You may not believe it, but on Earth, the lower the floor you live on, the slower time passes.
Although it sounds like metaphysics, this is a scientific conclusion that has been verified by scientists with atomic clocks that can be timed with high precision.
Einstein's greatness, I believe many people know, his most famous scientific theory is the theory of relativity. Relativity is divided into special relativity and general relativity. General relativity holds that the existence of matter can affect space-time and concludes that the stronger the gravitational field, the slower time is. The lower the floor, the stronger the earth's gravitational field, so there is a conclusion that the time of the lower floor is a little slower than the high floor.
However, this time change is very small, ordinary timers can not detect at all, to use atomic clocks.
In February 2022, the world-renowned scientific journal Nature published the latest research results of the American research team, which showed that the research team used strontium atomic clocks to verify the time expansion effect in general relativity for the first time at the millimeter scale.
Experiments have confirmed that on the millimeter scale, general relativity is also correct. Since the rate of time passing in different regions of gravitational field is different, this experiment measured that the height difference on Earth is one millimeter, and the time of the atomic clock that is one millimeter lower will be one hundred billionths of a billionth slower than the other. That is, a height difference of 1 millimeter, which will produce a time difference of one hundred billionths of a billionth.
The research team comes from the United States Joint Laboratory of Astrophysics (JILA). The lab is run jointly by the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.
According to Jila team member Ye Jun (a Chinese physicist, academician of the American Academy of Sciences, and foreign academician of the Chinese Academy of Sciences), the reason why the JILA team was able to verify general relativity at the millimeter scale was because it used the clock with the highest timing accuracy in the world. They made the strontium atomic clock to a very precise level, with an error of less than 0.1 second in the 13.8 billion years since the birth of the universe.
In 2010, physicists at NIST had used atomic clocks to test this conclusion in general relativity at a scale of 33 centimeters.
Atomic clocks are very precise timers, born in the 1950s, born out of the research of magnetic resonance by Rabbi and his students. Regardless of the timing instrument, it is essentially timed using periodic motion. Under the magnetic action of certain conditions, hydrogen, cesium, rubidium and other atoms will undergo ultra-fine transitions, which will radiate electromagnetic waves of a certain frequency, and this frequency can be used to perform very accurate timing. Prior to this, the world's most accurate timer was timed using the vibration frequency of a quartz crystal. Although its accuracy is far less than that of atomic clocks, it is still more practical in daily life, such as electronic watches.
In the modern International System of Units, the duration of 9192631770 cycles of radiation corresponding to the transition between the two superfine energy levels of a ground-state cesium-133 atom on a rotating geodetic level is defined as one second.
So how do scientists measure time differences using atomic clocks at the scale of 1 millimeter?
According to reports, Ye Jun's team used 6 laser beams to cool 100,000 strontium atoms successively, and used infrared lasers to maintain them in an ultra-cold state and load them into the optical lattice to form atomic groups. Then through a certain frequency of laser light, the atomic group is excited and the frequency of light emitted by different regions is measured.
According to general relativity, the stronger the gravitational field, not only will time slow down, but even the frequency of light will change. Under the action of gravity, the frequency of light will be redshifted, that is, the frequency becomes lower and the wavelength becomes longer, which is called gravitational redshift. The stronger the gravitational field, the greater this redshift.
In this experiment, the researchers did not use two separate atomic clocks, but indirectly measured time by measuring the amount of redshift in earth's gravitational field emitted by electromagnetic waves emitted by two regions of the same atomic cluster that differ by only one millimeter.
Scientists have known for a long time that general relativity is correct, so why bother to keep testing it?
In fact, the reason why science is the most reliable way for human beings to perceive the world lies in its rigor, and all scientific conclusions can be verified by experiments. The higher the accuracy of the experiment, the more correct it is to prove the correctness of the theory.
The more accurate verification of general relativity this time, the ultra-high precision atomic clock used by American scientists, is the most noteworthy. The application of GPS has the credit of general relativity, and the improvement of the accuracy of atomic clocks is also expected to improve the accuracy of GPS.
Because gravity is very weak in small-scale space, it was very difficult to measure in the past. Now this technology of using the same atomic clock to make high-precision measurements at tiny scales may help humans study the interaction of dark matter with ordinary matter, as well as gravitational waves.
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