Xiao Cha Ming Min was sent from The Temple of OuFei
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Do you know? On Earth, the lower the floors, the slower time passes.
This is not metaphysics, but the effect of dilation of time predicted by Einstein's general theory of relativity: the greater the gravitational pull, the slower time becomes.
△ Verify that the clock is faster at different height differences (Image from Nature)
An article on the cover of Nature today proves that even if the height difference is only one millimeter, the speed at which time passes is different, an experiment to date to test general relativity at the smallest scale.
The study came from Ye Jun's team in the JILA Laboratory at the University of Colorado.
He led the team to develop the world's most accurate atomic clock, and concluded that the time difference in one millimeter in height is about one hundred billionth of a billionth, that is, about 300 billion years is only 1 second apart, which is consistent with the prediction of general relativity.
This time difference due to different gravitational forces is called gravitational redshift, and although it has been verified countless times, it is the first time that such a high-precision detection is the first.
Gravity changes the frequency of light
General relativity states that the stronger the gravitational field, the slower time becomes, changing the frequency of electromagnetic waves.
If a beam of blue light shoots into the sky, under the action of gravity, it will move towards the red end, which is called "gravitational redshift".
At that time, scientists used rockets to send the atomic clock to an altitude of 10,000 kilometers and found that it was faster than the sea-level clock, about 73 years faster than a second.
Although this gap is not physically perceptible, it is closely related to our lives, because GPS must correct this extremely small time difference to accurately locate.
On the same day almost 12 years ago, a team from UC Berkeley measured the time difference between two atomic clocks with a height difference of 33 centimeters.
Now Ye Jun's team can measure the time difference between the upper and lower ends of an atomic gas in an atomic cloud, and the height difference between the two is only one millimeter!
Ultra-precise optical lattice clock
Why is Ye Jun's team so accurate? That's because they used a more accurate clock, the optical lattice clock.
The system first uses 6 laser beams to gradually cool 100,000 strontium atoms, and finally uses infrared lasers to maintain strontium atoms in an ultra-cold state.
Due to the coherence of the laser, there will be periodic areas of less energy in space, thus binding strontium atoms in a pancake-shaped space.
△ Principle of optical lattice clock (picture from NIST)
This design reduces lattice distortion caused by light and atomic scattering, homogenizes the sample, and expands the atom's wave of matter. The energy state of the atoms is so well controlled that it sets a record for a so-called quantum coherence time of 37 seconds.
Crucial to improving accuracy is the new imaging method developed by Ye Jun's team. This method provides a microscopic plot of the frequency distribution of the entire sample.
This way, they could compare two regions of an atomic cluster instead of the traditional method of using two separate atomic clocks.
After cooling the strontium atom, it is then excited by a laser beam, excitation of its outer electrons into a higher orbital.
Since only a very small range of laser frequencies can excite electrons, time can be measured extremely precisely by adjusting the laser to exactly the frequency of excitation and measuring.
△ Laser excitation strontium atom measurement frequency (picture from NIST)
Since the redshift in the one-millimeter range is small, about 0.00000000000000000001 (not counting, a total of 19 0s), in order to improve the accuracy, the research team solved this problem with an average of about 30 minutes.
After 90 hours of data analysis, their measurements were 9.8 (2.3) × 10-20mm-1, within the margin of error, and well in line with general relativity.
Connecting quantum mechanics and general relativity
Ye Jun, the corresponding author of the study, said that the breakthrough can improve the accuracy of the clock by 50 times.
This is expected to improve the accuracy of GPS.
Due to gravitational redshifting, the time correction of the GPS atomic clock must be made, and the more accurate the time correction, the higher the accuracy of the positioning.
This is even more significant for physics.
The most exciting thing is that we can now link quantum mechanics and gravity!
Ye Jun said that an accurate atomic clock will open up the possibility of exploring quantum mechanics in curved space-time, such as the complex physical state of particles distributed in different positions in curved space-time.
And, if the current measurements could be increased by a factor of 10, the team would be able to see the entire wave of matter of the atom as it passed through the curvature of space-time.
This means that gravitational effects at the quantum scale can begin to be explored.
Flannia Giacomini, a theoretical physicist at the University of Waterloo in Canada, also said atomic clocks are one of the most promising systems for exploring this problem.
Ye Jun said: Perhaps it is this tiny frequency difference that breaks the quantum coherence and makes macro time classic.
In addition, atomic clocks can be applied to microscopes to observe the subtle connections between quantum mechanics and gravity. It can also be applied to astronomical telescopes to observe the universe more accurately.
In fact, Professor Ye Jun is also using atomic clocks to find mysterious dark matter.
Even in geodesy, atomic clocks can help researchers make more precise measurements of the Earth and improve models.
Corresponding author Ye Jun
Finally, let's take a look at the corresponding author of this study, Ye Jun.
Jun Ye is a professor in the Department of Physics at the University of Colorado and a laboratory of experimental astrophysics (JILA) jointly established by the National Institute of Standards and Technology (NIST) and the University of Colorado.
Ye Jun graduated from the Department of Applied Physics of Shanghai Jiao Tong University with a bachelor's degree and from the University of Colorado under the supervision of Nobel Laureate in Physics John Hall.
Ye Jun has taught at the University of Colorado-Boulder since 1999 and took over the management of the lab after Hall retired in 2008.
In 2011, Ye Jun was elected as an academician of the National Academy of Sciences of the United States; in 2017, he was elected as a foreign academician of the Chinese Academy of Sciences; in 2020, he won the "Mozi Quantum Award", and in 2021, he won the Scientific Breakthrough Award for Basic Physics.
His main research areas are ultracold atom-molecule, precision measurement, and multibody quantum physics.
In 2007, Ye Jun and his research team made the world's first strontium atomic light clock that "has an error of only 1 second per 70 million years".
Since then, he has continued to set new records in this field.
In 2017, the new atomic clock designed by his team loaded strontium atoms into tiny three-dimensional cubes, with a density nearly 1,000 times higher than that of strontium atoms in the previous one-dimensional atomic clock design, further improving the measurement accuracy of atomic clocks.
In 2020, Ye Jun's team published Nature and Science papers in a row within 3 days.
Published in "Dipolar evaporation of reactive molecules to below the Fermi temperature," published in Nature, his team implemented quantum degeneracy gases for the first time.
Another paper published in Science, "Resonant collisional shielding of reactive molecules using electric fields," uses quantum mechanical theory to explain the collisions between molecules.
Address of thesis:
https://www.nature.com/articles/s41586-021-04349-7
Reference Links:
[1]https://www.nature.com/articles/d41586-022-00379-x
[2]https://www.sciencedaily.com/releases/2022/02/220216112213.htm
[3]https://www.quantamagazine.org/an-atomic-clock-promises-link-between-quantum-world-and-gravity-20211025/
[4]https://www.nist.gov/news-events/news/2022/02/jila-atomic-clocks-measure-einsteins-general-relativity-millimeter-scale
[5]https://news.berkeley.edu/2010/02/17/gravitational_redshift/