Albert Einstein’s theory of general relativity is packed with weird insights about our reality, but perhaps the most mind-boggling is the fact that strong gravitational fields or incredibly high speeds can warp the passage of time, an effect known as time dilation. For instance, clocks located onboard spacecraft might tick slightly faster or slower than those on Earth, depending on the distortive effects of their velocities and our planet’s gravity on time.
Now, in a major breakthrough, scientists at JILA, a joint operation between the National Institute of Standards and Technology and the University of Colorado Boulder, have measured time dilation at the smallest scale ever using the most accurate clocks in the world. The team showed that clocks located just a millimeter apart—about the width of a pencil tip—showed slightly different times due to the influence of Earth’s gravity.
The new experiment paves the way toward clocks with 50 times the precision of those available today, which could be used for a host of practical applications, while also shedding light on fundamental mysteries about our universe, including the long-sought “union of general relativity and quantum mechanics,” according to a study published on Wednesday in Nature.
“When you go to such a small scale, what does that mean? It means that the clock precision is better,” said Jun Ye, a JILA physicist who co-authored the study, in a call. “In some sense, what we are trying to say is that time and space are interconnected. As Einstein's relativity told us, time is space, space is time, and time is relative. There's no absolute concept of time.”
Ye and his colleagues at JILA have been pushing the frontiers of timekeeping and general relativity for several years by designing ever-more accurate atomic clocks. The role of the pendulum in these clocks is played by the shifting frequency of electrons in atoms that are carefully arrayed in lattices designed to control their chaotic energy and motion. These innovations distinguish atomic clocks as being by far the most accurate timekeepers ever devised, capable of losing just one second over 15 billion years, which is why they are used on Global Positioning Service (GPS) satellites and other systems that require hyper-precise time.
In 2010, JILA scientists used these clocks to measure time dilation at two points with a difference in elevation of 33 centimeters (roughly a foot), which was a big advance at that point. After a decade of fine-tuning their clocks, Ye and his colleagues have managed to track frequency shifts within a sample of 100,000 extremely cold strontium atoms, enabling them to snag the unprecedented millimeter-scale effects of dilation.
What’s more, the team managed to keep these atoms dancing in perfect unison for 37 seconds, setting a new record for the duration of “quantum coherence,” or the state in which the behavior of these atoms can be predicted.
“The first day, when we were able to start to see this long coherence time, we couldn't believe it,” Ye recalled. “Quantum coherence sounds very microscopic. The atom is revealing to you how electrons are moving around the nucleus, and so on. But it's incredible to think of 37 seconds, almost a minute—that’s a very macroscopic timescale, a ‘human being’ timescale.”
“When I was talking to my students, I said: ‘This is the first time in my life that I can imagine my atom ticking back and forth in a coherent fashion while I drink a cup of coffee,” he continued. “In the end, actually, this is the essence of this quantum revolution we are talking about now. It's fascinating to bring quantum phenomena, which is something very microscopic, into the worldview of the macroscopic.”
Bringing this bizarre quantum world into our more familiar surroundings can help scientists pursue some of the biggest open questions in science. For instance, researchers have tried for decades to make general relativity, which governs the large-scale cosmos of stars and galaxies, agree with quantum mechanics, which sets the rules in the tiny realm inside atoms. As atomic clocks become ever more accurate, scientists will be able to actually see the waves of atoms across the curvature of spacetime, where the classical and quantum worlds clash.
“That is fundamental as it gets, if we get to that point,” Ye said. “That's an area of physics that we have never explored.” If these clocks can be improved by another factor of 20, Ye added, “we get into a very, very interesting regime” that will yield “new insights when quantum mechanics finally meets up with general relativity.”
Beyond these new discoveries about the basic nature of our reality, the increasingly accurate clocks are inspiring a virtually endless list of potential applications that range from predictions of volcanic eruptions, to measurements of sea level rise, to deep space missions.
“The entire Earth is a living body; it's moving around very actively,” Ye said. “When you are talking about global warming and the changing Earth, we need better tools to be able to monitor what's going on. You can use clocks to actually measure the change in Earth because time and space are intrinsically connected.”
“We often like to make a joke: ‘We live a healthy life of a hundred years and during that period, your head ages a little faster than your feet by about half a microsecond,” he noted. “It’s minute on a biological scale—who cares about half a microsecond over a hundred years of my lifetime? But it makes all the difference in terms of measuring how the Earth is changing, and measuring how eventually, we can drive manned or unmanned vehicles to land on Mars or other distant planets. It's all based on this precise timing of information.”
Update: This article was updated to clarify that some clocks onboard spacecraft tick slower in space compared to those on Earth because they are traveling at high speeds, whereas some clocks tick faster in space than on Earth because they are further away from our planet’s gravitational field.