Newton, The Apple, & Gravity
By Danish Talwar
Cosmofluencer (Season 03)
We all have heard the story time and time again: an apple falls, it hits Newton on the head, and then “Gravity”.
It may have not gone exactly like that, and took him years to actually craft this theory, but it has been a vital theory. What actually is gravity or gravitational force?
Gravity is one of the four fundamental forces of the world, as well as the weakest. It is dependent upon the mass of both bodies and the square of the distance between them. That’s right, gravity doesn’t exist just between us and the earth, but every body attracts every other body in this universe. This theory took Newton 20 years to develop and is called Newton’s law of Gravitation.
Newton's Theory of Gravitation
It states that the force between two objects is proportional to the product of their masses and inversely to the square of distances between them, i.e.:
Gravitational Force ‘F’ = Gm1m2/ r2
Where ‘G’ is the gravitational constant.
(m1 and m2 are the masses of the two bodies, and r is the distance between them).
Gravitational force occurs due to the curvature of the fabric of spacetime of the universe. Mass bends spacetime in which other bodies are attracted to.
Think of it as a hill. If you were to place a ball on top of the hill, it would certainly roll down. Similarly, objects “fall” towards the centre to have the most stable position.
Spacetime around our solar system
You might wonder, “How does mass bend spacetime?” One theoretical explanation involves the concept of gravitons. These are hypothesized subatomic particles that carry the quantum of gravitational fields(these are the smallest force of gravity) and travel at the speed of light.”
Everything with mass emits them. The more the mass, the more gravitons are emitted by the body. These are subatomic in size and transfer between bodies.
But hey, that’s just a theory, a quantum theory.
Collision between gravitons releasing gravitational waves
You must be wondering, what does gravity even do? Well, it is one of the main reasons why you might be reading this blog right now. It holds everything in the universe together.
The planets, galaxies, the blanket of atmosphere, you to the earth, everything. It holds everything in place. But there is a force opposing this, for every action has an equal and opposite reaction. For us, this will be the normal force, which prevents us from slamming into the ground.
When two bodies are in contact, they both apply normal force on each other so that one doesn’t pass through the other and thus, oppose gravity that tries to attract everything to the center of the earth.
In orbit systems, this is the centripetal force that keeps the orbiting body around a body, well, in orbit. These two forces work against each other to keep everything in place.
When a body is in orbit, the centripetal force due to circular motion tries to throw the body outside the orbit while gravity tries to attract the body towards the center. This tug of war between forces helps keep the body in a stable orbit.
Centripetal and centrifugal force versus gravitational force
Since the Big Bang, the universe has been expanding uniformly throughout and this rate of expansion is speeding up. Gravity also competes with this force to keep everything intact. But this tussle could also lead to the beginning of the end.
Scenario 1: Rate of Expansion Overpowers Gravity (The Big Rip)
In such a scenario, everything drifts apart. Local Groups of Galaxy clusters become lone galaxies. Over time, the stars and the nebulae in the galaxy also move away from each other. Star systems become lonely and no new stars can form as the nebulae have turned into floating dust through space. With time, even these stars die out, matter turns into radiation and Black holes die out due to Hawking Radiation. All matter turns into radiation and photons until they also decay. Atoms break into subatomic particles and muons.
In the end, only energy is left. The universe becomes a dark and secluded place. This is called “ The Big Rip” Scenario as celestial bodies crumble away due to the ever-expanding rate of the universe.
Scenario 2: Gravity Overpowers the Rate of Expansion of the Universe (The Big Bounce)
In this case, gravity keeps everything together. As time passes, the rate of expansion decreases and eventually reverses. Everything moves closer. All matter begins to converge into a centre. Galaxies are ripped apart and planets in solar systems become rogue. Eventually, all matter compresses into a point-size body. And then begins the cycle of birth of a new universe. The matter explodes with a new Big Bang and the universe restarts.
This is The Big Bounce Scenario. From the destruction of one universe to the creation of a new one.
There are other theories that talk about the end of the universe like the Big Crunch, which is quite similar to the BIG BOUNCE, but that is beyond the scope of this blog.
However, all theoretical models about the end of the universe remain speculative as we can’t just look forward in time.
But you don’t have to worry about that yet, this will take so long that humanity will be long gone.
As technology gets better, more concise and accurate theories will be present in the future. Who knows how much of this blog would be true by then?
But that’s the beauty behind astronomy, it keeps changing. Every time it feels fresh and new.