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Megadrought On Earth Affected Gravity Waves At The Edge Of Space

                           

An intense drought that has persisted on Earth for over two decades is now thought to have affected gravity waves where our planet's atmosphere meets outer space.

The discovery happened by chance as a region in the southwest U.S. Transitioned to drought conditions in 2000. Researchers Chester Gardner of the University of Illinois and Chiao-Yao She of Colorado State University were already keeping an eye on the upper reaches of Earth's atmosphere with LIDAR (Light Detection and Ranging) when they observed a 30% decrease in gravity waves after the Southwestern North American (SWNA) megadrought began. 

"We never expected to make observations that would yield some insight into how a drought might affect Earth's upper atmosphere," Gardner said in a statement. Nevertheless, the researchers found that this precipitation deficit caused by the megadrought has been accompanied by a significant decrease in gravity waves at the edge of space, suggesting that changes in the lower atmosphere can affect the upper atmosphere more than was previously thought.

 

Gravity waves are different from the gravitational waves that warp spacetime. Instead, when two substances in the atmosphere are unbalanced, gravity waves form as the forces of gravity and buoyancy equalise and create vertical waves. (As a buoyant substance rises, gravity then pulls it back down before buoyancy takes effect once more and the process repeats.) These waves take more time to propagate through denser substances like water. In the furthest reaches of the upper atmosphere, they can spread much more easily (and in any direction) since the air is so thin at that altitude. 

Gardner and She first started using LIDAR in 1994 to monitor weather in the upper atmosphere. They never expected to see any drastic changes, and were surprised when they noticed that gravity waves were not so active in the years since the SWAN megadrought took over and storms in the lower atmosphere subsided. 

The researchers also found that the greatest amount of gravity wave activity happened during the winter, when powerful winds unleashed rain and snow, and in mid- to late summer, when rains swept in from the Pacific Ocean. This suggested to them that precipitation was a factor in gravity wave reduction. It appeared that storms had brought about gravity waves, and a downturn in precipitation during the drought meant fewer storms — and thus fewer gravity waves.

No one had previously made a connection between changes in weather in the lower atmosphere and subsequent changes in the upper atmosphere. While She and Gardner cautiously state that more research is still needed, their observations can set the bar for future computer models that track atmospheric changes that LIDAR can detect.

"Current atmospheric models can't see the waves because the resolution, even on the fastest computer models, is not sufficient to see the scale of these waves," Gardner said in the statement. "Now, scientists are developing regional models at very high resolution so that the models can see the larger-scale waves. Our observations can be used to test the accuracy of those models and to validate them."

Meanwhile, the seemingly endless drought still continues. More than 40% of the American Southwest has experienced 40 or more consecutive weeks of drought in 2022, according to a study by Climate Central. 

 

Does Gravity Make You Age More Slowly?

 

Einstein's theory of general relativity upended humanity's understanding of the universe more than a century ago, and since then, scientists have discovered that the steady march of time is anything but steady. Among the haunting implications of general relativity is that time passes more quickly at the top of every staircase in the world than it does at the bottom. 

This mind-bending phenomenon happens because the closer an object is to Earth, the stronger the impacts of gravity are. And because general relativity describes gravity as the warping of space and time, time itself travels more slowly at higher altitudes and greater distances from Earth, where gravity has less of an effect.

So, if time is linked to gravity, does that mean that people on top of mountains age faster than people at sea level do? Does increased gravity actually make people age more slowly?

Indeed, for all objects farther away from a gravitational field, such as Earth, time actually moves more slowly, James Chin-wen Chou, a physicist at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, told LiveScience in an email. That means people who live at high altitudes age a tad faster than those plodding through space-time at sea level.

"Gravity makes us age slower, in a relative term," Chou said. "Compared to someone not near any massive object, we are aging more slowly by a very tiny amount. In fact, for that someone, the whole world around us evolves more slowly under the effect of gravity."

Related: What is the shape of the universe?

The differences are minor but measurable. If you were to sit at the peak of Mount Everest — which is 29,000 feet (8,848 meters) above sea level — for 30 years, you would be 0.91 millisecond older than if you had spent those same 30 years at sea level, according to NIST. Similarly, if twins living at sea level were to part ways for 30 years, with one relocating to mile-high (1,600 m) Boulder, Colorado, and the other staying put, the high-elevation twin would be 0.17 millisecond older than their twin when they reunited.

In a striking experiment, NIST researchers used one of the most precise atomic clocks in the world to demonstrate that time runs faster even a mere 0.008 inch (0.2 millimeter) above the Earth's surface. 

"These aren't just calculations," said Tobias Bothwell, a physicist at NIST and co-author of a 2022 paper published in the journal Nature describing the experiment. "We have seen the change in the ticking of a clock at a distance roughly the width of a human hair," he told Live Science.

The key to understanding why massive objects warp the passage of time is recognizing that "space-time" is a four-dimensional tapestry woven from three space coordinates (up/down, right/left and forward/back) and one time coordinate (past/future). Gravity, in a relativistic model, is what we call it when any object with mass distorts that tapestry, curving space and time as one.

"Anything that possesses mass affects space-time," Andrew Norton, a professor of astrophysics at The Open University in the U.K., told Live Science in an email. In the vicinity of an object with mass, "space-time is distorted, resulting in the bending of space and the dilation of time. 

"The effect is real and measurable but negligible in everyday situations," Norton said.

When it comes to non-everyday situations, however, this phenomenon — also known as gravitational time dilation — can get messy. According to Norton, GPS satellites circling the globe at an altitude of 12,544 miles (20,186 kilometers) need to adjust for the fact that their clocks run 45.7 microseconds faster than clocks down here, over the course of 24 hours. 

"The most pressing effect of relativity over the passage of time is probably the accuracy of GPS," Chou said. "Because they [GPS satellites] are moving at high speeds and high up away from the earth, the relativistic effects from speed and gravity need to be carefully accounted for so that we are able to infer our position on the globe with high accuracy."

Closer to home, it is clear that gravity does, in fact, make us age more slowly. Sure, it's usually only a matter of milliseconds, and cowering at sea level is hardly a viable anti-aging strategy. But time is both precious and fleeting, especially when distant from any objects with mass.