Ye Jun, a physicist at the University of Colorado's Joint Laboratory in Astrophysics (JILA) and an alumnus of Shanghai Jiao Tong University, led a team to test Einstein's general theory of general relativity for the first time at the millimeter scale.
According to general relativity, atomic clocks at different heights in the gravitational field rotate at different speeds. When observed closer to Earth under stronger gravity, the frequency of atomic vibrations decreases—toward the red end of the electromagnetic spectrum— an effect known as gravitational redshift. That is, the clock travels more slowly at lower elevations.
The research team led by Ye Jun verified this time expansion effect on the smallest scale ever recorded, showing that two tiny atomic clocks, just one millimeter apart, also operate at different speeds. The results were published on the February 17 issue of the journal Nature and featured on the cover.
Image from Nature
The team proposes a way to make atomic clocks 50 times more accurate than before, and offers a way to potentially reveal how relativity and gravity interact with quantum mechanics, a major puzzle currently in physics research.
"The most important and exciting outcome is that we have the potential to link quantum physics to gravity, for example, to probe complex physics when particles are distributed in different locations in curved space-time." Ye Jun said, "In terms of timing, the results also show that making today's clocks 50 times more accurate without any obstacles - this is wonderful news." ”
Einstein's 1915 theory of general relativity revealed effects such as gravitational effects on time and had important practical applications, such as correcting GPS satellite measurements. Although the theory has been around for more than a century, physicists are still fascinated by it. Over the years, scientists at the National Institute of Standards and Technology (NIST) have used atomic clocks to measure relativity more and more precisely. For example, NIST physicists tested general relativity in 2010 by comparing two atomic clocks 33 centimeters apart.
The JILA Laboratory is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado-Boulder.
In this experiment, Ye Jun's team mainly used optical lattice clocks to conduct research. The team first used 6 laser beams to gradually cool 100,000 strontium atoms, and finally used infrared lasers to maintain the strontium atoms in an ultra-cold state and load them in an optical crystal lattice. The lattice can be imagined as a stack of pancakes produced by a laser beam, a design that 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 well controlled, setting a record for a quantum coherence time of 37 seconds.
The researchers measured the time expansion effect in this small cloud of strontium atoms, pictured by NIST
Crucial to improving accuracy is the team's innovative new imaging method. This method provides a microscopic map of the frequency distribution of the entire sample, enabling comparison of two regions of an atomic cluster, rather than following the traditional approach of two independent atomic clocks.
The redshift measured by the atomic group is small, in the range of 0.00000000000001, which is one hundred billionth of a billionth. Although this tiny scale cannot be directly perceived by humans, these differences combined have had a significant impact on the universe as well as technologies such as GPS. The research team used an average of about 30 minutes of data to solve this problem. After 90 hours of data processing, the measurement accuracy is 50 times higher than any clock ever before.
The atomic clock time difference of 100 billionths of a millimeter apart is from the paper
"This is a whole new race, a new way to explore quantum mechanics in curved spacetime," Ye said, "and if we can measure gravitational redshifts that are 10 times more precise than that, we can see the entire wave of matter of atoms that cross the curvature of space-time." For example, measuring jet lag at such tiny scales can lead us to discover that gravity breaks quantum coherence, which may be the root cause of our macroscopic world (still) the world of classical physics. ”
More accurate clocks have more than just timekeeping and navigation. Ye Jun believes that atomic clocks can be used both as microscopes to observe tiny connections between quantum mechanics and gravity, or as telescopes to observe the deepest parts of the universe. He is using atomic clocks to find mysterious dark matter, which scientists believe constitutes most of the matter in the universe. Atomic clocks can also further measure the shape of the Earth and improve models through "relativistic geodesy."