For years people Knew that when an object is thrown straight up, it falls back to Earth. People observed this phenomenon and could predict this force’s effect, but had no idea how it worked.

Since ancient times, there have been numerous theories and attempts to explain this mysterious force called “gravity”. During the early days, philosophers thought that the falling of objects toward the Earth was part of the gods’ realm and followed a natural motion. For instance, air belonged to the heavens, moving upward, while rocks belonged to the Earth, falling back to Earth.

“Take a moment to experience the effects of gravity. Lift your arm and feel how you are compelled to drop it again or use a ball or apple to throw straight up”

In the fourth century, Aristotle was the first to attempt a quantitative description of gravity. He wrote that an object fell at a constant speed, attained shortly after being released, and heavier things fell faster in proportion to their mass. Although this idea was wrong, it was stuck in mind until the 16th century.

In the 17th century, an Italian astronomer, Galileo Galilei, provided gravity’s first scientific explanation by conducting experiments and challenged the idea of gravity proposed by Aristotle.

Through his experiments, Galileo realized that a falling body picked up speed at a constant rate—in other words, it had constant acceleration. He also made the significant observation that, if air resistance can be neglected, all bodies fall with the same acceleration, bodies of different weights dropped together reach the ground simultaneously.

Galileo is said to have dropped balls of the same shape. Still, different weight from the leaning Tower of Pisa to demonstrate that the objects fell with the same acceleration, proving his prediction right while at the same time disproving Aristotle’s theory of gravity (which states that objects fall at speed proportional to their mass).

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“At the end of the last Apollo 15 moon walk, Commander David Scott (pictured below) performed a live demonstration for the television cameras. He held out a geologic hammer and a feather and dropped them at the same time. Because they were essentially in a vacuum, there was no air resistance, and the feather fell at the same rate as the hammer, as Galileo had concluded hundreds of years before – all objects released together fall at the same rate regardless of mass. Mission Controller Joe Allen described the demonstration in the “Apollo 15 Preliminary Science Report”:”

https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html

Galileo also performed another experiment to prove that gravity accelerates objects at a constant rate as it fell. However, there was one problem in testing this hypothesis. Galileo could not observe the object’s freefalling motion vertically. At the time, technology could not record such high speeds(lack of a clock or a timer that could measure high speed). As a result, Galileo tried to decelerate motion by replacing the falling object with a ball rolling down an inclined ramp. Since free falling is equivalent to a completely vertical ramp, he assumed that a ball rolling down an inclined ramp would speed up in the same way as a falling ball would.

Through this experiment, Galileo concluded that if an object is released from rest and gains speed at a steady rate, then the total distance traveled by the object is proportional to the time squared needed for that travel. For example, if an object released from rest travels for two seconds, it will travel 2 x 2 or 4 times as far as it would if it traveled for only one second after being released from rest.

The below video reproduces the above experiment performed by Galileo.

I believe we have a better understanding of how things move under the influence of gravity here on Earth. But how do we extend this understanding to the movement of other objects outside the planet earth? For example, how does the Moon revolve around Earth? Why does Earth orbit around the Sun? How do the stars move across the universe?

In the 17th century, Sir Isaac Newton, a mathematician, and physicist gave a new description of gravity. He believed that we live in a world where we can predict the movement of anything. Newton’s work on gravity is one of the critical turning points in physics.

Whenever we talk about Newton, the first thing that comes to our mind is Apple that fell on Newton’s head. This story is not entirely correct.
One day Newton sat thinking in his garden when he noticed an apple fell nearby. It wasn’t just that the apple fell made him excited, but that it tried to go to Earth’s center(straight down) and not in any other direction. That was vital Moment Newton realized that Earth had drawn the apple to it. He then realized that every object in the universe attracts every other object in proportion to its mass. The larger the mass of the object, its gravitational attraction will be much larger. He extended this idea to Earth, drawing Moon towards its center. It took almost 20 years from this day for newton to publish his work on gravity. While an apple might not have struck Sir Isaac Newton’s head as the myth suggests, the falling of one did inspire Newton to one of the great discoveries.

Newton’s universal law of gravitation states that the force (F) of attraction between two objects is equal to the product of their masses (m1 * m2), divided by the square of the distance between them (r). This formula also indicates that the distance between the object increases; the gravitational force gets weaker. (Remember the inverse square of the distance between the objects)

A question might arise why the Moon is not falling straight down to Earth as the apple did.

Imagine firing a cannonball horizontally from the top of a mountain like Mt.Everest on Earth. The ball would follow a curved trajectory as it moved forward and was attracted, by gravity, towards the ground at the same time. Fire the cannon ball with more energy, and it would land further away from the mountain, but it still would follow a curved trajectory in doing so.

Newton proposed that if you fired the cannon ball with enough energy, it could fly around the Earth and never lands because the Earth would be curving away underneath the ball at the same rate as the ball fell. In other words, the ball would now be in orbit around the Earth.
And this is what happens with the Moon – it is in free fall around the Earth, but it moves fast enough so that the Earth’s surface never quite “catches” it. The same principle is used even today for launching satellites in orbit around the Earth.

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Newton’s gravitation model worked pretty well for predicting the motion of most planets as well as objects on Earth. In fact, Newton’s gravitational law was used to discover the planet Neptune. But there is one problem that Newtown’s law of gravity could not predict nor explain Mercury’s orbit around the Sun correctly. The planet’s orbit shifted faster than the model derived using Newton’s law of gravity.

Another problem in Newton’s theory describes gravity as an instantaneous force of attraction between two massive objects. If one object moved away, the other object knows about the movement immediately due to the change in gravitation, irrespective of their distance.
For example, The Earth orbits around the Sun, and the distance between them is 91 Million Miles. Let’s assume that, suddenly, the Sun disappears. According to Newton’s Law of gravity, the Earth will instantly feel the impact of the disappearance of the Sun and loses its orbit, and Moves in a straight line. This behavior seems unrealistic and not explained well using Newton’s Law of Gravitation.

More importantly, Newton’s gravitation law describes gravity’s effects but didn’t explain how gravity worked. Newton was well aware of this and said:

For almost 200 Years after Netwon’s work on gravity was proposed, there were no breakthroughs to explain the violation identified in Newton’s Law of Gravity. In 1915 Albert Einstein, a German-born physicist redefines gravity by proposing the General theory of relativity. According to Einstein, gravity is not a force like Newton described; instead, a distortion in space-time created by massive objects. This distortion is felt as gravity.

I know it’s confusing. Why is gravity not a force? What is space-time, and what does the distortion mean?

Let’s try to understand space-time. Space is generally defined in three dimensions Length (Dimension 1), Width (Dimension 2), and Height (Dimension 3). All the work done by Newton referenced only the three dimensions of space, but Einstein came up with a New Idea of the 4th dimension called time.

Why Time? Let’s assume you invite your friend to meet for a coffee. What happens if you mention just the place to meet without mentioning the time or time to meet without mentioning the place. In both scenarios, you will fail to meet with your friend as there is no sufficient information unless you say both the place and the time to meet. According to Einstein, space without time is meaningless. You can’t describe motion without time, for example, meter per sec. Einstein defines space and time as an intertwined fabric. (See below sketch)

According to Einstein, when a large object is placed in a space-time fabric; the object distorts the fabric, creating curvature proportional to its mass. Any other object that gets into the space-time warp of a massive object will experience the phenomenon of gravity. For example, the Sun distorts the space-time around it, creating curvature, and all other planets rotate around the Sun not due to any other force but just because of the curvature around it.

According to Einstein, objects moving around the curvature are still moving along the straightest possible line, but due to a distortion in space-time, the straightest possible line is now along a spherical path.
For example, imagine a ball rolling across the trampoline it will simply follow the curve. From the ball’s perspective, it was always traveling in a straight line; it’s the warping of space-time that causes its deflection.

Also, Light rays, when crossing massive objects like the Sun, bends because of the space-time curvature. This phenomenon is called Gravitational lensing. This behavior was proved right during the solar eclipse, by measuring the bending of light from the stars around the Sun.

So, according to Einstein, gravity is not a force. It’s a curvature in space-time.

I hope by now, you have a good understanding of what gravity is.