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Falling Atoms Are Helping NASA Measure Earth’s Gravity

Measure Earth’s Gravity

You know what’s really keeping me down? Gravity. Everywhere I go, there it is. Scientists noticed this too, and since 2000 have used satellites to map the Earth’s gravitational field. Presently they've built up another instrument that utilizations quantum marvels to convey the most exact estimations yet. You may be wondering what point there is to map the Earth’s gravity. If you’ve taken high school physics you probably learned that Earth’s gravity pulls things toward its center at a rate of 9.8 m/s2. It turns out that while that’s a good enough approximation for the kinds of math problems you’ll tackle in high school, it’s not entirely accurate.
The Earth’s gravitational pull isn’t uniform, as it varies from one place to another. So, scientists developed a new instrument to deliver the most accurate measurements yet.
Falling Atoms Are Helping NASA Measure Earth’s Gravity
The Earth’s gravitational pull isn’t uniform, it varies from one place to another, and it can change over time. What might cause the Earth’s gravity to change? Well, gravity is directly related to mass, and the more massive an object is, the more it pulls on other objects. And what has a lot of mass that covers most of the Earth and can shift around? That’s right, water. Other things can contribute to a change in Earth’s gravity, but the shifting mass of water is the biggest factor. If a glacier melts or water gets trapped underground, its mass will be redistributed. So estimating the moment changes in the gravitational field is an approach to follow what Earth's water is doing, and how the water cycle is being influenced by environmental change.

Past NASA missions like GRACE and its successor GRACE-Follow On utilized twin satellites to outline field. The principle was one satellite would follow the other, and as they passed through fluctuations in the gravitational field, they would get closer or farther apart. Estimating the adjustment in separation would tell researchers how the gravity over that piece of the world contrasted. But since the goal is always more accuracy and precision, NASA teamed up with the company AOSense, Inc. to try an entirely different approach. 
prototype quantum sensor                  prototype quantum sensor
The new prototype sensor uses about 100 million cesium atoms sealed in a vacuum near absolute zero. This keeps the atoms insulated from outside factors that may affect the measurements. The only force pulling on these atoms is gravity.

Thanks to the bizarre nature of quantum physics, these atoms are coaxed into behaving like light waves. Lasers can split and send these waves down different paths, and as they travel they interact with gravity. Finally, the waves meet up again, and when they do, they form an interference pattern where some sections of the waves have a higher amplitude while others have a lower amplitude. By studying this interference pattern, the instrument can determine how gravity affected the atoms with unprecedented accuracy.
While past atom-based instruments needed components the size of a room, the new sensor is small enough to fit on a spacecraft. That doesn’t mean it’s sacrificing sensitivity; during testing, the device reported different gravitational measurements after the scientists came back from their lunch break... because it was detecting the added mass from the food in their stomachs. And once it’s up in space, the sensor will map the Earth’s gravity 10 times more precisely than GRACE-Follow On, at four times the spatial resolution.

An instrument like this could have uses beyond Earth one day, like measuring the interiors of other planets, moons, and comets. It could also be tuned to detect gravitational waves in new frequency ranges that we haven’t been able to see yet. Whatever it ends up being used for, the new instrument is an ingenious application of scientific principles to solve a problem and a testament to just how clever scientists are. 

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