Gravity

"You don't understand the gravity of the situation!"

But do we understand the concept of gravity at all?

"Newton invented discovered gravity when an apple fell on his head."

The apple didn't fall on his head and he had to work for twenty years straight to formulate the concept of gravity.

"The attractive force by the virtue of mass is called gravitational force".

What is force? F=m*a? Push or pull that brings in motion in an object when unopposed? Force is a very broad term. When I say "Put more force" in layman's terms, I mean, spend more energy on that particular object to bring in a physical change in it. Force can also be without actual physical contact, like the magnetic force or the nuclear force. So, force has a very vast set of definitions. Each of them might originate from the same idea, but are used in their own unique ways. But is gravity really a force?

The first theory of gravity was stated by Aristotle in the 4th century BC. He stated that there can be no effect of motion without any cause. The cause of motion for bodies to fall down was because it was the tendency of all bodies to fall towards the centre of the universe and obviously, Earth was considered to be the centre of the universe back then. Many other famous astronomers like Bhaskaracharya theorized geocentric theory and defined gravity as an attractive force directed only towards Earth.

Coming to the modern era, scientists like Galileo, Copernicus, Kepler and Newton made significant observations and contributions to this deeply misunderstood concept. According to the widely accepted Newton's theory of gravity, gravity is a force caused by matter due to its mass. The attractive force is the weakest force in nature and exists between all bodies in the universe. This concept was so ahead of its time that with minimal flaws, it was accepted and in fact still in use. The Newtonian concept of gravity obeys rules in practical and classical mechanics. However, when it comes to the cosmos, the reality is broken. This theory does not account for black holes or other complicated cosmic phenomena. But why should mass give rise to a universal attractive force? There must be a reason to it, right?

In recent times, the concept of gravity is dissected using the theory of space-time. Space-time is a four-dimensional concept that considers space and time collectively to explain the concept of gravity. According to the concept of space-time, matter is wrapped around by an invisible fabric. Any matter that obeys the space-time continuum creates a bend that is proportional to its mass. The heavier the matter, the deeper the bend. This curve on the space-time influences an attractive force on every other object in the universe. To understand this better, the space-time continuum is considered as a trampoline. Now, objects create a larger bend and that bend attracts the other objects towards it. On a larger scale, the distance between cosmic objects is very big. So, the attractive force is less and takes time.

But does this mean that the heavier bodies tend to attract lighter ones till there's a collision? Not necessarily. Remember, the concept of gravity is two directional. That is, the lighter object also inflicts a certain amount of force on the heavier object, making it orbit around the heavier object. This is the reason why the Earth never collides with the Sun. The Earth has its own gravitational force which it uses to orbit around the Sun. However, this understanding is broken when the limits are exceeded. If something as dense as a black hole exists near anything, it'd suck in everything because the space-time bend is so deep that it collapses into a singularity. This singularity is a four-dimensional concept and requires a very deep knowledge about the topic. Let's stick to the simple terms for our good.

So, black holes, right? What makes them suck up everything including light? Firstly, no one has officially seen any black hole or went into it (God forbid!). So, no one really knows what happens to the matter that goes in. Then, black holes absorb everything. And everything means literally everything, from stars to even light. So, there is no way to send radio waves to a black hole and get them back. The only way to confirm the presence of black holes accurately is by detecting what is now called 'Gravitational waves' which is a distortion in the space-time continuum.

So, how is Einstein's theory different from Newton's theory? Both of them fail to explain the existence and the gravity of black holes. The advantage of the space-time theory is that it takes in time as a dimension to explain relative motions of cosmic bodies. This gives us an advantage of verifying the laws of physics from different perspectives.

Take an example of two people moving at different speeds. If both of them are projecting a light ray at each other, and the speed of light from one person's point of view is calculated, it varies for both of them! But the laws of physics say that the value of relative speed of any object is same when observed from different perspectives. To save physics from being broken, Einstein stitched up the three-dimensional space with time and called it space-time. When the speed of light was calculated this time, it was a constant and physics was saved! Of course, this is just a simple analogy to understand the importance of the continuum and there's so much more!

So, what is gravity? We began by defining force and then went on till the present-day theory and all about black holes. We learnt that gravity is directly linked to the space-time continuum. All the theories that we use today are true in their own terms. We have learnt so much about this and yet we have just scratched the surface. The more we discover, we realize there's more to discover. This is the beauty of science. It never exhausts ever.

There's much more to tell you and I'll talk to you in the next one!

- Sameer Devipur