## Gravity - force or curvature of spacetime

As defined by newton, gravity is an attractive force which acts between two objects and this force is given by F=Gm1m2/r^2.

Where, m1 & m2 are mass of the objects and r is the distance between them. We can see that the force is inversely proportional to the distance between the objects and it reduces exponentially as the distance between the objects increases. We can clearly see that gravity is a force as it provides acceleration which again justify the Newton’s second law of motion, F = m*a.

Now if we look gravity from different perspective as described by Einstein, it is a curvature of spacetime and not a force.Its bit difficult to understand at the first go as in where there is discrepancy in Einstein’s law of gravity and Newton’s law of gravity. Before we deep dive more into it, let’s understand what is spacetime.

When we talk about space, normally we think it’s an isolated place where nothing exists. But in reality, space has nothing to do about nothingness (not even close). If you take out all the planets, galaxies, nebulas, stars, black hole and every matter present outside the universe, you will still be left with something called Space. It’s a real entity and is as real as gravity. We call it as fabric of space time. Why time is a part of space, we will discuss in another topic.

Fabric of spacetime can bend and twist just like a sheet of fabric. Now imagine a sheet of fabric made of elastic material and when you place a heavy ball in between of the sheet, we can see that bends the fabric of sheet. This is what matter do to the fabric of spacetime. Matter bends the fabric of space time and the more massive object is, the more it stretches the space.

We can clearly see that gravity is a force as it provides acceleration which again justify the Newton’s second law of motion, F = m*a.

Now if we look gravity from different perspective as described by Einstein, it is a curvature of spacetime and not a force.

Its bit difficult to understand at the first go as in where there is discrepancy in Einstein’s law of gravity and Newton’s law of gravity. Before we deep dive more into it, let’s understand what is spacetime.

When we talk about space, normally we think it’s an isolated place where nothing exists. But in reality, space has nothing to do about nothingness (not even close). If you take out all the planets, galaxies, nebulas, stars, black hole and every matter present outside the universe, you will still be left with something called Space. It’s a real entity and is as real as gravity. We call it as fabric of space time. Why time is a part of space, we will discuss in another topic.

Fabric of spacetime can bend and twist just like a sheet of fabric. Now imagine a sheet of fabric made of elastic material and when you place a heavy ball in between of the sheet, we can see that bends the fabric of sheet. This is what matter do to the fabric of spacetime. Matter bends the fabric of space time and the more massive object is, the more it stretches the space. Now if we look back in the equation formulated by Newton, it states that the gravitational force acts instantaneously and there is no factor of time present in it. In simple language, according to newton if somehow Sun disappear right at this moment then earth will follow its tangential path and will start move in a straight line. But as we know that’s not true as gravity travels at speed of light and if sun happens to disappear right at this moment then the earth will still revolve in its original path for about 8 minutes as the gravity takes time to reach and the final effect of gravity by sun will disappear after 8 minutes as light takes same amount of time to travel from sun to earth.

Einstein thought that if there is a force then there must be some kind of source for it just like electromagnetic forces created due to the charged particle. When we take gravity into consideration there is no such elementary particle which carries this force and the only thing which interact between two objects is the “Space”. Image credit: https://www.science.org.au/

Gravity can also be considered as a constant acceleration through spacetime. Imagine you are in a spaceship and it is constantly accelerating upwards at a rate of 9.8 m/s^2. You will feel the same force of gravity that you feel on earth. Einstein realised that the effects of acceleration and gravity were indistinguishable too. This is called the equivalence principle.

Now the question is if Gravity is a result of curvature of space time then why does it have an acceleration and from where does this acceleration comes from? Which force is acting behind this acceleration? as without any force we can’t have acceleration.

If we consider the experiment of elastic sheet wherein a heavy ball is placed in between and when we place a small ball around on the sheet it starts revolving around the heavy ball due to the curvature of the sheet but again, we are forgetting one thing that we are conducting the experiment in the presence of gravity. It is like we are using gravity to explain gravity.

Will the experiment give the same result if we conduct it in space and not on earth? What if we place a heavy ball on elastic sheet in space and then place the small ball around it, will it revolve around the heavy ball? Will the heavy ball will create a curvature on the fabric of elastic sheet?

It won’t as there is no force in space to pull the heavy ball down to create a curvature on the elastic sheet.

Gravity is one heck of a mystery to all of us. If it is a curvature of space created by celestial bodies, then why does the rest of the celestial body around it follow that curvature? Which force is driving them to orbit around the curvature? If gravity is not a force, then why does it show its effect on nearby bodies? What can be the elementary particle behind this mysterious force?