Gravity.

Everywhere I go, there it is.

Scientists noticed this too, and since 2000 have used satellites to map the Earth’s

gravitational field.

Now they’ve developed a new instrument that uses quantum phenomena to deliver the most

accurate measurements yet.

You may be wondering what point there is to mapping 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, 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.

There are other things that 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 measuring the minute changes in the gravitational field is a way to track what Earth’s water

is doing, and how the water cycle is being affected by climate change.

Previous NASA missions like GRACE and its successor GRACE-Follow On used twin satellites

to map the 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.

Measuring the change in distance would tell scientists how the gravity over that part

of the world differed.

But since the goal is always more accuracy and precision, NASA teamed up with the company

AOSense, Inc. to try an entirely different approach.

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.

Gravity mapping satellites can also help us probe the Earth's secrets, like what’s happening

under Antarctica.

Maren has a video on that here.

If you liked this video, let us know down in the comments, and make sure to subscribe

to get all your quantum news here.

Thanks for watching and I’ll see you next time.