# Astronomy Wonder Project How was the concept of gravity realized, and how is it measured?

Gravity is defined to be the force by which a planet or other body draws objects towards its center. The same force that allows for planets in our solar system to orbit the sun, or what makes things fall on the ground here on Earth.

How was it discovered?

In the 1600s, the concept of gravity was realized when Isaac Newton witness a falling apple from a tree while he thought about the forces of nature. He developed the theory that defined gravity as a universal force acting on all matter, the farther apart two particles are and the less mass they have, the less gravitational force they posses. The theory was left unchallenged for 3 centuries, until in the 1900s Albert Einsten extended Newton’s theory with his general theory of relativity. He argued that gravity was more than just a force; it was a curve in the fourth dimension of space and time. Given enough mass, an object can cause a straight beam of light to curve, this effect is what astronomers call gravitational lensing, which was one of the methods leading to the discovery of black holes.

Here is a video explaining more about Einsten’s theory of Relativity >>

How can gravity be measured?

It was said that the greater the mass and the closer together, the more stronger the gravitational pull, so with that how exactly were they able to determine the strength of gravity on Earth? It was assumed that the acceleration of a body due to gravity is a constant 9.81 meters per second squared, by every square meter gained toward the planet’s centre, the faster the object goes down. However the assumption would be considered true only if the planet was completely smooth and contains equal amounts of elements and minerals. Earths consists of mountains, caves, difference in terrain, oceans, valleys, etc, all containing different amounts of mass which influences the gravitational pull in different regions.

One of the way to find where on Earth is gravity the strongest or weakest, in 2002 NASA and German Aerospace Centre launch a joint mission names GRACE (short for Gravity Recovery and Climate Experiment), sending 2 satellites into the same orbit around Earth. One about 220 kilometers (137 miles) in front of the other at an altitude of 460 kilometers (286 miles) above the Earth’s surface. As the leading satellite passes over a stronger area of gravity, it would detect the gravitational pull and increase in speed slightly, thus decreasing distance from the tailing satellite, and the opposite happens when the lead satellite passes over a weaker gravitational pull, having distance increase between the satellites. The changes in distance between the satellites are very small– about one-tenth the width of a human hair — that they are undetectable by the human eye. GRACE measures these changes by generating pulses of microwave energy — a highly energetic form of electromagnetic radiation — that bounce back and forth between the two satellites. The distance between the satellites is determined by the time a microwave pulse takes to travel from one satellite to the other and back. Along with this, GRACE maps the entire gravitational field of the planet every 30 days, and with changes in gravity happening over time reveals detail about polar ice, sea level, ocean currents, Earth’s water cycle and the interior structure of the Earth.

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Overall, one of the studies of gravity involves a branch of Quantum Theory, which is known as quantum gravity, in which scientists attempt to generally relate to each other. Quantum Theory refers to how does the universe work in the smallest level (subatomic) and it helps scientists develop standard models for particle physics which details more about the inner workings of the universe. However quantum gravity has one exception – it doesn’t explain much about gravity.

Both theories of Quantum Theory and relativity do explain most about the observable universe (the “horizon” of  the universe we are able to see from Earth), and contradict each other like in the study of black holes. Not unexpected, numerous scientist do continuous work toward a unified theory. Whatever theories are adopted, it is difficult to overstate the importance of gravity, it is the glue that holds the cosmos together, even if its leaves unanswered questions about the universe.

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Here is a video demonstrating gravity>>

Resources

Image – Gravity

Image – Gravity Map

What is Gravity – Howstuffworks.com

What is Gravity – spaceplace.nasa.gov

GRACE – NASA