## Gravity and Orbits

The universe is made up of many different classes of objects, stars, comets, asteroids, black holes, nebulae, and planets. Planets are seemingly mundane compared to the other names mentioned above, but they pose huge mathematical problems within physics. It is a learned fact that Planets are celestial bodies made up of elements and that there are two types of planets, etc. But, can we actually define a planet? No. Here is the scientific definition – A planet is any celestial body that revolves around a star in an elliptical orbit, has enough gravity to maintain a spherical shape and clears its surroundings by the strength of its gravity.

Let us dive into that definition a bit deeper. First, what is an elliptical orbit? Simply put, an elliptical orbit is an oval-shaped path the planet follows around its star. Second, what is gravity? I know, all of you think that you know what gravity is, but, let us look at it from a different perspective, let’s look at it through geometry. We know that Force is a vector quantity, which means we must mention its strength and direction while defining it. So, Earth’s gravity is a force of X strength *in the direction normal and towards the surface of the Earth. *This last bit is crucial, without it, the force is not clearly defined. Let us take an example.

Think of the Sun and the Earth in space, with no other body of any sort in between. The sun is exerting a gravitational force on the Earth, making it come towards it. We can calculate the strength of this force, but what is its direction? If it is towards the centre of the sun (imagine the sun pulling the Earth), then why does the Earth go around the sun and not towards it? Isaac Newton acknowledged this flaw in his model of gravity. He knew that the direction he gave to gravity was only partially correct, and stated that he knew gravity existed, but could not understand how it affects other bodies. How is the sun able to pull the Earth when it is millions of miles away? While Newton couldn’t solve that problem, another genius could.

Albert Alva Einstein, in his paper on General Relativity, published a geometric understanding of gravity. He said – If you imagine space to be a trampoline, and the sun is a heavy ball on said trampoline, the ball (sun) would create a depression around itself. This is the area that the Sun’s gravity can affect other objects, AKA its gravitational field. If you gently roll a marble with some initial speed into that depression, you will see it move in an elliptical path. This marble represents the Earth. While the marble eventually collapses into the ball, the Earth has such a path around the sun, with such a gravitational force and acceleration, it can continuously revolve around the sun without being absorbed by it. The path of all the planets in our solar system, not only the Earth, is so precise that neither do they break free from the Sun’s gravity and disappear into space, nor do they be absorbed by the sun’s gravity. They maintain an intricate balance so that they can continuously orbit the sun and absorb energy from it.

The concept of gravity has been continuously evolving and is still evolving today. Researchers are now exploring the possibilities of ‘gravitons’ (particles that can help mesh gravity with quantum physics) and ‘negative gravity’ within antimatter particles.

We started out this article by trying to form a definition of a planet, and in the course of doing so, we explored Newton’s theories on gravity, Einstein’s Theory of General Relativity and the n – body problem. We also discussed new areas of research within gravity, relating to its effect on anti-matter particles and how gravity can be moulded with quantum theory to form the theory of ‘everything’. We saw how one simple definition relies on centuries of work and research by scientists. Physics is a never-ending quest for the truth, and the journey has just begun.

### – Aryan Agarwal

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