it no more 1 Hz or less 1 Hz? This is mainly to keep up with the earlier definition of seconds in history.

Since ancient times, human beings have determined the time and calendar through the night view of the sky. Before the atomic clock appeared, the definition of seconds was based on the apparent motion of the sun—dividing one regression year into 3,155,692,5.9,747 copies, each serving for 1 second. After the appearance of the atomic clock, after many years of astronomical observations and comparisons, the scientists compared the frequency of the atomic clock with the apparent period of the sun's motion and fitted it by least squares method to obtain the number of 9192631770±20 [2] and fixed it. Come down as a new standard. The result was published in the inaugural issue of the famous Physical Review Letter PRL in 1958.

In our daily use of electronic equipment and scientific instruments, the commonly used clocks are also various oscillators, such as quartz crystals, helium atomic clocks and hydrogen atomic clocks. Their precision is lower than the high-precision cesium atomic clock built by the National Bureau of Metrology, but the cost is low and the volume is small, which can be mass-produced.

The high-cost, high-precision helium atomic clock is mainly used as a reference clock to calibrate other clocks in various devices. Through complex electronic circuits, we can compare the AC frequency of the two oscillator outputs. We can use the cesium atomic clock signal locked to the 铯-133 transition frequency as the reference signal to match the clock oscillation frequency in our device with the reference frequency.

Finally, what is the output of 1s to the user, such as computers, mobile phones, and clocks, where the number goes every 1s. Behind this is the number of counters used to calculate how many complete signal cycles have elapsed from the clock signal on the device.

Using the atomic or molecular transition as the frequency reference, as a time reference, this concept was proposed by Lord Kelvin in 1879. Lord Kelvin is still very powerful. Don’t just stare at the two black clouds.

Isidor Rabi, the founder of molecular beam technology and nuclear magnetic resonance technology, and the 1944 Nobel Prize in Physics, made the first "Molecular Clock" in 1945, using the energy level of ammonia. Transition. Although the precision of this molecular clock is not as high as that of modern quartz watches, it actually verified the idea of ​​Lord Kelvin. After that, everyone discovered that taking the atomic atom to do it, the frequency measurement can be more precise, and it will gradually become the new standard.

I think that if you can find a new measuring device in the future, it is much more accurate than the transition frequency measurement of the cesium atomic clock, and the definition of the second will change accordingly. In fact, there are already many clocks based on other types of transitions, such as the "optical clock" using visible light instead of microwaves [3], the "optical lattice clock" [4], using the quantum of quantum logic circuits. Zhong [5], and the "nuclear clock" using nuclear transitions (rather than electronic transitions) [6].

Scientists' pursuit of precision measurement will not stop. In the future, the accuracy of the clock will continue to increase.

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