The World’s Most Precise Clock Proved Einstein Right Again
For most of human history, we’ve kept time using Earth’s location in space. First, we divided a day on Earth and then a year into seconds. This time interval was determined by the location of the Earth. Then atomic clocks appeared.
By examining the atoms of the element cesium with a process called ultra-fine transition, which emits and absorbs microwaves, researchers can keep time very precisely with the help of vibrating quartz crystals. This method, which forms the basis of time measurement made by scientists, provided a more accurate definition of the second in 1967.
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However, for half a century, neither this definition nor the setting of the atomic clocks used in the definition has changed. The clocks in question would not have deviated by a second since the dinosaurs disappeared. Now, better atomic clocks have been developed that can do more than just keep time. Moreover, these clocks can become marvelous tools in physical science.
In their study published five days ago, two different research groups report that they developed clocks that can measure the difficult physical processes that occur within these clocks. The research teams published their results in two separate articles in Nature on February 16. These new watches can also measure the gravitational time dilation, one of Albert Einstein’s predictions, at the smallest scale ever.
No cesium or quartz is used in such cutting-edge atomic clocks. The clocks are based on pancake-like structures made of supercooled strontium atoms. Users of the watch can control these atoms with the help of a laser that emits visible light. Therefore, clocks are called “optical clocks”.
Such an optical clock is located at the University of Wisconsin, Madison. This watch can house six strontium pancakes (actually six small watches) in the same structure. (This number is nothing special; more hours can be added or subtracted. “Six is an arbitrary number,” says Shimon Kolkowitz, a physicist at the University of Wisconsin, Madison.)
This clock in Madison deviates by only one second in 300 billion years; that is more than 20 times the age of the universe. While this is a world record, it is not itself the most powerful watch. Another clock at the Joint Institute for Laboratory Astrophysics (JILA), a joint project of the US National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, surpasses it.
Having more than one “clock” on the same device may not be useful for keeping time. (For example, which time will you follow?) But it allows the time to be compared with each other. Because these clocks are very, very precise, they can measure some very precise physical processes. For example, the group working in Boulder can test the time dilation inside the device.
“This is something you’ve found so far by comparing separate clocks over long distances,” says Tobias Bothwell, a graduate student at JILA and NIST.
According to the theory of relativity, the faster you go, the closer you get to the speed of light, the slower time slows down. Gravitational fields can cause the same slowdown: the stronger the field, the greater the time dilation. For example, let’s consider Earth. The closer you are to the center of the Earth, the more Earth’s gravity pulls you down and the more time dilation you experience.
In fact, you live time slower than the birds overhead. Things under your feet also experience time slower than you. The Earth’s core is actually 2.5 years older than the Earth’s crust. This amount may sound like a lot, but when we look at the 4.6 billion year history of our planet, it is not a drop in the time bucket. But scientists have been measuring such subtle differences for decades, using everything from gamma rays to radio signals sent to Mars to atomic clocks.
In 1971, two scientists took atomic clocks with them and boarded passenger planes and took each of these clocks around the world in different directions. As predicted, they saw a slight difference of a few thousand nanoseconds. In 2020, scientists using two clocks, one on the Tokyo Skytree tower and the other 451 meters above it, detected a difference that reaffirmed Einstein’s predictions.
These experiments show that relativity is universal. “It’s basically the same all over the world,” says Alexander Aeppli, a graduate student at JILA and NIST. “If you’re measuring a centimeter here, you’re measuring a centimeter elsewhere.”
NIST has already gone down to the centimeter level. Scientists working at NIST performed a similar measurement in 2010 using different clocks located about 30 centimeters away.
In one of the new experiments, two strontium pancakes in a single device were brought much closer together: about a millimeter. Working in Boulder, the group was able to distinguish the slight difference in light after 90 hours of data collection, and performed a measurement 50 times more sensitive than previous ones.
The previous measurement record they broke in expansion (a difference in the frequency of light) was 19 decimal places, Bothwell says. “Now we have reached 21 numbers… Normally, if you advance even one digit, you will be excited. But we were lucky that we advanced two steps at the same time.”
According to Kolkowitz, these are “very beautiful and exciting results.”
But Kolkowitz, who was not involved in the study at NIST, says that NIST’s clock has one drawback: It is not easy to get the clock out of the lab. “The NIST group has the world’s best laser, but it’s not very portable,” he says.
He thinks that the work of the two groups complements each other. Developed at Boulder, the watch can measure time and other physical properties more precisely than ever before. Meanwhile, Kolkowitz thinks a more portable watch like the one from Madison could be taken to different environments; Among them is space. With the watch taken into space, dark matter or gravitational waves can be searched.
While it would be pretty cool to prove that basic physical processes work as Einstein and his colleagues predicted, there is actually quite a bit of application for this kind of science in actual world conditions. For example, more precise clocks can be useful for navigation systems; GPS needs to correct for time dilation. Also, measuring the force of time dilation could allow gravitational fields to be measured more precisely, allowing for example to look below the Earth’s surface.
“You can look at magma vents below Earth and maybe calculate when a volcano might erupt,” Aeppli says. “Something like that…”
Author: Rahul Rao/Popular Science. Atomic clock at the US National Institute of Standards and Technology. Photo: Jacobson/NIST https://popsci.com.tr/